diff --git a/src/java.base/share/classes/java/io/ObjectInputStream.java b/src/java.base/share/classes/java/io/ObjectInputStream.java
index 520eb3f9e89..fc53a806c4c 100644
--- a/src/java.base/share/classes/java/io/ObjectInputStream.java
+++ b/src/java.base/share/classes/java/io/ObjectInputStream.java
@@ -479,7 +479,7 @@ protected ObjectInputStream() throws IOException, SecurityException {
      *          stream instead of objects.
      * @throws  IOException Any of the usual Input/Output related exceptions.
      */
-    public final @Nullable Object readObject()
+    public final Object readObject()
         throws IOException, ClassNotFoundException {
         return readObject(Object.class);
     }
diff --git a/src/java.base/share/classes/java/lang/AbstractStringBuilder.java b/src/java.base/share/classes/java/lang/AbstractStringBuilder.java
index a5f5a3a9f79..135221bcbc7 100644
--- a/src/java.base/share/classes/java/lang/AbstractStringBuilder.java
+++ b/src/java.base/share/classes/java/lang/AbstractStringBuilder.java
@@ -612,7 +612,7 @@ public AbstractStringBuilder append(@Nullable StringBuffer sb) {
     /**
      * @since 1.8
      */
-    AbstractStringBuilder append(AbstractStringBuilder asb) {
+    AbstractStringBuilder append(@Nullable AbstractStringBuilder asb) {
         if (asb == null) {
             return appendNull();
         }
@@ -1676,6 +1676,7 @@ void getBytes(byte[] dst, int dstBegin, byte coder) {
     }
 
     /* for readObject() */
+    @SuppressWarnings("nullness:assignment.type.incompatible") // This like a false positive to me
     void initBytes(char[] value, int off, int len) {
         if (String.COMPACT_STRINGS) {
             this.value = StringUTF16.compress(value, off, len);
diff --git a/src/java.base/share/classes/java/lang/ApplicationShutdownHooks.java b/src/java.base/share/classes/java/lang/ApplicationShutdownHooks.java
index 7d8b25aac94..87a7b5636b5 100644
--- a/src/java.base/share/classes/java/lang/ApplicationShutdownHooks.java
+++ b/src/java.base/share/classes/java/lang/ApplicationShutdownHooks.java
@@ -25,6 +25,7 @@
 package java.lang;
 
 import org.checkerframework.checker.interning.qual.UsesObjectEquals;
+import org.checkerframework.checker.nullness.qual.Nullable;
 import org.checkerframework.framework.qual.AnnotatedFor;
 
 import java.util.*;
@@ -40,7 +41,7 @@
 @AnnotatedFor({"interning"})
 @UsesObjectEquals class ApplicationShutdownHooks {
     /* The set of registered hooks */
-    private static IdentityHashMap<Thread, Thread> hooks;
+    private static @Nullable IdentityHashMap<Thread, Thread> hooks;
     static {
         try {
             Shutdown.add(1 /* shutdown hook invocation order */,
@@ -95,6 +96,7 @@ static synchronized boolean remove(Thread hook) {
      * to run in. Hooks are run concurrently and this method waits for
      * them to finish.
      */
+    @SuppressWarnings("nullness:dereference.of.nullable") // AOSEN: hooks is null only if illegal state exception is thrown
     static void runHooks() {
         Collection<Thread> threads;
         synchronized(ApplicationShutdownHooks.class) {
diff --git a/src/java.base/share/classes/java/lang/Boolean.java b/src/java.base/share/classes/java/lang/Boolean.java
index d3b6b532b97..d1804a914af 100644
--- a/src/java.base/share/classes/java/lang/Boolean.java
+++ b/src/java.base/share/classes/java/lang/Boolean.java
@@ -313,7 +313,7 @@ public boolean equals(@Nullable Object obj) {
     @Pure
     @StaticallyExecutable
     @EnsuresNonNullIf(expression="#1", result=true)
-    public static boolean getBoolean(@Nullable String name) {
+    public static boolean getBoolean(String name) {
         boolean result = false;
         try {
             result = parseBoolean(System.getProperty(name));
diff --git a/src/java.base/share/classes/java/lang/Character.java b/src/java.base/share/classes/java/lang/Character.java
index 37f024e1d59..1d824c30ff5 100644
--- a/src/java.base/share/classes/java/lang/Character.java
+++ b/src/java.base/share/classes/java/lang/Character.java
@@ -3813,7 +3813,7 @@ private UnicodeBlock(String idName, String... aliases) {
             0x100000, // 100000..10FFFF; Supplementary Private Use Area-B
         };
 
-        private static final UnicodeBlock[] blocks = {
+        private static final @Nullable UnicodeBlock[] blocks = {
             BASIC_LATIN,
             LATIN_1_SUPPLEMENT,
             LATIN_EXTENDED_A,
@@ -11474,7 +11474,7 @@ static int toUpperCaseEx(int codePoint) {
      */
     @SideEffectFree
     @StaticallyExecutable
-    static char[] toUpperCaseCharArray(int codePoint) {
+    static char @Nullable [] toUpperCaseCharArray(int codePoint) {
         // As of Unicode 6.0, 1:M uppercasings only happen in the BMP.
         assert isBmpCodePoint(codePoint);
         return CharacterData.of(codePoint).toUpperCaseCharArray(codePoint);
@@ -11542,7 +11542,7 @@ public static char reverseBytes(char ch) {
      */
     @Pure
     @StaticallyExecutable
-    public static String getName(int codePoint) {
+    public static @Nullable String getName(int codePoint) {
         if (!isValidCodePoint(codePoint)) {
             throw new IllegalArgumentException(
                 String.format("Not a valid Unicode code point: 0x%X", codePoint));
diff --git a/src/java.base/share/classes/java/lang/CharacterData.java b/src/java.base/share/classes/java/lang/CharacterData.java
index 566dbbabe5d..43a5e68b07d 100644
--- a/src/java.base/share/classes/java/lang/CharacterData.java
+++ b/src/java.base/share/classes/java/lang/CharacterData.java
@@ -26,6 +26,7 @@
 package java.lang;
 
 import org.checkerframework.checker.interning.qual.UsesObjectEquals;
+import org.checkerframework.checker.nullness.qual.Nullable;
 import org.checkerframework.common.value.qual.IntRange;
 import org.checkerframework.framework.qual.AnnotatedFor;
 
@@ -55,7 +56,7 @@ int toUpperCaseEx(int ch) {
         return toUpperCase(ch);
     }
 
-    char[] toUpperCaseCharArray(int ch) {
+    char @Nullable [] toUpperCaseCharArray(int ch) {
         return null;
     }
 
diff --git a/src/java.base/share/classes/java/lang/CharacterName.java b/src/java.base/share/classes/java/lang/CharacterName.java
index 007ddf57a33..8009239b61a 100644
--- a/src/java.base/share/classes/java/lang/CharacterName.java
+++ b/src/java.base/share/classes/java/lang/CharacterName.java
@@ -26,6 +26,7 @@
 package java.lang;
 
 import org.checkerframework.checker.interning.qual.UsesObjectEquals;
+import org.checkerframework.checker.nullness.qual.Nullable;
 import org.checkerframework.framework.qual.AnnotatedFor;
 
 import java.io.DataInputStream;
@@ -54,6 +55,7 @@
     private CharacterName()  {
         try (@SuppressWarnings("removal") DataInputStream dis = new DataInputStream(new InflaterInputStream(
             AccessController.doPrivileged(new PrivilegedAction<>() {
+                @SuppressWarnings("nullness:return.type.incompatible")
                 public InputStream run() {
                     return getClass().getResourceAsStream("uniName.dat");
                 }
@@ -144,7 +146,7 @@ public static CharacterName getInstance() {
         return cname;
     }
 
-    public String getName(int cp) {
+    public @Nullable String getName(int cp) {
         int off = 0;
         int bk = bkIndices[cp >> 8];
         if (bk == -1 || (off = lookup[(bk << 8) + (cp & 0xff)]) == 0)
diff --git a/src/java.base/share/classes/java/lang/Class.java b/src/java.base/share/classes/java/lang/Class.java
index dbb50a98bb3..eff3ecfc71b 100644
--- a/src/java.base/share/classes/java/lang/Class.java
+++ b/src/java.base/share/classes/java/lang/Class.java
@@ -302,6 +302,7 @@ public String toString(@GuardSatisfied Class<T> this) {
      *
      * @since 1.8
      */
+    @SuppressWarnings("nullness:dereference.of.nullable") // False positive
     public String toGenericString() {
         if (isPrimitive()) {
             return toString();
@@ -403,7 +404,7 @@ static String typeVarBounds(TypeVariable<?> typeVar) {
      */
     @ForName
     @CallerSensitive
-    public static Class<? extends Object> forName(@ClassGetName String className)
+    public static Class<? extends @Nullable Object> forName(@ClassGetName String className)
                 throws ClassNotFoundException {
         Class<?> caller = Reflection.getCallerClass();
         return forName0(className, true, ClassLoader.getClassLoader(caller), caller);
@@ -479,7 +480,7 @@ public static Class<? extends Object> forName(@ClassGetName String className)
      * @since     1.2
      */
     @CallerSensitive
-    public static Class<? extends Object> forName(@ClassGetName String name, boolean initialize,
+    public static Class<? extends @Nullable Object> forName(@ClassGetName String name, boolean initialize,
                                    @Nullable ClassLoader loader)
         throws ClassNotFoundException
     {
@@ -503,8 +504,8 @@ public static Class<? extends Object> forName(@ClassGetName String name, boolean
 
     /** Called after security check for system loader access checks have been made. */
     private static native Class<?> forName0(String name, boolean initialize,
-                                            ClassLoader loader,
-                                            Class<?> caller)
+                                            @Nullable ClassLoader loader,
+                                            @Nullable Class<?> caller)
         throws ClassNotFoundException;
 
 
@@ -556,7 +557,7 @@ private static native Class<?> forName0(String name, boolean initialize,
      */
     @SuppressWarnings("removal")
     @CallerSensitive
-    public static Class<? extends Object> forName(Module module, String name) {
+    public static @Nullable Class<? extends @Nullable Object> forName(Module module, String name) {
         Objects.requireNonNull(module);
         Objects.requireNonNull(name);
 
@@ -569,7 +570,7 @@ public static Class<? extends Object> forName(Module module, String name) {
                 // java.base has all permissions
                 sm.checkPermission(SecurityConstants.GET_CLASSLOADER_PERMISSION);
             }
-            PrivilegedAction<ClassLoader> pa = module::getClassLoader;
+            PrivilegedAction<@Nullable ClassLoader> pa = module::getClassLoader;
             cl = AccessController.doPrivileged(pa);
         } else {
             cl = module.getClassLoader();
@@ -635,7 +636,7 @@ public static Class<? extends Object> forName(Module module, String name) {
      *          s.checkPackageAccess()} denies access to the package
      *          of this class.
      */
-    @SuppressWarnings("removal")
+    @SuppressWarnings({"removal","nullness:return.type.incompatible"})
     @CallerSensitive
     @Deprecated(since="9")
     public @NonNull T newInstance()
@@ -662,7 +663,7 @@ public static Class<? extends Object> forName(Module module, String name) {
                 // access check is done with the true caller
                 java.security.AccessController.doPrivileged(
                     new java.security.PrivilegedAction<>() {
-                        public Void run() {
+                        public @NonNull Void run() {
                                 c.setAccessible(true);
                                 return null;
                             }
@@ -676,7 +677,7 @@ public Void run() {
 
         try {
             Class<?> caller = Reflection.getCallerClass();
-            return getReflectionFactory().newInstance(tmpConstructor, null, caller);
+            return getReflectionFactory().<T>3(tmpConstructor, null, caller);
         } catch (InvocationTargetException e) {
             Unsafe.getUnsafe().throwException(e.getTargetException());
             // Not reached
@@ -719,6 +720,7 @@ public Void run() {
     @Pure
     @EnsuresNonNullIf(expression={"#1"}, result=true)
     @IntrinsicCandidate
+    @SuppressWarnings("flowexpr.parameter.not.final") // AOSEN: I don't understand this warning
     public native boolean isInstance(@GuardSatisfied Class<T> this, @Nullable Object obj);
 
 
@@ -1120,6 +1122,7 @@ public TypeVariable<Class<T>>[] getTypeParameters() {
      * @since 9
      * @jls 6.7 Fully Qualified Names
      */
+    @SuppressWarnings("nullness:dereference.of.nullable") // c is not null if isArray() is true
     public @DotSeparatedIdentifiers String getPackageName() {
         String pn = this.packageName;
         if (pn == null) {
@@ -1184,7 +1187,7 @@ public TypeVariable<Class<T>>[] getTypeParameters() {
      * @return an array of interfaces directly implemented by this class
      */
     @SideEffectFree
-    public Class<? extends Object>[] getInterfaces(@GuardSatisfied Class<T> this) {
+    public Class<? extends @Nullable Object>[] getInterfaces(@GuardSatisfied Class<T> this) {
         // defensively copy before handing over to user code
         return getInterfaces(true);
     }
@@ -1272,7 +1275,7 @@ public Type[] getGenericInterfaces() {
      * @since 1.1
      */
     @Pure
-    public @Nullable Class<? extends Object> getComponentType(@GuardSatisfied Class<T> this) {
+    public @Nullable Class<? extends @Nullable Object> getComponentType(@GuardSatisfied Class<T> this) {
         // Only return for array types. Storage may be reused for Class for instance types.
         if (isArray()) {
             return componentType;
@@ -1288,7 +1291,7 @@ public Type[] getGenericInterfaces() {
      * If this class does not represent an array class, then this method returns
      * {@code null}.
      */
-    private Class<?> elementType() {
+    private @Nullable Class<?> elementType() {
         if (!isArray()) return null;
 
         Class<?> c = this;
@@ -1444,7 +1447,7 @@ private Class<?> elementType() {
 
     private native Object[] getEnclosingMethod0();
 
-    private EnclosingMethodInfo getEnclosingMethodInfo() {
+    private @Nullable EnclosingMethodInfo getEnclosingMethodInfo() {
         Object[] enclosingInfo = getEnclosingMethod0();
         if (enclosingInfo == null)
             return null;
@@ -1547,7 +1550,7 @@ private static Class<?> toClass(Type o) {
      * @since 1.5
      */
     @CallerSensitive
-    public @Nullable Constructor<? extends Object> getEnclosingConstructor() throws SecurityException {
+    public @Nullable Constructor<? extends @Nullable Object> getEnclosingConstructor() throws SecurityException {
         EnclosingMethodInfo enclosingInfo = getEnclosingMethodInfo();
 
         if (enclosingInfo == null)
@@ -1613,7 +1616,7 @@ private static Class<?> toClass(Type o) {
      * @since 1.1
      */
     @CallerSensitive
-    public @Nullable Class<? extends Object> getDeclaringClass() throws SecurityException {
+    public @Nullable Class<? extends @Nullable Object> getDeclaringClass() throws SecurityException {
         final Class<?> candidate = getDeclaringClass0();
 
         if (candidate != null) {
@@ -1645,7 +1648,7 @@ private static Class<?> toClass(Type o) {
      */
     @Pure
     @CallerSensitive
-    public @Nullable Class<? extends Object> getEnclosingClass() throws SecurityException {
+    public @Nullable Class<? extends @Nullable Object> getEnclosingClass() throws SecurityException {
         // There are five kinds of classes (or interfaces):
         // a) Top level classes
         // b) Nested classes (static member classes)
@@ -1725,6 +1728,7 @@ private String getSimpleName0() {
      * @return an informative string for the name of this class or interface
      * @since 1.8
      */
+    @SuppressWarnings("nullness:dereference.of.nullable") // cl is not null if isArray() is true
     public String getTypeName() {
         if (isArray()) {
             try {
@@ -1842,7 +1846,7 @@ public boolean isMemberClass(@GuardSatisfied Class<T> this) {
      * Returns {@code null} if the underlying class is a top level
      * class.
      */
-    private String getSimpleBinaryName() {
+    private @Nullable String getSimpleBinaryName() {
         if (isTopLevelClass())
             return null;
         String name = getSimpleBinaryName0();
@@ -1904,8 +1908,8 @@ private boolean hasEnclosingMethodInfo() {
      *
      * @since 1.1
      */
-    @SuppressWarnings("removal")
     @CallerSensitive
+    @SuppressWarnings({"removal", "nullness:override.return.invalid"})
     public Class<? extends Object>[] getClasses() {
         SecurityManager sm = System.getSecurityManager();
         if (sm != null) {
@@ -2107,7 +2111,7 @@ public Method[] getMethods() throws SecurityException {
      * @since 1.1
      */
     @CallerSensitive
-    public Constructor<? extends Object>[] getConstructors() throws SecurityException {
+    public Constructor<? extends @Nullable Object>[] getConstructors() throws SecurityException {
         @SuppressWarnings("removal")
         SecurityManager sm = System.getSecurityManager();
         if (sm != null) {
@@ -2369,7 +2373,7 @@ public Constructor<T> getConstructor(Class<?>... parameterTypes)
      * @jls 8.5 Member Type Declarations
      */
     @CallerSensitive
-    public Class<? extends Object>[] getDeclaredClasses() throws SecurityException {
+    public Class<? extends @Nullable Object>[] getDeclaredClasses() throws SecurityException {
         @SuppressWarnings("removal")
         SecurityManager sm = System.getSecurityManager();
         if (sm != null) {
@@ -2484,7 +2488,7 @@ public Field[] getDeclaredFields() throws SecurityException {
      * @since 16
      */
     @CallerSensitive
-    public RecordComponent[] getRecordComponents() {
+    public RecordComponent @Nullable [] getRecordComponents() {
         @SuppressWarnings("removal")
         SecurityManager sm = System.getSecurityManager();
         if (sm != null) {
@@ -3007,6 +3011,10 @@ public Constructor<T> getDeclaredConstructor(Class<?>... parameterTypes)
      * the module. For other callers, then the package needs to be open to
      * the caller.
      */
+    @SuppressWarnings({"nullness:argument.type.incompatible", "nullness:dereference.of.nullable"})
+    // This code is updated in the latest JDK for the first warning
+    // https://github.com/openjdk/jdk/blob/4a4209ffef8f8d65054cbf46ebf8e169d100c0d8/src/java.base/share/classes/java/lang/Class.java#L2689-L2696
+    // The second warning is a false positive because this method is only called by when guarded with getModule().isNamed()
     private boolean isOpenToCaller(String name, Class<?> caller) {
         // assert getModule().isNamed();
         Module thisModule = getModule();
@@ -3122,7 +3130,7 @@ private void checkMemberAccess(@SuppressWarnings("removal") SecurityManager sm,
      *
      * NOTE: this method should only be called if a SecurityManager is active
      */
-    private void checkPackageAccess(@SuppressWarnings("removal") SecurityManager sm, final ClassLoader ccl,
+    private void checkPackageAccess(@SuppressWarnings("removal") SecurityManager sm, final @Nullable ClassLoader ccl,
                                     boolean checkProxyInterfaces) {
         final ClassLoader cl = getClassLoader0();
 
@@ -3152,7 +3160,7 @@ private void checkPackageAccess(@SuppressWarnings("removal") SecurityManager sm,
      * NOTE: this method does not support Proxy classes
      */
     private static void checkPackageAccessForPermittedSubclasses(@SuppressWarnings("removal") SecurityManager sm,
-                                    final ClassLoader ccl, Class<?>[] subClasses) {
+                                    final @Nullable ClassLoader ccl, Class<?>[] subClasses) {
         final ClassLoader cl = subClasses[0].getClassLoader0();
 
         if (ReflectUtil.needsPackageAccessCheck(ccl, cl)) {
@@ -3315,7 +3323,7 @@ private GenericsFactory getFactory() {
 
     // accessor for generic info repository;
     // generic info is lazily initialized
-    private ClassRepository getGenericInfo() {
+    private @Nullable ClassRepository getGenericInfo() {
         ClassRepository genericInfo = this.genericInfo;
         if (genericInfo == null) {
             String signature = getGenericSignature0();
@@ -3519,7 +3527,7 @@ private Method[] privateGetPublicMethods() {
     //
 
     // This method does not copy the returned Field object!
-    private static Field searchFields(Field[] fields, String name) {
+    private static @Nullable Field searchFields(Field[] fields, String name) {
         for (Field field : fields) {
             if (field.getName().equals(name)) {
                 return field;
@@ -3531,7 +3539,7 @@ private static Field searchFields(Field[] fields, String name) {
     // Returns a "root" Field object. This Field object must NOT
     // be propagated to the outside world, but must instead be copied
     // via ReflectionFactory.copyField.
-    private Field getField0(String name) {
+    private @Nullable Field getField0(String name) {
         // Note: the intent is that the search algorithm this routine
         // uses be equivalent to the ordering imposed by
         // privateGetPublicFields(). It fetches only the declared
@@ -3564,7 +3572,7 @@ private Field getField0(String name) {
     }
 
     // This method does not copy the returned Method object!
-    private static Method searchMethods(Method[] methods,
+    private static @Nullable Method searchMethods(Method[] methods,
                                         String name,
                                         Class<?>[] parameterTypes)
     {
@@ -3587,7 +3595,7 @@ && arrayContentsEq(parameterTypes,
     // Returns a "root" Method object. This Method object must NOT
     // be propagated to the outside world, but must instead be copied
     // via ReflectionFactory.copyMethod.
-    private Method getMethod0(String name, Class<?>[] parameterTypes) {
+    private @Nullable Method getMethod0(String name, Class<?> @Nullable [] parameterTypes) {
         PublicMethods.MethodList res = getMethodsRecursive(
             name,
             parameterTypes == null ? EMPTY_CLASS_ARRAY : parameterTypes,
@@ -3598,7 +3606,7 @@ private Method getMethod0(String name, Class<?>[] parameterTypes) {
     // Returns a list of "root" Method objects. These Method objects must NOT
     // be propagated to the outside world, but must instead be copied
     // via ReflectionFactory.copyMethod.
-    private PublicMethods.MethodList getMethodsRecursive(String name,
+    private PublicMethods.@Nullable MethodList getMethodsRecursive(String name,
                                                          Class<?>[] parameterTypes,
                                                          boolean includeStatic) {
         // 1st check declared public methods
@@ -3718,7 +3726,7 @@ private static <U> Constructor<U>[] copyConstructors(Constructor<U>[] arg) {
     /**
      * Helper method to get the method name from arguments.
      */
-    private String methodToString(String name, Class<?>[] argTypes) {
+    private String methodToString(String name, Class<?> @Nullable [] argTypes) {
         return getName() + '.' + name +
                 ((argTypes == null || argTypes.length == 0) ?
                 "()" :
@@ -3778,6 +3786,7 @@ private String methodToString(String name, Class<?>[] argTypes) {
      * @see    java.lang.ClassLoader#setDefaultAssertionStatus
      * @since  1.4
      */
+    @SuppressWarnings("contracts.precondition.not.satisfied")
     public boolean desiredAssertionStatus() {
         ClassLoader loader = getClassLoader0();
         // If the loader is null this is a system class, so ask the VM
@@ -3872,7 +3881,7 @@ private static ReflectionFactory getReflectionFactory() {
      * @since 1.5
      * @jls 8.9.1 Enum Constants
      */
-    public @NonNull T @Nullable [] getEnumConstants() {
+    public T @Nullable [] getEnumConstants() {
         T[] values = getEnumConstantsShared();
         return (values != null) ? values.clone() : null;
     }
@@ -3884,14 +3893,14 @@ private static ReflectionFactory getReflectionFactory() {
      * uncloned, cached, and shared by all callers.
      */
     @SuppressWarnings("removal")
-    T[] getEnumConstantsShared() {
+    T @Nullable [] getEnumConstantsShared() {
         T[] constants = enumConstants;
         if (constants == null) {
             if (!isEnum()) return null;
             try {
                 final Method values = getMethod("values");
                 java.security.AccessController.doPrivileged(
-                    new java.security.PrivilegedAction<>() {
+                    new java.security.PrivilegedAction<@Nullable Void>() {
                         public Void run() {
                                 values.setAccessible(true);
                                 return null;
@@ -3908,7 +3917,7 @@ public Void run() {
         }
         return constants;
     }
-    private transient volatile T[] enumConstants;
+    private transient volatile T @Nullable [] enumConstants;
 
     /**
      * Returns a map from simple name to enum constant.  This package-private
@@ -3917,7 +3926,8 @@ public Void run() {
      * efficiently.  Note that the map is returned by this method is
      * created lazily on first use.  Typically it won't ever get created.
      */
-    Map<String, @NonNull T> enumConstantDirectory() {
+    @SuppressWarnings("nullness:dereference.of.nullable") // Should we require T to extend @NonNull?
+    Map<String, T> enumConstantDirectory() {
         Map<String, T> directory = enumConstantDirectory;
         if (directory == null) {
             T[] universe = getEnumConstantsShared();
@@ -3951,7 +3961,7 @@ public Void run() {
     public @PolyNull @Signed T cast(@PolyNull Object obj) {
         if (obj != null && !isInstance(obj))
             throw new ClassCastException(cannotCastMsg(obj));
-        return (T) obj;
+        return (@PolyNull T) obj;
     }
 
     private String cannotCastMsg(Object obj) {
@@ -3998,7 +4008,7 @@ public <U> Class<? extends U> asSubclass(Class<U> clazz) {
      */
     @Override
     @SuppressWarnings("unchecked")
-    public <A extends Annotation> @Nullable A getAnnotation(Class<A> annotationClass) {
+    public <A extends @Nullable Annotation> @Nullable A getAnnotation(Class<A> annotationClass) {
         Objects.requireNonNull(annotationClass);
 
         return (A) annotationData().annotations.get(annotationClass);
@@ -4024,7 +4034,7 @@ public boolean isAnnotationPresent(@GuardSatisfied Class<T> this, @GuardSatisfie
      * @since 1.8
      */
     @Override
-    public <A extends Annotation> A[] getAnnotationsByType(Class<A> annotationClass) {
+    public <A extends @Nullable Annotation> A[] getAnnotationsByType(Class<A> annotationClass) {
         Objects.requireNonNull(annotationClass);
 
         AnnotationData annotationData = annotationData();
@@ -4055,7 +4065,7 @@ public Annotation[] getAnnotations() {
      */
     @Override
     @SuppressWarnings("unchecked")
-    public <A extends Annotation> @Nullable A getDeclaredAnnotation(Class<A> annotationClass) {
+    public <A extends @Nullable Annotation> @Nullable A getDeclaredAnnotation(Class<A> annotationClass) {
         Objects.requireNonNull(annotationClass);
 
         return (A) annotationData().declaredAnnotations.get(annotationClass);
@@ -4070,7 +4080,7 @@ public Annotation[] getAnnotations() {
      * @since 1.8
      */
     @Override
-    public <A extends Annotation> A[] getDeclaredAnnotationsByType(Class<A> annotationClass) {
+    public <A extends @Nullable Annotation> A[] getDeclaredAnnotationsByType(Class<A> annotationClass) {
         Objects.requireNonNull(annotationClass);
 
         return AnnotationSupport.getDirectlyAndIndirectlyPresent(annotationData().declaredAnnotations,
@@ -4091,14 +4101,14 @@ public Annotation[] getDeclaredAnnotations()  {
 
     // annotation data that might get invalidated when JVM TI RedefineClasses() is called
     private static class AnnotationData {
-        final Map<Class<? extends Annotation>, Annotation> annotations;
-        final Map<Class<? extends Annotation>, Annotation> declaredAnnotations;
+        final Map<Class<? extends @Nullable Annotation>, Annotation> annotations;
+        final Map<Class<? extends @Nullable Annotation>, Annotation> declaredAnnotations;
 
         // Value of classRedefinedCount when we created this AnnotationData instance
         final int redefinedCount;
 
-        AnnotationData(Map<Class<? extends Annotation>, Annotation> annotations,
-                       Map<Class<? extends Annotation>, Annotation> declaredAnnotations,
+        AnnotationData(Map<Class<? extends @Nullable Annotation>, Annotation> annotations,
+                       Map<Class<? extends @Nullable Annotation>, Annotation> declaredAnnotations,
                        int redefinedCount) {
             this.annotations = annotations;
             this.declaredAnnotations = declaredAnnotations;
@@ -4129,15 +4139,15 @@ private AnnotationData annotationData() {
     }
 
     private AnnotationData createAnnotationData(int classRedefinedCount) {
-        Map<Class<? extends Annotation>, Annotation> declaredAnnotations =
+        Map<Class<? extends @Nullable Annotation>, Annotation> declaredAnnotations =
             AnnotationParser.parseAnnotations(getRawAnnotations(), getConstantPool(), this);
         Class<?> superClass = getSuperclass();
-        Map<Class<? extends Annotation>, Annotation> annotations = null;
+        Map<Class<? extends @Nullable Annotation>, Annotation> annotations = null;
         if (superClass != null) {
-            Map<Class<? extends Annotation>, Annotation> superAnnotations =
+            Map<Class<? extends @Nullable Annotation>, Annotation> superAnnotations =
                 superClass.annotationData().annotations;
-            for (Map.Entry<Class<? extends Annotation>, Annotation> e : superAnnotations.entrySet()) {
-                Class<? extends Annotation> annotationClass = e.getKey();
+            for (Map.Entry<Class<? extends @Nullable Annotation>, Annotation> e : superAnnotations.entrySet()) {
+                Class<? extends @Nullable Annotation> annotationClass = e.getKey();
                 if (AnnotationType.getInstance(annotationClass).isInherited()) {
                     if (annotations == null) { // lazy construction
                         annotations = new LinkedHashMap<>((Math.max(
@@ -4173,7 +4183,7 @@ AnnotationType getAnnotationType() {
         return annotationType;
     }
 
-    Map<Class<? extends Annotation>, Annotation> getDeclaredAnnotationMap() {
+    Map<Class<? extends @Nullable Annotation>, Annotation> getDeclaredAnnotationMap() {
         return annotationData().declaredAnnotations;
     }
 
@@ -4201,7 +4211,7 @@ Map<Class<? extends Annotation>, Annotation> getDeclaredAnnotationMap() {
      * @return an object representing the superclass
      * @since 1.8
      */
-    public AnnotatedType getAnnotatedSuperclass() {
+    public @Nullable AnnotatedType getAnnotatedSuperclass() {
         if (this == Object.class ||
                 isInterface() ||
                 isArray() ||
@@ -4285,7 +4295,7 @@ public AnnotatedType[] getAnnotatedInterfaces() {
      * @jvms 5.4.4 Access Control
      */
     @CallerSensitive
-    public Class<? extends Object> getNestHost() {
+    public Class<? extends @Nullable Object> getNestHost() {
         if (isPrimitive() || isArray()) {
             return this;
         }
@@ -4377,7 +4387,7 @@ public boolean isNestmateOf(Class<?> c) {
      * @jvms 4.7.29 The {@code NestMembers} Attribute
      */
     @CallerSensitive
-    public Class<? extends Object>[] getNestMembers() {
+    public Class<? extends @Nullable Object>[] getNestMembers() {
         if (isPrimitive() || isArray()) {
             return new Class<?>[] { this };
         }
@@ -4494,7 +4504,7 @@ public String descriptorString() {
      */
     @Override
     @Pure
-    public @Nullable Class<? extends Object> componentType() {
+    public @Nullable Class<? extends @Nullable Object> componentType() {
         return isArray() ? componentType : null;
     }
 
@@ -4520,6 +4530,7 @@ public Class<? extends Object> arrayType() {
      * @since 12
      */
     @Override
+    @SuppressWarnings("nullness:dereference.of.nullable") // False positive, c is only null if isArray() is false
     public Optional<ClassDesc> describeConstable() {
         Class<?> c = isArray() ? elementType() : this;
         return c.isHidden() ? Optional.empty()
@@ -4575,7 +4586,7 @@ public Optional<ClassDesc> describeConstable() {
      * @since 17
      */
     @CallerSensitive
-    public Class<? extends Object>[] getPermittedSubclasses() {
+    public Class<? extends @Nullable Object> @Nullable [] getPermittedSubclasses() {
         Class<?>[] subClasses;
         if (isArray() || isPrimitive() || (subClasses = getPermittedSubclasses0()) == null) {
             return null;
diff --git a/src/java.base/share/classes/java/lang/ClassLoader.java b/src/java.base/share/classes/java/lang/ClassLoader.java
index 5e197dfa8e1..481de8af507 100644
--- a/src/java.base/share/classes/java/lang/ClassLoader.java
+++ b/src/java.base/share/classes/java/lang/ClassLoader.java
@@ -30,6 +30,7 @@
 import org.checkerframework.checker.nonempty.qual.EnsuresNonEmptyIf;
 import org.checkerframework.checker.nonempty.qual.NonEmpty;
 import org.checkerframework.checker.nullness.qual.EnsuresNonNull;
+import org.checkerframework.checker.nullness.qual.MonotonicNonNull;
 import org.checkerframework.checker.nullness.qual.Nullable;
 import org.checkerframework.checker.nullness.qual.RequiresNonNull;
 import org.checkerframework.checker.signature.qual.BinaryName;
@@ -254,7 +255,7 @@
     private final @Nullable ClassLoader parent;
 
     // class loader name
-    private final String name;
+    private final @Nullable String name;
 
     // the unnamed module for this ClassLoader
     private final Module unnamedModule;
@@ -280,6 +281,7 @@ private ParallelLoaders() {}
          * Returns {@code true} is successfully registered; {@code false} if
          * loader's super class is not registered.
          */
+        @SuppressWarnings("nullness:argument.type.incompatible") // False positive because set interface is used.
         static boolean register(Class<? extends ClassLoader> c) {
             synchronized (loaderTypes) {
                 if (loaderTypes.contains(c.getSuperclass())) {
@@ -373,7 +375,7 @@ private NamedPackage getNamedPackage(String pn, Module m) {
         return checkCreateClassLoader(null);
     }
 
-    private static Void checkCreateClassLoader(String name) {
+    private static Void checkCreateClassLoader(@Nullable String name) {
         if (name != null && name.isEmpty()) {
             throw new IllegalArgumentException("name must be non-empty or null");
         }
@@ -386,7 +388,7 @@ private static Void checkCreateClassLoader(String name) {
         return null;
     }
 
-    private ClassLoader(Void unused, String name, ClassLoader parent) {
+    private ClassLoader(Void unused, @Nullable String name, @Nullable ClassLoader parent) {
         this.name = name;
         this.parent = parent;
         this.unnamedModule = new Module(this);
@@ -507,7 +509,7 @@ protected ClassLoader() {
 
     // package-private used by StackTraceElement to avoid
     // calling the overrideable getName method
-    final String name() {
+    final @Nullable String name() {
         return name;
     }
 
@@ -640,7 +642,7 @@ protected Class<?> loadClass(@BinaryName String name, boolean resolve)
      *         this class loader, or {@code null} if the class could not be found.
      */
     @ForName
-    final Class<?> loadClass(Module module, @BinaryName String name) {
+    final @Nullable Class<?> loadClass(Module module, @BinaryName String name) {
         synchronized (getClassLoadingLock(name)) {
             // First, check if the class has already been loaded
             Class<?> c = findLoadedClass(name);
@@ -701,7 +703,7 @@ private void checkPackageAccess(Class<?> cls, ProtectionDomain pd) {
 
             final String packageName = cls.getPackageName();
             if (!packageName.isEmpty()) {
-                AccessController.doPrivileged(new PrivilegedAction<>() {
+                AccessController.doPrivileged(new PrivilegedAction<@Nullable Void>() {
                     public Void run() {
                         sm.checkPackageAccess(packageName);
                         return null;
@@ -759,7 +761,7 @@ protected Class<?> findClass(@BinaryName String name) throws ClassNotFoundExcept
      *
      * @since 9
      */
-    protected Class<?> findClass(String moduleName, String name) {
+    protected @Nullable Class<?> findClass(@Nullable String moduleName, String name) {
         if (moduleName == null) {
             try {
                 return findClass(name);
@@ -1122,12 +1124,12 @@ protected final Class<?> defineClass(@Nullable @BinaryName String name, java.nio
         return c;
     }
 
-    static native Class<?> defineClass1(ClassLoader loader, @BinaryName String name, byte[] b, int off, int len,
-                                        ProtectionDomain pd, String source);
+    static native Class<?> defineClass1(ClassLoader loader, @BinaryName @Nullable String name, byte[] b, int off, int len,
+                                        @Nullable ProtectionDomain pd, @Nullable String source);
 
-    static native Class<?> defineClass2(ClassLoader loader, @BinaryName String name, java.nio.ByteBuffer b,
-                                        int off, int len, ProtectionDomain pd,
-                                        String source);
+    static native Class<?> defineClass2(ClassLoader loader, @BinaryName @Nullable String name, java.nio.ByteBuffer b,
+                                        int off, int len, @Nullable ProtectionDomain pd,
+                                        @Nullable String source);
 
     /**
      * Defines a class of the given flags via Lookup.defineClass.
@@ -1182,7 +1184,7 @@ private void checkCerts(String name, @Nullable CodeSource cs) {
      * check to make sure the certs for the new class (certs) are the same as
      * the certs for the first class inserted in the package (pcerts)
      */
-    private boolean compareCerts(Certificate[] pcerts, Certificate @Nullable [] certs) {
+    private boolean compareCerts(Certificate[] pcerts, Certificate[] certs) {
         // empty array fast-path
         if (certs.length == 0)
             return pcerts.length == 0;
@@ -1357,7 +1359,7 @@ protected final void setSigners(Class<?> c, Object[] signers) {
      * @see java.lang.module.ModuleReader#find(String)
      * @since 9
      */
-    protected URL findResource(String moduleName, String name) throws IOException {
+    protected @Nullable URL findResource(@Nullable String moduleName, String name) throws IOException {
         if (moduleName == null) {
             return findResource(name);
         } else {
@@ -1977,6 +1979,7 @@ static ClassLoader getBuiltinAppClassLoader() {
      *
      * @see java.lang.System#initPhase3
      */
+    @SuppressWarnings("nullness:dereference.of.nullable") // AOSEN: I think this is a real possible NPE.
     static synchronized ClassLoader initSystemClassLoader() {
         if (VM.initLevel() != 3) {
             throw new InternalError("system class loader cannot be set at initLevel " +
@@ -2088,7 +2091,7 @@ static void checkClassLoaderPermission(ClassLoader cl, Class<?> caller) {
      *
      * This method does not throw IllegalArgumentException.
      */
-    Package definePackage(Class<?> c) {
+    @Nullable Package definePackage(Class<?> c) {
         if (c.isPrimitive() || c.isArray()) {
             return null;
         }
@@ -2120,7 +2123,7 @@ Package definePackage(String name, Module m) {
     /*
      * Returns a Package object for the named package
      */
-    private Package toPackage(String name, NamedPackage p, Module m) {
+    private Package toPackage(String name, @Nullable NamedPackage p, Module m) {
         // define Package object if the named package is not yet defined
         if (p == null)
             return NamedPackage.toPackage(name, m);
@@ -2245,7 +2248,7 @@ protected Package definePackage(@FullyQualifiedName String name, @Nullable Strin
      *
      * @since  9
      */
-    public final Package getDefinedPackage(String name) {
+    public final @Nullable Package getDefinedPackage(String name) {
         Objects.requireNonNull(name, "name cannot be null");
 
         NamedPackage p = packages.get(name);
@@ -2477,14 +2480,14 @@ static long findNative(@Nullable ClassLoader loader, String entryName) {
     // none of this ClassLoader's assertion status modification methods have
     // been invoked.
     // @GuardedBy("assertionLock")
-    private @Nullable Map<@Nullable String, Boolean> packageAssertionStatus = null;
+    private @MonotonicNonNull Map<@Nullable String, Boolean> packageAssertionStatus = null;
 
     // Maps String fullyQualifiedClassName to Boolean assertionStatus If this
     // field is null then we are delegating assertion status queries to the VM,
     // i.e., none of this ClassLoader's assertion status modification methods
     // have been invoked.
     // @GuardedBy("assertionLock")
-    @Nullable Map<String, Boolean> classAssertionStatus = null;
+    @MonotonicNonNull Map<String, Boolean> classAssertionStatus = null;
 
     /**
      * Sets the default assertion status for this class loader.  This setting
@@ -2632,6 +2635,7 @@ public void clearAssertionStatus() {
      * @since  1.4
      */
     @RequiresNonNull({"classAssertionStatus", "packageAssertionStatus"})
+    @SuppressWarnings("nullness:argument.type.incompatible") // False positive because map interface is used.
     boolean desiredAssertionStatus(String className) {
         synchronized (assertionLock) {
             // assert classAssertionStatus   != null;
@@ -2726,6 +2730,9 @@ private boolean trySetObjectField(String name, Object obj) {
     /**
      * Called by the VM, during -Xshare:dump
      */
+    @SuppressWarnings({"nullness:assignment.type.incompatible","nullness:dereference.of.nullable"})
+    // classLoaderValueMap is null only when reset
+    // AOSEN: parallelLockMap.clear(); is this a real NPE?
     private void resetArchivedStates() {
         parallelLockMap.clear();
         packages.clear();
diff --git a/src/java.base/share/classes/java/lang/ClassNotFoundException.java b/src/java.base/share/classes/java/lang/ClassNotFoundException.java
index 6c719b216cd..0ca108ce7a1 100644
--- a/src/java.base/share/classes/java/lang/ClassNotFoundException.java
+++ b/src/java.base/share/classes/java/lang/ClassNotFoundException.java
@@ -104,7 +104,7 @@ public ClassNotFoundException(@Nullable String s, @Nullable Throwable ex) {
      * @return the {@code Exception} that was raised while loading a class
      * @since 1.2
      */
-    public Throwable getException() {
+    public @Nullable Throwable getException() {
         return super.getCause();
     }
 
diff --git a/src/java.base/share/classes/java/lang/ClassValue.java b/src/java.base/share/classes/java/lang/ClassValue.java
index c92e4f4749f..d7181c3ab03 100644
--- a/src/java.base/share/classes/java/lang/ClassValue.java
+++ b/src/java.base/share/classes/java/lang/ClassValue.java
@@ -26,6 +26,8 @@
 package java.lang;
 
 import org.checkerframework.checker.interning.qual.UsesObjectEquals;
+import org.checkerframework.checker.nullness.qual.Nullable;
+import org.checkerframework.checker.nullness.qual.PolyNull;
 import org.checkerframework.framework.qual.AnnotatedFor;
 
 import java.util.WeakHashMap;
@@ -101,9 +103,10 @@ protected ClassValue() {
      * @see #remove
      * @see #computeValue
      */
+    @SuppressWarnings("nullness:dereference.of.nullable") // Could the entry be null?
     public T get(Class<?> type) {
         // non-racing this.hashCodeForCache : final int
-        Entry<?>[] cache;
+        @Nullable Entry<?>[] cache;
         Entry<T> e = probeHomeLocation(cache = getCacheCarefully(type), this);
         // racing e : current value <=> stale value from current cache or from stale cache
         // invariant:  e is null or an Entry with readable Entry.version and Entry.value
@@ -189,7 +192,7 @@ public void remove(Class<?> type) {
     /// --------
 
     /** Return the cache, if it exists, else a dummy empty cache. */
-    private static Entry<?>[] getCacheCarefully(Class<?> type) {
+    private static @Nullable Entry<?>[] getCacheCarefully(Class<?> type) {
         // racing type.classValueMap{.cacheArray} : null => new Entry[X] <=> new Entry[Y]
         ClassValueMap map = type.classValueMap;
         if (map == null)  return EMPTY_CACHE;
@@ -199,7 +202,7 @@ private static Entry<?>[] getCacheCarefully(Class<?> type) {
     }
 
     /** Initial, one-element, empty cache used by all Class instances.  Must never be filled. */
-    private static final Entry<?>[] EMPTY_CACHE = { null };
+    private static final @Nullable Entry<?>[] EMPTY_CACHE = { null };
 
     /**
      * Slow tail of ClassValue.get to retry at nearby locations in the cache,
@@ -207,7 +210,7 @@ private static Entry<?>[] getCacheCarefully(Class<?> type) {
      * Called only if the first probe was empty or a collision.
      * This is a separate method, so compilers can process it independently.
      */
-    private T getFromBackup(Entry<?>[] cache, Class<?> type) {
+    private T getFromBackup(@Nullable Entry<?>[] cache, Class<?> type) {
         Entry<T> e = probeBackupLocations(cache, this);
         if (e != null)
             return e.value();
@@ -216,7 +219,7 @@ private T getFromBackup(Entry<?>[] cache, Class<?> type) {
 
     // Hack to suppress warnings on the (T) cast, which is a no-op.
     @SuppressWarnings("unchecked")
-    Entry<T> castEntry(Entry<?> e) { return (Entry<T>) e; }
+    @PolyNull Entry<T> castEntry(@PolyNull Entry<?> e) { return (Entry<T>) e; }
 
     /** Called when the fast path of get fails, and cache reprobe also fails.
      */
@@ -242,7 +245,7 @@ private T getFromHashMap(Class<?> type) {
     }
 
     /** Check that e is non-null, matches this ClassValue, and is live. */
-    boolean match(Entry<?> e) {
+    boolean match(@Nullable Entry<?> e) {
         // racing e.version : null (blank) => unique Version token => null (GC-ed version)
         // non-racing this.version : v1 => v2 => ... (updates are read faithfully from volatile)
         return (e != null && e.get() == this.version);
@@ -326,8 +329,8 @@ static class Version<T> {
      *  into the dead state.
      */
     static class Entry<T> extends WeakReference<Version<T>> {
-        final Object value;  // usually of type T, but sometimes (Entry)this
-        Entry(Version<T> version, T value) {
+        final @Nullable Object value;  // usually of type T, but sometimes (Entry)this
+        Entry(@Nullable Version<T> version, T value) {
             super(version);
             this.value = value;  // for a regular entry, value is of type T
         }
@@ -341,8 +344,8 @@ static class Entry<T> extends WeakReference<Version<T>> {
         @SuppressWarnings("unchecked")  // if !isPromise, type is T
         T value() { assertNotPromise(); return (T) value; }
         boolean isPromise() { return value == this; }
-        Version<T> version() { return get(); }
-        ClassValue<T> classValueOrNull() {
+        @Nullable Version<T> version() { return get(); }
+        @Nullable ClassValue<T> classValueOrNull() {
             Version<T> v = version();
             return (v == null) ? null : v.classValue();
         }
@@ -395,7 +398,7 @@ private static ClassValueMap initializeMap(Class<?> type) {
         return map;
     }
 
-    static <T> Entry<T> makeEntry(Version<T> explicitVersion, T value) {
+    static <T> Entry<T> makeEntry(@Nullable Version<T> explicitVersion, T value) {
         // Note that explicitVersion might be different from this.version.
         return new Entry<>(explicitVersion, value);
 
@@ -474,7 +477,7 @@ <T> Entry<T> startEntry(ClassValue<T> classValue) {
 
         /** Finish a query.  Overwrite a matching placeholder.  Drop stale incoming values. */
         synchronized
-        <T> Entry<T> finishEntry(ClassValue<T> classValue, Entry<T> e) {
+        <T> @Nullable Entry<T> finishEntry(ClassValue<T> classValue, Entry<T> e) {
             @SuppressWarnings("unchecked")  // one map has entries for all value types <T>
             Entry<T> e0 = (Entry<T>) get(classValue.identity);
             if (e == e0) {
@@ -545,7 +548,7 @@ <T> void changeEntry(ClassValue<T> classValue, T value) {
         // Statics do not need synchronization.
 
         /** Load the cache entry at the given (hashed) location. */
-        static Entry<?> loadFromCache(Entry<?>[] cache, int i) {
+        static @Nullable Entry<?> loadFromCache(@Nullable Entry<?>[] cache, int i) {
             // non-racing cache.length : constant
             // racing cache[i & (mask)] : null <=> Entry
             return cache[i & (cache.length-1)];
@@ -553,12 +556,12 @@ static Entry<?> loadFromCache(Entry<?>[] cache, int i) {
         }
 
         /** Look in the cache, at the home location for the given ClassValue. */
-        static <T> Entry<T> probeHomeLocation(Entry<?>[] cache, ClassValue<T> classValue) {
+        static <T> @Nullable Entry<T> probeHomeLocation(@Nullable  Entry<?>[] cache, ClassValue<T> classValue) {
             return classValue.castEntry(loadFromCache(cache, classValue.hashCodeForCache));
         }
 
         /** Given that first probe was a collision, retry at nearby locations. */
-        static <T> Entry<T> probeBackupLocations(Entry<?>[] cache, ClassValue<T> classValue) {
+        static <T> @Nullable Entry<T> probeBackupLocations(@Nullable Entry<?>[] cache, ClassValue<T> classValue) {
             if (PROBE_LIMIT <= 0)  return null;
             // Probe the cache carefully, in a range of slots.
             int mask = (cache.length-1);
@@ -594,7 +597,7 @@ static <T> Entry<T> probeBackupLocations(Entry<?>[] cache, ClassValue<T> classVa
         }
 
         /** How far out of place is e? */
-        private static int entryDislocation(Entry<?>[] cache, int pos, Entry<?> e) {
+        private static int entryDislocation(@Nullable Entry<?>[] cache, int pos, Entry<?> e) {
             ClassValue<?> cv = e.classValueOrNull();
             if (cv == null)  return 0;  // entry is not live!
             int mask = (cache.length-1);
@@ -636,7 +639,7 @@ private void reduceCacheLoad() {
         /** Remove stale entries in the given range.
          *  Should be executed under a Map lock.
          */
-        private void removeStaleEntries(Entry<?>[] cache, int begin, int count) {
+        private void removeStaleEntries(@Nullable Entry<?>[] cache, int begin, int count) {
             if (PROBE_LIMIT <= 0)  return;
             int mask = (cache.length-1);
             int removed = 0;
@@ -660,7 +663,7 @@ private void removeStaleEntries(Entry<?>[] cache, int begin, int count) {
          *  to be found via reprobes.  Find an entry after cache[begin]
          *  to plug into the hole, or return null if none is needed.
          */
-        private Entry<?> findReplacement(Entry<?>[] cache, int home1) {
+        private @Nullable Entry<?> findReplacement(@Nullable Entry<?>[] cache, int home1) {
             Entry<?> replacement = null;
             int haveReplacement = -1, replacementPos = 0;
             int mask = (cache.length-1);
@@ -741,7 +744,7 @@ private <T> void addToCache(ClassValue<T> classValue, Entry<T> e) {
         /** Store the given entry.  Update cacheLoad, and return any live victim.
          *  'Gently' means return self rather than dislocating a live victim.
          */
-        private Entry<?> placeInCache(Entry<?>[] cache, int pos, Entry<?> e, boolean gently) {
+        private @Nullable Entry<?> placeInCache(Entry<?>[] cache, int pos, Entry<?> e, boolean gently) {
             Entry<?> e2 = overwrittenEntry(cache[pos]);
             if (gently && e2 != null) {
                 // do not overwrite a live entry
@@ -758,7 +761,7 @@ private Entry<?> placeInCache(Entry<?>[] cache, int pos, Entry<?> e, boolean gen
          *  because the caller is going to store something
          *  in its place.
          */
-        private <T> Entry<T> overwrittenEntry(Entry<T> e2) {
+        private <T> @Nullable Entry<T> overwrittenEntry(Entry<T> e2) {
             if (e2 == null)  cacheLoad += 1;
             else if (e2.isLive())  return e2;
             return null;
diff --git a/src/java.base/share/classes/java/lang/ConditionalSpecialCasing.java b/src/java.base/share/classes/java/lang/ConditionalSpecialCasing.java
index 386cbe416cc..6e81de8b2c0 100644
--- a/src/java.base/share/classes/java/lang/ConditionalSpecialCasing.java
+++ b/src/java.base/share/classes/java/lang/ConditionalSpecialCasing.java
@@ -26,6 +26,7 @@
 package java.lang;
 
 import org.checkerframework.checker.interning.qual.UsesObjectEquals;
+import org.checkerframework.checker.nullness.qual.Nullable;
 import org.checkerframework.framework.qual.AnnotatedFor;
 
 import java.text.BreakIterator;
@@ -137,11 +138,11 @@ static int toUpperCaseEx(String src, int index, Locale locale) {
         }
     }
 
-    static char[] toLowerCaseCharArray(String src, int index, Locale locale) {
+    static char @Nullable [] toLowerCaseCharArray(String src, int index, Locale locale) {
         return lookUpTable(src, index, locale, true);
     }
 
-    static char[] toUpperCaseCharArray(String src, int index, Locale locale) {
+    static char @Nullable [] toUpperCaseCharArray(String src, int index, Locale locale) {
         char[] result = lookUpTable(src, index, locale, false);
         if (result != null) {
             return result;
@@ -150,7 +151,7 @@ static char[] toUpperCaseCharArray(String src, int index, Locale locale) {
         }
     }
 
-    private static char[] lookUpTable(String src, int index, Locale locale, boolean bLowerCasing) {
+    private static char @Nullable [] lookUpTable(String src, int index, Locale locale, boolean bLowerCasing) {
         HashSet<Entry> set = entryTable.get(src.codePointAt(index));
         char[] ret = null;
 
@@ -430,10 +431,10 @@ static class Entry {
         int ch;
         char [] lower;
         char [] upper;
-        String lang;
+        @Nullable String lang;
         int condition;
 
-        Entry(int ch, char[] lower, char[] upper, String lang, int condition) {
+        Entry(int ch, char[] lower, char[] upper, @Nullable String lang, int condition) {
             this.ch = ch;
             this.lower = lower;
             this.upper = upper;
@@ -453,7 +454,7 @@ char[] getUpperCase() {
             return upper;
         }
 
-        String getLanguage() {
+        @Nullable String getLanguage() {
             return lang;
         }
 
diff --git a/src/java.base/share/classes/java/lang/Enum.java b/src/java.base/share/classes/java/lang/Enum.java
index 3628f0f3bf3..89c2c97929b 100644
--- a/src/java.base/share/classes/java/lang/Enum.java
+++ b/src/java.base/share/classes/java/lang/Enum.java
@@ -235,7 +235,7 @@ public final int compareTo(@UnknownKeyFor @Tainted E o) {
      *     enum type
      */
     @SuppressWarnings("unchecked")
-    public final Class<@Tainted E> getDeclaringClass() {
+    public final @Nullable Class<@Tainted E> getDeclaringClass() {
         Class<?> clazz = getClass();
         Class<?> zuper = clazz.getSuperclass();
         return (zuper == Enum.class) ? (Class<E>)clazz : (Class<E>)zuper;
@@ -250,6 +250,7 @@ public final int compareTo(@UnknownKeyFor @Tainted E o) {
      * @since 12
      */
     @Override
+    @SuppressWarnings("nullness:dereference.of.nullable") // AOSEN: is this a real NPE?
     public final Optional<EnumDesc<E>> describeConstable() {
         return getDeclaringClass()
                 .describeConstable()
diff --git a/src/java.base/share/classes/java/lang/InheritableThreadLocal.java b/src/java.base/share/classes/java/lang/InheritableThreadLocal.java
index f54101e2659..4c504934f37 100644
--- a/src/java.base/share/classes/java/lang/InheritableThreadLocal.java
+++ b/src/java.base/share/classes/java/lang/InheritableThreadLocal.java
@@ -84,7 +84,7 @@ protected T childValue(T parentValue) {
      *
      * @param t the current thread
      */
-    ThreadLocalMap getMap(Thread t) {
+    @Nullable ThreadLocalMap getMap(Thread t) {
        return t.inheritableThreadLocals;
     }
 
diff --git a/src/java.base/share/classes/java/lang/Integer.java b/src/java.base/share/classes/java/lang/Integer.java
index 7917ad6dbe9..dceb7e8158e 100644
--- a/src/java.base/share/classes/java/lang/Integer.java
+++ b/src/java.base/share/classes/java/lang/Integer.java
@@ -1353,7 +1353,7 @@ public boolean equals(@Nullable Object obj) {
      */
     @SideEffectFree
     @StaticallyExecutable
-    public static @Nullable Integer getInteger(@Nullable String nm) {
+    public static @Nullable Integer getInteger(String nm) {
         return getInteger(nm, null);
     }
 
@@ -1401,7 +1401,7 @@ public boolean equals(@Nullable Object obj) {
      */
     @SideEffectFree
     @StaticallyExecutable
-    public static Integer getInteger(@Nullable String nm, int val) {
+    public static Integer getInteger(String nm, int val) {
         Integer result = getInteger(nm, null);
         return (result == null) ? Integer.valueOf(val) : result;
     }
@@ -1445,7 +1445,7 @@ public static Integer getInteger(@Nullable String nm, int val) {
      */
     @SideEffectFree
     @StaticallyExecutable
-    public static @PolyNull Integer getInteger(@Nullable String nm, @PolyNull Integer val) {
+    public static @PolyNull Integer getInteger(String nm, @PolyNull Integer val) {
         String v = null;
         try {
             v = System.getProperty(nm);
diff --git a/src/java.base/share/classes/java/lang/LayerInstantiationException.java b/src/java.base/share/classes/java/lang/LayerInstantiationException.java
index 6c1469df79b..5bdafbc1241 100644
--- a/src/java.base/share/classes/java/lang/LayerInstantiationException.java
+++ b/src/java.base/share/classes/java/lang/LayerInstantiationException.java
@@ -25,6 +25,8 @@
 
 package java.lang;
 
+import org.checkerframework.checker.nullness.qual.Nullable;
+
 /**
  * Thrown when creating a {@linkplain ModuleLayer module layer} fails.
  *
@@ -48,7 +50,7 @@ public LayerInstantiationException() {
      * @param msg
      *        The detail message; can be {@code null}
      */
-    public LayerInstantiationException(String msg) {
+    public LayerInstantiationException(@Nullable String msg) {
         super(msg);
     }
 
diff --git a/src/java.base/share/classes/java/lang/Long.java b/src/java.base/share/classes/java/lang/Long.java
index 33abaa3d59d..63153e1ab37 100644
--- a/src/java.base/share/classes/java/lang/Long.java
+++ b/src/java.base/share/classes/java/lang/Long.java
@@ -1575,7 +1575,7 @@ public boolean equals(@Nullable Object obj) {
      */
     @SideEffectFree
     @StaticallyExecutable
-    public static @Nullable Long getLong(@Nullable String nm) {
+    public static @Nullable Long getLong(String nm) {
         return getLong(nm, null);
     }
 
@@ -1622,7 +1622,7 @@ public boolean equals(@Nullable Object obj) {
      */
     @SideEffectFree
     @StaticallyExecutable
-    public static Long getLong(@Nullable String nm, long val) {
+    public static Long getLong(String nm, long val) {
         Long result = Long.getLong(nm, null);
         return (result == null) ? Long.valueOf(val) : result;
     }
@@ -1673,7 +1673,7 @@ public static Long getLong(@Nullable String nm, long val) {
      */
     @SideEffectFree
     @StaticallyExecutable
-    public static @PolyNull Long getLong(@Nullable String nm, @PolyNull Long val) {
+    public static @PolyNull Long getLong(String nm, @PolyNull Long val) {
         String v = null;
         try {
             v = System.getProperty(nm);
diff --git a/src/java.base/share/classes/java/lang/Module.java b/src/java.base/share/classes/java/lang/Module.java
index ab7d2103850..febbfeba30b 100644
--- a/src/java.base/share/classes/java/lang/Module.java
+++ b/src/java.base/share/classes/java/lang/Module.java
@@ -25,6 +25,10 @@
 
 package java.lang;
 
+import org.checkerframework.checker.nullness.qual.Nullable;
+import org.checkerframework.checker.nullness.qual.EnsuresNonNull;
+import org.checkerframework.dataflow.qual.Pure;
+
 import java.io.IOException;
 import java.io.InputStream;
 import java.lang.annotation.Annotation;
@@ -100,14 +104,14 @@
 public final class Module implements AnnotatedElement {
 
     // the layer that contains this module, can be null
-    private final ModuleLayer layer;
+    private final @Nullable ModuleLayer layer;
 
     // module name and loader, these fields are read by VM
-    private final String name;
-    private final ClassLoader loader;
+    private final @Nullable String name;
+    private final @Nullable ClassLoader loader;
 
     // the module descriptor
-    private final ModuleDescriptor descriptor;
+    private final @Nullable ModuleDescriptor descriptor;
 
     // true, if this module allows restricted native access
     private volatile boolean enableNativeAccess;
@@ -117,10 +121,10 @@ public final class Module implements AnnotatedElement {
      * VM but will not read any other modules, will not have any exports setup
      * and will not be registered in the service catalog.
      */
-    Module(ModuleLayer layer,
+    Module(@Nullable ModuleLayer layer,
            ClassLoader loader,
            ModuleDescriptor descriptor,
-           URI uri)
+           @Nullable URI uri)
     {
         this.layer = layer;
         this.name = descriptor.name();
@@ -147,7 +151,7 @@ public final class Module implements AnnotatedElement {
      *
      * @see ClassLoader#getUnnamedModule
      */
-    Module(ClassLoader loader) {
+    Module(@Nullable ClassLoader loader) {
         this.layer = null;
         this.name = null;
         this.loader = loader;
@@ -160,7 +164,7 @@ public final class Module implements AnnotatedElement {
      *
      * @apiNote This constructor is for VM white-box testing.
      */
-    Module(ClassLoader loader, ModuleDescriptor descriptor) {
+    Module(@Nullable ClassLoader loader, ModuleDescriptor descriptor) {
         this.layer = null;
         this.name = descriptor.name();
         this.loader = loader;
@@ -176,6 +180,9 @@ public final class Module implements AnnotatedElement {
      * @see ClassLoader#getUnnamedModule()
      * @jls 7.7.5 Unnamed Modules
      */
+    @SuppressWarnings({"contracts.postcondition.not.satisfied"})
+    @EnsuresNonNull({"name", "descriptor"}) // Named modules always have non-null name and descriptor
+    @Pure
     public boolean isNamed() {
         return name != null;
     }
@@ -186,7 +193,8 @@ public boolean isNamed() {
      *
      * @return The module name
      */
-    public String getName() {
+    @Pure
+    public @Nullable String getName() {
         return name;
     }
 
@@ -203,7 +211,7 @@ public String getName() {
      * @throws SecurityException
      *         If denied by the security manager
      */
-    public ClassLoader getClassLoader() {
+    public @Nullable ClassLoader getClassLoader() {
         @SuppressWarnings("removal")
         SecurityManager sm = System.getSecurityManager();
         if (sm != null) {
@@ -218,11 +226,11 @@ public ClassLoader getClassLoader() {
      *
      * @return The module descriptor for this module
      */
-    public ModuleDescriptor getDescriptor() {
+    public @Nullable ModuleDescriptor getDescriptor() {
         return descriptor;
     }
 
-    /**
+    /**1
      * Returns the module layer that contains this module or {@code null} if
      * this module is not in a module layer.
      *
@@ -237,7 +245,7 @@ public ModuleDescriptor getDescriptor() {
      *
      * @see java.lang.reflect.Proxy
      */
-    public ModuleLayer getLayer() {
+    public @Nullable ModuleLayer getLayer() {
         if (isNamed()) {
             ModuleLayer layer = this.layer;
             if (layer != null)
@@ -1124,6 +1132,7 @@ public Set<String> getPackages() {
      *         If the module cannot be defined to the VM or its packages overlap
      *         with another module mapped to the same class loader
      */
+    @SuppressWarnings("nullness:assignment.type.incompatible") // toArray returns a non-null element array
     static Map<String, Module> defineModules(Configuration cf,
                                              Function<String, ClassLoader> clf,
                                              ModuleLayer layer)
@@ -1273,7 +1282,7 @@ static Map<String, Module> defineModules(Configuration cf,
      * Find the runtime Module corresponding to the given ResolvedModule
      * in the given parent layer (or its parents).
      */
-    private static Module findModule(ModuleLayer parent,
+    private static @Nullable Module findModule(ModuleLayer parent,
                                      ResolvedModule resolvedModule) {
         Configuration cf = resolvedModule.configuration();
         String dn = resolvedModule.name();
@@ -1296,6 +1305,7 @@ private static Module findModule(ModuleLayer parent,
      * @param m the module
      * @param nameToModule map of module name to Module (for qualified exports)
      */
+    @SuppressWarnings("nullness:dereference.of.nullable") // AOSEN: is this a NPE?
     private static void initExports(Module m, Map<String, Module> nameToModule) {
         Map<String, Set<Module>> exportedPackages = new HashMap<>();
 
@@ -1334,6 +1344,7 @@ private static void initExports(Module m, Map<String, Module> nameToModule) {
      *                     under construction
      * @param parents the parent layers
      */
+    @SuppressWarnings("nullness:dereference.of.nullable") // AOSEN: is this a NPE?
     private static void initExportsAndOpens(Module m,
                                             Map<String, Module> nameToSource,
                                             Map<String, Module> nameToModule,
@@ -1413,7 +1424,7 @@ private static void initExportsAndOpens(Module m,
      * @param nameToModule The modules in the layer under construction
      * @param parents The parent layers
      */
-    private static Module findModule(String target,
+    private static @Nullable Module findModule(String target,
                                      Map<String, Module> nameToSource,
                                      Map<String, Module> nameToModule,
                                      List<ModuleLayer> parents) {
@@ -1441,7 +1452,7 @@ private static Module findModule(String target,
      * declaration annotation.
      */
     @Override
-    public <T extends Annotation> T getAnnotation(Class<T> annotationClass) {
+    public <T extends @Nullable Annotation> @Nullable T getAnnotation(Class<T> annotationClass) {
         return moduleInfoClass().getDeclaredAnnotation(annotationClass);
     }
 
@@ -1482,7 +1493,7 @@ private Class<?> moduleInfoClass() {
             clazz = this.moduleInfoClass;
             if (clazz == null) {
                 if (isNamed()) {
-                    PrivilegedAction<Class<?>> pa = this::loadModuleInfoClass;
+                    PrivilegedAction<@Nullable Class<?>> pa = this::loadModuleInfoClass;
                     clazz = AccessController.doPrivileged(pa);
                 }
                 if (clazz == null) {
@@ -1495,7 +1506,7 @@ class DummyModuleInfo { }
         }
     }
 
-    private Class<?> loadModuleInfoClass() {
+    private @Nullable Class<?> loadModuleInfoClass() {
         Class<?> clazz = null;
         try (InputStream in = getResourceAsStream("module-info.class")) {
             if (in != null)
@@ -1541,7 +1552,7 @@ public void visitAttribute(Attribute attr) {
                 // drop non-annotation attributes
             }
             @Override
-            public ModuleVisitor visitModule(String name, int flags, String version) {
+            public @Nullable ModuleVisitor visitModule(String name, int flags, String version) {
                 // drop Module attribute
                 return null;
             }
@@ -1642,7 +1653,9 @@ protected Class<?> loadClass(String cn, boolean resolve)
      * @see Class#getResourceAsStream(String)
      */
     @CallerSensitive
-    public InputStream getResourceAsStream(String name) throws IOException {
+    @SuppressWarnings("nullness:argument.type.incompatible") // This code is updated in the latest JDK.
+    //https://github.com/openjdk/jdk/blob/4a4209ffef8f8d65054cbf46ebf8e169d100c0d8/src/java.base/share/classes/java/lang/Module.java#L1688C21-L1695C22
+    public @Nullable InputStream getResourceAsStream(String name) throws IOException {
         if (name.startsWith("/")) {
             name = name.substring(1);
         }
@@ -1707,7 +1720,7 @@ public String toString() {
      * Returns the module that a given caller class is a member of. Returns
      * {@code null} if the caller is {@code null}.
      */
-    private Module getCallerModule(Class<?> caller) {
+    private @Nullable Module getCallerModule(Class<?> caller) {
         return (caller != null) ? caller.getModule() : null;
     }
 
@@ -1717,12 +1730,12 @@ private Module getCallerModule(Class<?> caller) {
     // JVM_DefineModule
     private static native void defineModule0(Module module,
                                              boolean isOpen,
-                                             String version,
-                                             String location,
+                                             @Nullable String version,
+                                             @Nullable String location,
                                              Object[] pns);
 
     // JVM_AddReadsModule
-    private static native void addReads0(Module from, Module to);
+    private static native void addReads0(Module from, @Nullable Module to);
 
     // JVM_AddModuleExports
     private static native void addExports0(Module from, String pn, Module to);
diff --git a/src/java.base/share/classes/java/lang/ModuleLayer.java b/src/java.base/share/classes/java/lang/ModuleLayer.java
index 6cecfe6d388..6a347fe5c76 100644
--- a/src/java.base/share/classes/java/lang/ModuleLayer.java
+++ b/src/java.base/share/classes/java/lang/ModuleLayer.java
@@ -25,6 +25,8 @@
 
 package java.lang;
 
+import org.checkerframework.checker.nullness.qual.Nullable;
+
 import java.lang.module.Configuration;
 import java.lang.module.ModuleDescriptor;
 import java.lang.module.ResolvedModule;
@@ -171,9 +173,10 @@ public final class ModuleLayer {
     /**
      * Creates a new module layer from the modules in the given configuration.
      */
+    @SuppressWarnings("nullness:argument.type.incompatible") // AOSEN: there could be possible NPE if clf is null
     private ModuleLayer(Configuration cf,
                         List<ModuleLayer> parents,
-                        Function<String, ClassLoader> clf)
+                        @Nullable Function<String, ClassLoader> clf)
     {
         this.cf = cf;
         this.parents = parents; // no need to do defensive copy
@@ -880,7 +883,7 @@ public Optional<Module> findModule(String name) {
      *
      * @throws SecurityException if denied by the security manager
      */
-    public ClassLoader findLoader(String name) {
+    public @Nullable ClassLoader findLoader(String name) {
         Optional<Module> om = findModule(name);
 
         // can't use map(Module::getClassLoader) as class loader can be null
diff --git a/src/java.base/share/classes/java/lang/NamedPackage.java b/src/java.base/share/classes/java/lang/NamedPackage.java
index 6234b949e65..335beb2e66c 100644
--- a/src/java.base/share/classes/java/lang/NamedPackage.java
+++ b/src/java.base/share/classes/java/lang/NamedPackage.java
@@ -24,6 +24,8 @@
  */
 package java.lang;
 
+import org.checkerframework.checker.nullness.qual.Nullable;
+
 import java.lang.module.Configuration;
 import java.lang.module.ModuleReference;
 import java.net.URI;
@@ -70,7 +72,8 @@ Module module() {
      * Returns the location of the module if this named package is in
      * a named module; otherwise, returns null.
      */
-    URI location() {
+    @SuppressWarnings({"nullness:dereference.of.nullable", "nullness:argument.type.incompatible"}) // The problem does not apply to named packages
+    @Nullable URI location() {
         if (module.isNamed() && module.getLayer() != null) {
             Configuration cf = module.getLayer().configuration();
             ModuleReference mref
diff --git a/src/java.base/share/classes/java/lang/Package.java b/src/java.base/share/classes/java/lang/Package.java
index 5682bb8be75..08f8ba6751e 100644
--- a/src/java.base/share/classes/java/lang/Package.java
+++ b/src/java.base/share/classes/java/lang/Package.java
@@ -440,7 +440,7 @@ private Class<?> getPackageInfo() {
             // find package-info.class defined by loader
             String cn = packageName() + ".package-info";
             Module module = module();
-            PrivilegedAction<ClassLoader> pa = module::getClassLoader;
+            PrivilegedAction<@Nullable ClassLoader> pa = module::getClassLoader;
             @SuppressWarnings("removal")
             ClassLoader loader = AccessController.doPrivileged(pa);
             Class<?> c;
@@ -470,7 +470,7 @@ class PackageInfoProxy {}
      * @since 1.5
      */
     @Override
-    public <A extends Annotation> @Nullable A getAnnotation(Class<A> annotationClass) {
+    public <A extends @Nullable Annotation> @Nullable A getAnnotation(Class<A> annotationClass) {
         return getPackageInfo().getAnnotation(annotationClass);
     }
 
@@ -494,7 +494,7 @@ public boolean isAnnotationPresent(@GuardSatisfied Package this, @GuardSatisfied
      * @since 1.8
      */
     @Override
-    public  <A extends Annotation> A[] getAnnotationsByType(Class<A> annotationClass) {
+    public  <A extends @Nullable Annotation> A[] getAnnotationsByType(Class<A> annotationClass) {
         return getPackageInfo().getAnnotationsByType(annotationClass);
     }
 
@@ -518,7 +518,7 @@ public Annotation[] getAnnotations() {
      * @since 1.8
      */
     @Override
-    public <A extends Annotation> @Nullable A getDeclaredAnnotation(Class<A> annotationClass) {
+    public <A extends @Nullable Annotation> @Nullable A getDeclaredAnnotation(Class<A> annotationClass) {
         return getPackageInfo().getDeclaredAnnotation(annotationClass);
     }
 
@@ -527,7 +527,7 @@ public Annotation[] getAnnotations() {
      * @since 1.8
      */
     @Override
-    public <A extends Annotation> A[] getDeclaredAnnotationsByType(Class<A> annotationClass) {
+    public <A extends @Nullable Annotation> A[] getDeclaredAnnotationsByType(Class<A> annotationClass) {
         return getPackageInfo().getDeclaredAnnotationsByType(annotationClass);
     }
 
@@ -560,9 +560,9 @@ public Annotation[] getDeclaredAnnotations()  {
      * @param loader defining class loader
      */
     Package(@DotSeparatedIdentifiers String name,
-            String spectitle, String specversion, String specvendor,
-            String impltitle, String implversion, String implvendor,
-            URL sealbase, ClassLoader loader)
+            @Nullable String spectitle, @Nullable String specversion, @Nullable String specvendor,
+            @Nullable String impltitle, @Nullable String implversion, @Nullable String implvendor,
+            @Nullable URL sealbase, ClassLoader loader)
     {
         super(Objects.requireNonNull(name),
               loader != null ? loader.getUnnamedModule()
@@ -586,18 +586,18 @@ static class VersionInfo {
         static final VersionInfo NULL_VERSION_INFO
             = new VersionInfo(null, null, null, null, null, null, null);
 
-        private final String specTitle;
-        private final String specVersion;
-        private final String specVendor;
-        private final String implTitle;
-        private final String implVersion;
-        private final String implVendor;
-        private final URL sealBase;
-
-        static VersionInfo getInstance(String spectitle, String specversion,
-                                       String specvendor, String impltitle,
-                                       String implversion, String implvendor,
-                                       URL sealbase) {
+        private final @Nullable String specTitle;
+        private final @Nullable String specVersion;
+        private final @Nullable String specVendor;
+        private final @Nullable String implTitle;
+        private final @Nullable String implVersion;
+        private final @Nullable String implVendor;
+        private final @Nullable URL sealBase;
+
+        static VersionInfo getInstance(@Nullable String spectitle, @Nullable String specversion,
+                                       @Nullable String specvendor, @Nullable String impltitle,
+                                       @Nullable String implversion, @Nullable String implvendor,
+                                       @Nullable URL sealbase) {
             if (spectitle == null && specversion == null &&
                     specvendor == null && impltitle == null &&
                     implversion == null && implvendor == null &&
@@ -609,10 +609,10 @@ static VersionInfo getInstance(String spectitle, String specversion,
                     sealbase);
         }
 
-        private VersionInfo(String spectitle, String specversion,
-                            String specvendor, String impltitle,
-                            String implversion, String implvendor,
-                            URL sealbase)
+        private VersionInfo(@Nullable String spectitle, @Nullable String specversion,
+                            @Nullable String specvendor, @Nullable String impltitle,
+                            @Nullable String implversion, @Nullable String implvendor,
+                            @Nullable URL sealbase)
         {
             this.implTitle = impltitle;
             this.implVersion = implversion;
diff --git a/src/java.base/share/classes/java/lang/ProcessBuilder.java b/src/java.base/share/classes/java/lang/ProcessBuilder.java
index 85a2fe3b703..9f88ef5139d 100644
--- a/src/java.base/share/classes/java/lang/ProcessBuilder.java
+++ b/src/java.base/share/classes/java/lang/ProcessBuilder.java
@@ -28,6 +28,7 @@
 import org.checkerframework.checker.mustcall.qual.MustCall;
 import org.checkerframework.checker.nullness.qual.Nullable;
 import org.checkerframework.checker.tainting.qual.Untainted;
+import org.checkerframework.dataflow.qual.Pure;
 import org.checkerframework.framework.qual.AnnotatedFor;
 
 import java.io.File;
@@ -198,7 +199,7 @@
 public final class ProcessBuilder
 {
     private List<String> command;
-    private File directory;
+    private @Nullable File directory;
     private Map<String,String> environment;
     private boolean redirectErrorStream;
     private Redirect[] redirects;
@@ -368,7 +369,7 @@ public Map<String,String> environment() {
     }
 
     // Only for use by Runtime.exec(...envp...)
-    ProcessBuilder environment(String[] envp) {
+    ProcessBuilder environment(String @Nullable [] envp) {
         assert environment == null;
         if (envp != null) {
             environment = ProcessEnvironment.emptyEnvironment(envp.length);
@@ -581,7 +582,8 @@ public enum Type {
          * @return the file associated with this redirect,
          *         or {@code null} if there is no such file
          */
-        public File file() { return null; }
+        @Pure
+        public @Nullable File file() { return null; }
 
         /**
          * When redirected to a destination file, indicates if the output
@@ -679,7 +681,7 @@ public String toString() {
          * instances of the same type associated with non-null equal
          * {@code File} instances.
          */
-        public boolean equals(Object obj) {
+        public boolean equals(@Nullable Object obj) {
             if (obj == this)
                 return true;
             if (! (obj instanceof Redirect r))
@@ -1089,7 +1091,7 @@ public Process start() throws IOException {
      * @return the new Process
      * @throws IOException if an I/O error occurs
      */
-    private Process start(Redirect[] redirects) throws IOException {
+    private Process start(@Nullable Redirect[] redirects) throws IOException {
         // Must convert to array first -- a malicious user-supplied
         // list might try to circumvent the security check.
         String[] cmdarray = command.toArray(new String[command.size()]);
@@ -1284,7 +1286,7 @@ public static List<Process> startPipeline(List<ProcessBuilder> builders) throws
             Redirect prevOutput = null;
             for (int index = 0; index < builders.size(); index++) {
                 ProcessBuilder builder = builders.get(index);
-                Redirect[] redirects = builder.redirects();
+                @Nullable Redirect[] redirects = builder.redirects();
                 if (index > 0) {
                     // check the current Builder to see if it can take input from the previous
                     if (builder.redirectInput() != Redirect.PIPE) {
diff --git a/src/java.base/share/classes/java/lang/ProcessHandle.java b/src/java.base/share/classes/java/lang/ProcessHandle.java
index 229f98d0b6f..aff82c93b9f 100644
--- a/src/java.base/share/classes/java/lang/ProcessHandle.java
+++ b/src/java.base/share/classes/java/lang/ProcessHandle.java
@@ -24,6 +24,8 @@
  */
 package java.lang;
 
+import org.checkerframework.checker.nullness.qual.Nullable;
+
 import java.time.Duration;
 import java.time.Instant;
 import java.util.Optional;
@@ -415,7 +417,7 @@ public interface Info {
      *         the same system process; otherwise returns {@code false}
      */
     @Override
-    boolean equals(Object other);
+    boolean equals(@Nullable Object other);
 
     /**
      * Compares this ProcessHandle with the specified ProcessHandle for order.
diff --git a/src/java.base/share/classes/java/lang/ProcessHandleImpl.java b/src/java.base/share/classes/java/lang/ProcessHandleImpl.java
index 593ee48f27d..fb7d71a5c9b 100644
--- a/src/java.base/share/classes/java/lang/ProcessHandleImpl.java
+++ b/src/java.base/share/classes/java/lang/ProcessHandleImpl.java
@@ -88,7 +88,7 @@ final class ProcessHandleImpl implements ProcessHandle {
     /**
      * The thread pool of "process reaper" daemon threads.
      */
-    @SuppressWarnings("removal")
+    @SuppressWarnings({"removal", "nullness:dereference.of.nullable"}) // AOSEN: Is this a real NPE?
     private static final Executor processReaperExecutor =
             AccessController.doPrivileged((PrivilegedAction<Executor>) () -> {
                 // Initialize ThreadLocalRandom now to avoid using the smaller stack
@@ -341,7 +341,7 @@ public Optional<ProcessHandle> parent() {
      *      if greater than the length of the arrays, the arrays are too small
      */
     private static native int getProcessPids0(long pid, long[] pids,
-                                              long[] ppids, long[] starttimes);
+                                              long @Nullable [] ppids, long[] starttimes);
 
     /**
      * Destroy the process for this ProcessHandle.
@@ -427,6 +427,7 @@ public Stream<ProcessHandle> children() {
      *            0 for all processes
      * @return a stream of ProcessHandles
      */
+    @SuppressWarnings("nullness:accessing.nullable") // AOSEN: this is a false positive to me
     static Stream<ProcessHandle> children(long pid) {
         @SuppressWarnings("removal")
         SecurityManager sm = System.getSecurityManager();
@@ -448,6 +449,7 @@ static Stream<ProcessHandle> children(long pid) {
     }
 
     @Override
+    @SuppressWarnings("nullness:accessing.nullable") // AOSEN: this is a false positive to me
     public Stream<ProcessHandle> descendants() {
         @SuppressWarnings("removal")
         SecurityManager sm = System.getSecurityManager();
@@ -563,12 +565,12 @@ static class Info implements ProcessHandle.Info {
          */
         private native void info0(long pid);
 
-        String command;
-        String commandLine;
-        String[] arguments;
+        @Nullable String command;
+        @Nullable String commandLine;
+        String @Nullable [] arguments;
         long startTime;
         long totalTime;
-        String user;
+        @Nullable String user;
 
         Info() {
             command = null;
diff --git a/src/java.base/share/classes/java/lang/PublicMethods.java b/src/java.base/share/classes/java/lang/PublicMethods.java
index b9851e2f049..54f0effe5a7 100644
--- a/src/java.base/share/classes/java/lang/PublicMethods.java
+++ b/src/java.base/share/classes/java/lang/PublicMethods.java
@@ -24,6 +24,8 @@
  */
 package java.lang;
 
+import org.checkerframework.checker.nullness.qual.Nullable;
+
 import jdk.internal.reflect.ReflectionFactory;
 
 import java.lang.reflect.Method;
@@ -58,6 +60,7 @@ final class PublicMethods {
      * than added method.
      * See comments in code...
      */
+    @SuppressWarnings("nullness:dereference.of.nullable") // AOSEN: is this a real NPE?
     void merge(Method method) {
         Key key = new Key(method);
         MethodList existing = map.get(key);
@@ -112,7 +115,7 @@ static boolean matches(Method method,
         }
 
         @Override
-        public boolean equals(Object o) {
+        public boolean equals(@Nullable Object o) {
             if (this == o) return true;
             //noinspection StringEquality (guaranteed interned String(s))
             return (o instanceof Key that)
@@ -145,7 +148,7 @@ private MethodList(Method method) {
          *         {@code ptypes} and including or excluding static methods as
          *         requested by {@code includeStatic} flag.
          */
-        static MethodList filter(Method[] methods, String name,
+        static @Nullable MethodList filter(Method[] methods, String name,
                                  Class<?>[] ptypes, boolean includeStatic) {
             MethodList head = null, tail = null;
             for (Method method : methods) {
@@ -170,16 +173,16 @@ static MethodList filter(Method[] methods, String name,
          * containing only the most specific methods for each signature
          * (i.e. return type). The returned head of the merged list may or
          * may not be the same as the {@code head} of the given list.
-         * The given {@code methodList} is not modified.
+         * The given {@code methodList} is not modified.1
          */
-        static MethodList merge(MethodList head, MethodList methodList) {
+        static @Nullable MethodList merge(@Nullable MethodList head, @Nullable MethodList methodList) {
             for (MethodList ml = methodList; ml != null; ml = ml.next) {
                 head = merge(head, ml.method);
             }
             return head;
         }
 
-        private static MethodList merge(MethodList head, Method method) {
+        private static @Nullable MethodList merge(@Nullable MethodList head, Method method) {
             Class<?> dclass = method.getDeclaringClass();
             Class<?> rtype = method.getReturnType();
             MethodList prev = null;
diff --git a/src/java.base/share/classes/java/lang/ReflectiveOperationException.java b/src/java.base/share/classes/java/lang/ReflectiveOperationException.java
index 0c79de008be..2296582813c 100644
--- a/src/java.base/share/classes/java/lang/ReflectiveOperationException.java
+++ b/src/java.base/share/classes/java/lang/ReflectiveOperationException.java
@@ -25,6 +25,8 @@
 
 package java.lang;
 
+import org.checkerframework.checker.nullness.qual.Nullable;
+
 /**
  * Common superclass of exceptions thrown by reflective operations in
  * core reflection.
@@ -53,7 +55,7 @@ public ReflectiveOperationException() {
      * @param   message   the detail message. The detail message is saved for
      *          later retrieval by the {@link #getMessage()} method.
      */
-    public ReflectiveOperationException(String message) {
+    public ReflectiveOperationException(@Nullable String message) {
         super(message);
     }
 
@@ -72,7 +74,7 @@ public ReflectiveOperationException(String message) {
      *         permitted, and indicates that the cause is nonexistent or
      *         unknown.)
      */
-    public ReflectiveOperationException(String message, Throwable cause) {
+    public ReflectiveOperationException(@Nullable String message, @Nullable Throwable cause) {
         super(message, cause);
     }
 
@@ -86,7 +88,7 @@ public ReflectiveOperationException(String message, Throwable cause) {
      *         permitted, and indicates that the cause is nonexistent or
      *         unknown.)
      */
-    public ReflectiveOperationException(Throwable cause) {
+    public ReflectiveOperationException(@Nullable Throwable cause) {
         super(cause);
     }
 }
diff --git a/src/java.base/share/classes/java/lang/Runtime.java b/src/java.base/share/classes/java/lang/Runtime.java
index 9e20a7e0a31..bd4444be140 100644
--- a/src/java.base/share/classes/java/lang/Runtime.java
+++ b/src/java.base/share/classes/java/lang/Runtime.java
@@ -1017,6 +1017,7 @@ private Version(List<Integer> unmodifiableListOfVersions,
          *
          * @return  The Version of the given string
          */
+        @SuppressWarnings("nullness:dereference.of.nullable") //AOSEN: possible NPE? I am not sure `m.group(VersionPattern.VNUM_GROUP)`
         public static Version parse(String s) {
             if (s == null)
                 throw new NullPointerException();
@@ -1418,6 +1419,7 @@ public String toString() {
          *
          */
         @Override
+        @SuppressWarnings("nullness:override.param.invalid") // AOSEN: Don't use @Nullable here as null value will produce NPE.
         public boolean equals(Object obj) {
             boolean ret = equalsIgnoreOptional(obj);
             if (!ret)
@@ -1442,7 +1444,7 @@ public boolean equals(Object obj) {
          *          ignoring the optional build information
          *
          */
-        public boolean equalsIgnoreOptional(Object obj) {
+        public boolean equalsIgnoreOptional(@Nullable Object obj) {
             if (this == obj)
                 return true;
             return (obj instanceof Version that)
diff --git a/src/java.base/share/classes/java/lang/RuntimePermission.java b/src/java.base/share/classes/java/lang/RuntimePermission.java
index d19b1125fa3..19f0d85f804 100644
--- a/src/java.base/share/classes/java/lang/RuntimePermission.java
+++ b/src/java.base/share/classes/java/lang/RuntimePermission.java
@@ -419,7 +419,7 @@ public final class RuntimePermission extends BasicPermission {
      * @throws IllegalArgumentException if {@code name} is empty.
      */
 
-    public RuntimePermission(String name)
+    public RuntimePermission(@Nullable String name)
     {
         super(name);
     }
@@ -436,7 +436,7 @@ public RuntimePermission(String name)
      * @throws IllegalArgumentException if {@code name} is empty.
      */
 
-    public RuntimePermission(String name, @Nullable String actions)
+    public RuntimePermission(@Nullable String name, @Nullable String actions)
     {
         super(name, actions);
     }
diff --git a/src/java.base/share/classes/java/lang/SecurityManager.java b/src/java.base/share/classes/java/lang/SecurityManager.java
index a98860f6436..56715710476 100644
--- a/src/java.base/share/classes/java/lang/SecurityManager.java
+++ b/src/java.base/share/classes/java/lang/SecurityManager.java
@@ -26,6 +26,7 @@
 package java.lang;
 
 import org.checkerframework.checker.interning.qual.UsesObjectEquals;
+import org.checkerframework.checker.nullness.qual.Nullable;
 import org.checkerframework.framework.qual.AnnotatedFor;
 
 import java.lang.module.ModuleDescriptor;
@@ -490,6 +491,7 @@ public void checkCreateClassLoader() {
 
     private static ThreadGroup rootGroup = getRootGroup();
 
+    @SuppressWarnings("nullness:dereference.of.nullable") // AOSEN: is this a real bug?
     private static ThreadGroup getRootGroup() {
         ThreadGroup root =  Thread.currentThread().getThreadGroup();
         while (root.getParent() != null) {
@@ -1244,6 +1246,7 @@ private static String[] getPackages(String p) {
     /**
      * Record the non-exported packages of the modules in the given layer
      */
+    @SuppressWarnings("type.arguments.not.inferred")
     static void addNonExportedPackages(ModuleLayer layer) {
         Set<String> bootModules = ModuleLoaderMap.bootModules();
         Set<String> platformModules = ModuleLoaderMap.platformModules();
@@ -1546,7 +1549,7 @@ public void checkSecurityAccess(String target) {
      * @since   1.1
      * @see     java.lang.ThreadGroup
      */
-    public ThreadGroup getThreadGroup() {
+    public @Nullable ThreadGroup getThreadGroup() {
         return Thread.currentThread().getThreadGroup();
     }
 
diff --git a/src/java.base/share/classes/java/lang/StackFrameInfo.java b/src/java.base/share/classes/java/lang/StackFrameInfo.java
index acdd222121f..eaec9d824a2 100644
--- a/src/java.base/share/classes/java/lang/StackFrameInfo.java
+++ b/src/java.base/share/classes/java/lang/StackFrameInfo.java
@@ -24,6 +24,8 @@
  */
 package java.lang;
 
+import org.checkerframework.checker.nullness.qual.Nullable;
+
 import jdk.internal.access.JavaLangInvokeAccess;
 import jdk.internal.access.SharedSecrets;
 
@@ -96,7 +98,7 @@ public int getByteCodeIndex() {
     }
 
     @Override
-    public String getFileName() {
+    public @Nullable String getFileName() {
         return toStackTraceElement().getFileName();
     }
 
diff --git a/src/java.base/share/classes/java/lang/StackStreamFactory.java b/src/java.base/share/classes/java/lang/StackStreamFactory.java
index 55b276e393f..4c480c6da58 100644
--- a/src/java.base/share/classes/java/lang/StackStreamFactory.java
+++ b/src/java.base/share/classes/java/lang/StackStreamFactory.java
@@ -24,8 +24,10 @@
  */
 package java.lang;
 
+import org.checkerframework.checker.nullness.qual.Nullable;
 import org.checkerframework.checker.nonempty.qual.EnsuresNonEmptyIf;
 import org.checkerframework.dataflow.qual.Pure;
+
 import jdk.internal.reflect.MethodAccessor;
 import jdk.internal.reflect.ConstructorAccessor;
 import java.lang.StackWalker.Option;
@@ -258,7 +260,7 @@ private boolean skipReflectionFrames() {
          * or {@code null} if no more frame. If advanceToNextBatch is true,
          * it will only fetch the next batch.
          */
-        final Class<?> peekFrame() {
+        final @Nullable Class<?> peekFrame() {
             while (frameBuffer.isActive() && depth < maxDepth) {
                 if (frameBuffer.isEmpty()) {
                     // fetch another batch of stack frames
@@ -290,7 +292,8 @@ final Class<?> peekFrame() {
          * 2. reuse or expand the allocated buffers
          * 3. create specialized StackFrame objects
          */
-        private Object doStackWalk(long anchor, int skipFrames, int batchSize,
+        // AOSEN: let's try nullable first.
+        private @Nullable Object doStackWalk(long anchor, int skipFrames, int batchSize,
                                                 int bufStartIndex, int bufEndIndex) {
             checkState(NEW);
 
@@ -334,7 +337,7 @@ private int getNextBatch() {
          *
          * @see #tryNextFrame
          */
-        final Class<?> nextFrame() {
+        final @Nullable Class<?> nextFrame() {
             if (!hasNext()) {
                 return null;
             }
@@ -519,7 +522,7 @@ final Class<?> at(int index) {
          * Returns next StackFrame object in the current batch of stack frames;
          * or null if no more stack frame.
          */
-        StackFrame nextStackFrame() {
+        @Nullable StackFrame nextStackFrame() {
             if (!hasNext()) {
                 return null;
             }
@@ -563,7 +566,7 @@ protected int batchSize(int lastBatchFrameCount) {
         // ------- Implementation of Spliterator
 
         @Override
-        public Spliterator<StackFrame> trySplit() {
+        public @Nullable Spliterator<StackFrame> trySplit() {
             return null;   // ordered stream and do not allow to split
         }
 
@@ -589,6 +592,7 @@ public void forEachRemaining(Consumer<? super StackFrame> action) {
         }
 
         @Override
+        @SuppressWarnings("nullness:argument.type.incompatible") // AOSEN: why this error?
         public boolean tryAdvance(Consumer<? super StackFrame> action) {
             checkState(OPEN);
 
@@ -675,6 +679,7 @@ Class<?> findCaller() {
         }
 
         @Override
+        @SuppressWarnings("nullness:assignment.type.incompatible") // AOSEN: caller is only null if nextFrame() returns null
         protected Integer consumeFrames() {
             checkState(OPEN);
             int n = 0;
diff --git a/src/java.base/share/classes/java/lang/StackTraceElement.java b/src/java.base/share/classes/java/lang/StackTraceElement.java
index 27b5bd237a2..9a11cd450f6 100644
--- a/src/java.base/share/classes/java/lang/StackTraceElement.java
+++ b/src/java.base/share/classes/java/lang/StackTraceElement.java
@@ -71,15 +71,15 @@ public final class StackTraceElement implements java.io.Serializable {
     /**
      * The name of the class loader.
      */
-    private String classLoaderName;
+    private @Nullable String classLoaderName;
     /**
      * The module name.
      */
-    private String moduleName;
+    private @Nullable String moduleName;
     /**
      * The module version.
      */
-    private String moduleVersion;
+    private @Nullable String moduleVersion;
     /**
      * The declaring class.
      */
@@ -91,7 +91,7 @@ public final class StackTraceElement implements java.io.Serializable {
     /**
      * The source file name.
      */
-    private String fileName;
+    private @Nullable String fileName;
     /**
      * The source line number.
      */
@@ -160,10 +160,10 @@ public StackTraceElement(@FullyQualifiedName String declaringClass, String metho
      *
      * @since 9
      */
-    public StackTraceElement(String classLoaderName,
-                             String moduleName, String moduleVersion,
+    public StackTraceElement(@Nullable String classLoaderName,
+                             @Nullable String moduleName, @Nullable String moduleVersion,
                              String declaringClass, String methodName,
-                             String fileName, int lineNumber) {
+                             @Nullable String fileName, int lineNumber) {
         this.classLoaderName = classLoaderName;
         this.moduleName      = moduleName;
         this.moduleVersion   = moduleVersion;
@@ -220,7 +220,7 @@ public int getLineNumber(@GuardSatisfied StackTraceElement this) {
      * @since 9
      * @see Module#getName()
      */
-    public String getModuleName() {
+    public @Nullable String getModuleName() {
         return moduleName;
     }
 
@@ -234,7 +234,7 @@ public String getModuleName() {
      * @since 9
      * @see java.lang.module.ModuleDescriptor.Version
      */
-    public String getModuleVersion() {
+    public @Nullable String getModuleVersion() {
         return moduleVersion;
     }
 
@@ -249,7 +249,7 @@ public String getModuleVersion() {
      * @since 9
      * @see java.lang.ClassLoader#getName()
      */
-    public String getClassLoaderName() {
+    public @Nullable String getClassLoaderName() {
         return classLoaderName;
     }
 
@@ -453,6 +453,7 @@ public int hashCode(@GuardSatisfied StackTraceElement this) {
      * If the loader is one of the built-in loaders (`boot`, `platform`, or `app`)
      * then set BUILTIN_CLASS_LOADER to omit the first element (`<loader>/`).
      */
+    @SuppressWarnings("nullness:assignment.type.incompatible") // AOSEN: declaringClassObject set to null only when clearing it
     private synchronized void computeFormat() {
         try {
             Class<?> cls = (Class<?>) declaringClassObject;
@@ -534,6 +535,7 @@ static Set<String> hashedModules() {
         }
 
         @Pure
+        @SuppressWarnings("nullness:argument.type.incompatible") // False positive because set interface is used.
         static boolean contains(Module m) {
             return HASHED_MODULES.contains(m.getName());
         }
diff --git a/src/java.base/share/classes/java/lang/StackWalker.java b/src/java.base/share/classes/java/lang/StackWalker.java
index c703e4caf6f..ac44b6f50be 100644
--- a/src/java.base/share/classes/java/lang/StackWalker.java
+++ b/src/java.base/share/classes/java/lang/StackWalker.java
@@ -200,7 +200,7 @@ public default String getDescriptor() {
          *
          * @jvms 4.7.10 The {@code SourceFile} Attribute
          */
-        public String getFileName();
+        public @Nullable String getFileName();
 
         /**
          * Returns the line number of the source line containing the execution
@@ -296,7 +296,7 @@ enum ExtendedOption {
         new StackWalker(DEFAULT_EMPTY_OPTION);
 
     private final Set<Option> options;
-    private final ExtendedOption extendedOption;
+    private final @Nullable ExtendedOption extendedOption;
     private final int estimateDepth;
     final boolean retainClassRef; // cached for performance
 
@@ -334,6 +334,7 @@ public static StackWalker getInstance() {
      * @throws SecurityException if a security manager exists and its
      *         {@code checkPermission} method denies access.
      */
+    @SuppressWarnings("nullness:argument.type.incompatible") // AOSEN: I don't understand this error
     public static StackWalker getInstance(Option option) {
         return getInstance(EnumSet.of(Objects.requireNonNull(option)));
     }
@@ -411,7 +412,7 @@ private StackWalker(EnumSet<Option> options) {
     private StackWalker(EnumSet<Option> options, int estimateDepth) {
         this(options, estimateDepth, null);
     }
-    private StackWalker(EnumSet<Option> options, int estimateDepth, ExtendedOption extendedOption) {
+    private StackWalker(EnumSet<Option> options, int estimateDepth, @Nullable ExtendedOption extendedOption) {
         this.options = options;
         this.estimateDepth = estimateDepth;
         this.extendedOption = extendedOption;
diff --git a/src/java.base/share/classes/java/lang/String.java b/src/java.base/share/classes/java/lang/String.java
index f091617a8f4..0f106d4ef26 100644
--- a/src/java.base/share/classes/java/lang/String.java
+++ b/src/java.base/share/classes/java/lang/String.java
@@ -786,6 +786,7 @@ static String newStringUTF8NoRepl(byte[] bytes, int offset, int length) {
         }
     }
 
+    @SuppressWarnings("throwing.nullable") // Should we call this as a true bug?
     static String newStringNoRepl(byte[] src, Charset cs) throws CharacterCodingException {
         try {
             return newStringNoRepl1(src, cs);
@@ -956,6 +957,7 @@ private static boolean isASCII(byte[] src) {
     /*
      * Throws CCE, instead of replacing, if unmappable.
      */
+    @SuppressWarnings("throwing.nullable") // Should we call this as a true bug?
     static byte[] getBytesNoRepl(String s, Charset cs) throws CharacterCodingException {
         try {
             return getBytesNoRepl1(s, cs);
@@ -3249,6 +3251,7 @@ public String replace(@GuardSatisfied CharSequence target, @GuardSatisfied CharS
      */
     @SideEffectFree
     @StaticallyExecutable
+    @SuppressWarnings("nullness:return.type.incompatible") // False positive as the returned array's element is always nonnull
     public String @MinLen(1) [] split(@Regex String regex, int limit) {
         /* fastpath if the regex is a
          * (1) one-char String and this character is not one of the
@@ -3379,7 +3382,7 @@ public String replace(@GuardSatisfied CharSequence target, @GuardSatisfied CharS
      */
     @SideEffectFree
     @StaticallyExecutable
-    public static String join(CharSequence delimiter, @Nullable CharSequence... elements) {
+    public static String join(CharSequence delimiter, CharSequence... elements) {
         var delim = delimiter.toString();
         var elems = new String[elements.length];
         for (int i = 0; i < elements.length; i++) {
@@ -3474,6 +3477,7 @@ static String join(String prefix, String suffix, String delimiter, String[] elem
      */
     @SideEffectFree
     @StaticallyExecutable
+    @SuppressWarnings("nullness:assignment.type.incompatible") // AOSEN: this is a real NPE for me
     public static String join(CharSequence delimiter,
             Iterable<? extends @Nullable CharSequence> elements) {
         Objects.requireNonNull(delimiter);
diff --git a/src/java.base/share/classes/java/lang/StringBuffer.java b/src/java.base/share/classes/java/lang/StringBuffer.java
index 989cac71188..cfa0c0c98bb 100644
--- a/src/java.base/share/classes/java/lang/StringBuffer.java
+++ b/src/java.base/share/classes/java/lang/StringBuffer.java
@@ -126,7 +126,7 @@ public final class StringBuffer
      * A cache of the last value returned by toString. Cleared
      * whenever the StringBuffer is modified.
      */
-    private transient String toStringCache;
+    private transient @Nullable String toStringCache;
 
     /** use serialVersionUID from JDK 1.0.2 for interoperability */
     @java.io.Serial
@@ -357,7 +357,7 @@ public synchronized StringBuffer append(@LeakedToResult StringBuffer this, @NonL
      * @since 1.8
      */
     @Override
-    synchronized StringBuffer append(@LeakedToResult StringBuffer this, @NonLeaked AbstractStringBuilder asb) {
+    synchronized StringBuffer append(@LeakedToResult StringBuffer this, @NonLeaked @Nullable AbstractStringBuilder asb) {
         toStringCache = null;
         super.append(asb);
         return this;
@@ -783,6 +783,8 @@ private synchronized void writeObject(java.io.ObjectOutputStream s)
      * @throws IOException if an I/O error occurs
      * @throws ClassNotFoundException if a serialized class cannot be loaded
      */
+    @SuppressWarnings({"nullness:argument.type.incompatible", "nullness:dereference.of.nullable"})
+    // AOSEN: if get method can return null, will there be a NPE?
     @java.io.Serial
     private void readObject(java.io.ObjectInputStream s)
         throws java.io.IOException, ClassNotFoundException {
diff --git a/src/java.base/share/classes/java/lang/StringLatin1.java b/src/java.base/share/classes/java/lang/StringLatin1.java
index 726e51322d0..2333c6782ec 100644
--- a/src/java.base/share/classes/java/lang/StringLatin1.java
+++ b/src/java.base/share/classes/java/lang/StringLatin1.java
@@ -25,6 +25,8 @@
 
 package java.lang;
 
+import org.checkerframework.checker.nullness.qual.Nullable;
+
 import java.util.Arrays;
 import java.util.Locale;
 import java.util.Objects;
@@ -299,7 +301,7 @@ public static int lastIndexOf(final byte[] value, int ch, int fromIndex) {
         return -1;
     }
 
-    public static String replace(byte[] value, char oldChar, char newChar) {
+    public static @Nullable String replace(byte[] value, char oldChar, char newChar) {
         if (canEncode(oldChar)) {
             int len = value.length;
             int i = -1;
@@ -336,7 +338,7 @@ public static String replace(byte[] value, char oldChar, char newChar) {
         return null; // for string to return this;
     }
 
-    public static String replace(byte[] value, int valLen, byte[] targ,
+    public static @Nullable String replace(byte[] value, int valLen, byte[] targ,
                                  int targLen, byte[] repl, int replLen)
     {
         assert targLen > 0;
@@ -464,6 +466,7 @@ public static String toLowerCase(String str, byte[] value, Locale locale) {
         return new String(result, LATIN1);
     }
 
+    @SuppressWarnings("nullness:dereference.of.nullable") // AOSEN: ConditionalSpecialCasing.toLowerCaseCharArray could return null
     private static String toLowerCaseEx(String str, byte[] value,
                                         int first, Locale locale, boolean localeDependent)
     {
@@ -539,6 +542,7 @@ public static String toUpperCase(String str, byte[] value, Locale locale) {
         return new String(result, LATIN1);
     }
 
+    @SuppressWarnings("nullness:dereference.of.nullable") // AOSEN: ConditionalSpecialCasing.toUpperCaseCharArray could return null
     private static String toUpperCaseEx(String str, byte[] value,
                                         int first, Locale locale, boolean localeDependent)
     {
@@ -584,7 +588,7 @@ private static String toUpperCaseEx(String str, byte[] value,
         return StringUTF16.newString(result, 0, resultOffset);
     }
 
-    public static String trim(byte[] value) {
+    public static @Nullable String trim(byte[] value) {
         int len = value.length;
         int st = 0;
         while ((st < len) && ((value[st] & 0xff) <= ' ')) {
@@ -623,7 +627,7 @@ public static int lastIndexOfNonWhitespace(byte[] value) {
         return right;
     }
 
-    public static String strip(byte[] value) {
+    public static @Nullable String strip(byte[] value) {
         int left = indexOfNonWhitespace(value);
         if (left == value.length) {
             return "";
@@ -633,12 +637,12 @@ public static String strip(byte[] value) {
         return ifChanged ? newString(value, left, right - left) : null;
     }
 
-    public static String stripLeading(byte[] value) {
+    public static @Nullable String stripLeading(byte[] value) {
         int left = indexOfNonWhitespace(value);
         return (left != 0) ? newString(value, left, value.length - left) : null;
     }
 
-    public static String stripTrailing(byte[] value) {
+    public static @Nullable String stripTrailing(byte[] value) {
         int right = lastIndexOfNonWhitespace(value);
         return (right != value.length) ? newString(value, 0, right) : null;
     }
@@ -707,7 +711,7 @@ public void forEachRemaining(Consumer<? super String> action) {
         }
 
         @Override
-        public Spliterator<String> trySplit() {
+        public @Nullable Spliterator<String> trySplit() {
             int half = (fence + index) >>> 1;
             int mid = skipLineSeparator(indexOfLineSeparator(half));
             if (mid < fence) {
@@ -746,7 +750,7 @@ public static char getChar(byte[] val, int index) {
         return (char)(val[index] & 0xff);
     }
 
-    public static byte[] toBytes(int[] val, int off, int len) {
+    public static byte @Nullable [] toBytes(int[] val, int off, int len) {
         byte[] ret = new byte[len];
         for (int i = 0; i < len; i++) {
             int cp = val[off++];
@@ -807,7 +811,7 @@ static class CharsSpliterator implements Spliterator.OfInt {
         }
 
         @Override
-        public OfInt trySplit() {
+        public @Nullable OfInt trySplit() {
             int lo = index, mid = (lo + fence) >>> 1;
             return (lo >= mid)
                    ? null
diff --git a/src/java.base/share/classes/java/lang/StringUTF16.java b/src/java.base/share/classes/java/lang/StringUTF16.java
index 6f3aff9e48e..507ad8dd03c 100644
--- a/src/java.base/share/classes/java/lang/StringUTF16.java
+++ b/src/java.base/share/classes/java/lang/StringUTF16.java
@@ -25,6 +25,8 @@
 
 package java.lang;
 
+import org.checkerframework.checker.nullness.qual.Nullable;
+
 import java.util.Arrays;
 import java.util.Locale;
 import java.util.Spliterator;
@@ -157,7 +159,7 @@ public static byte[] toBytes(char[] value, int off, int len) {
         return val;
     }
 
-    public static byte[] compress(char[] val, int off, int len) {
+    public static byte @Nullable [] compress(char[] val, int off, int len) {
         byte[] ret = new byte[len];
         if (compress(val, off, ret, 0, len) == len) {
             return ret;
@@ -165,7 +167,7 @@ public static byte[] compress(char[] val, int off, int len) {
         return null;
     }
 
-    public static byte[] compress(byte[] val, int off, int len) {
+    public static byte @Nullable [] compress(byte[] val, int off, int len) {
         byte[] ret = new byte[len];
         if (compress(val, off, ret, 0, len) == len) {
             return ret;
@@ -633,7 +635,7 @@ private static int lastIndexOfSupplementary(final byte[] value, int ch, int from
         return -1;
     }
 
-    public static String replace(byte[] value, char oldChar, char newChar) {
+    public static @Nullable String replace(byte[] value, char oldChar, char newChar) {
         int len = value.length >> 1;
         int i = -1;
         while (++i < len) {
@@ -665,7 +667,7 @@ public static String replace(byte[] value, char oldChar, char newChar) {
         return null;
     }
 
-    public static String replace(byte[] value, int valLen, boolean valLat1,
+    public static @Nullable String replace(byte[] value, int valLen, boolean valLat1,
                                  byte[] targ, int targLen, boolean targLat1,
                                  byte[] repl, int replLen, boolean replLat1)
     {
@@ -847,6 +849,7 @@ public static String toLowerCase(String str, byte[] value, Locale locale) {
         }
     }
 
+    @SuppressWarnings("nullness:dereference.of.nullable") // AOSEN: ConditionalSpecialCasing.toLowerCaseCharArray could return null
     private static String toLowerCaseEx(String str, byte[] value,
                                         byte[] result, int first, Locale locale,
                                         boolean localeDependent) {
@@ -948,6 +951,7 @@ public static String toUpperCase(String str, byte[] value, Locale locale) {
         }
     }
 
+    @SuppressWarnings("nullness:dereference.of.nullable") // AOSEN: ConditionalSpecialCasing.toUpperCaseCharArray could return null
     private static String toUpperCaseEx(String str, byte[] value,
                                         byte[] result, int first,
                                         Locale locale, boolean localeDependent)
@@ -1001,7 +1005,7 @@ private static String toUpperCaseEx(String str, byte[] value,
         return newString(result, 0, resultOffset);
     }
 
-    public static String trim(byte[] value) {
+    public static @Nullable String trim(byte[] value) {
         int length = value.length >> 1;
         int len = length;
         int st = 0;
@@ -1042,7 +1046,7 @@ public static int lastIndexOfNonWhitespace(byte[] value) {
         return right;
     }
 
-    public static String strip(byte[] value) {
+    public static @Nullable String strip(byte[] value) {
         int length = value.length >>> 1;
         int left = indexOfNonWhitespace(value);
         if (left == length) {
@@ -1053,13 +1057,13 @@ public static String strip(byte[] value) {
         return ifChanged ? newString(value, left, right - left) : null;
     }
 
-    public static String stripLeading(byte[] value) {
+    public static @Nullable String stripLeading(byte[] value) {
         int length = value.length >>> 1;
         int left = indexOfNonWhitespace(value);
         return (left != 0) ? newString(value, left, length - left) : null;
     }
 
-    public static String stripTrailing(byte[] value) {
+    public static @Nullable String stripTrailing(byte[] value) {
         int length = value.length >>> 1;
         int right = lastIndexOfNonWhitespace(value);
         return (right != length) ? newString(value, 0, right) : null;
@@ -1129,7 +1133,7 @@ public void forEachRemaining(Consumer<? super String> action) {
         }
 
         @Override
-        public Spliterator<String> trySplit() {
+        public @Nullable Spliterator<String> trySplit() {
             int half = (fence + index) >>> 1;
             int mid = skipLineSeparator(indexOfLineSeparator(half));
             if (mid < fence) {
@@ -1202,7 +1206,7 @@ static class CharsSpliterator implements Spliterator.OfInt {
         }
 
         @Override
-        public OfInt trySplit() {
+        public @Nullable OfInt trySplit() {
             int lo = index, mid = (lo + fence) >>> 1;
             return (lo >= mid)
                    ? null
@@ -1262,7 +1266,7 @@ static class CodePointsSpliterator implements Spliterator.OfInt {
         }
 
         @Override
-        public OfInt trySplit() {
+        public @Nullable OfInt trySplit() {
             int lo = index, mid = (lo + fence) >>> 1;
             if (lo >= mid)
                 return null;
diff --git a/src/java.base/share/classes/java/lang/System.java b/src/java.base/share/classes/java/lang/System.java
index 19439ccff41..c47432ef27d 100644
--- a/src/java.base/share/classes/java/lang/System.java
+++ b/src/java.base/share/classes/java/lang/System.java
@@ -141,6 +141,7 @@ private System() {
      * @see Console#reader()
      */
     @CFComment("This field can be null. The Checker Framework conservatively annotates it as @NonNull, forbidding programs that set it to null.")
+    @SuppressWarnings("nullness:assignment.type.incompatible")
     public static final @MustCall({}) InputStream in = null;
 
     /**
@@ -174,6 +175,7 @@ private System() {
      * @see     Charset#defaultCharset()
      */
     @CFComment("This field can be null. The Checker Framework conservatively annotates it as @NonNull, forbidding programs that set it to null.")
+    @SuppressWarnings("nullness:assignment.type.incompatible")
     public static final @MustCall({}) PrintStream out = null;
 
     /**
@@ -194,7 +196,8 @@ private System() {
      * @see     Console#charset()
      * @see     Charset#defaultCharset()
      */
-    @CFComment("This field can be null. The Checker Framework conservatively annotates it as @NonNull, forbidding programs that set it to null.")
+    @CFComment("This field can be null. The Checker Framework conservatively annotates it as @NonNull, forbidding programs that set it to null. Should we do it?")
+    @SuppressWarnings("nullness:assignment.type.incompatible")
     public static final @MustCall({}) PrintStream err = null;
 
     // indicates if a security manager is possible
@@ -202,11 +205,12 @@ private System() {
     private static final int MAYBE = 2;
     private static @Stable int allowSecurityManager;
 
-    // current security manager
+    // current security manage9
     @SuppressWarnings("removal")
-    private static volatile SecurityManager security;   // read by VM
+    private static volatile @Nullable SecurityManager security;   // read by VM
 
     // return true if a security manager is allowed
+    @Pure
     private static boolean allowSecurityManager() {
         return (allowSecurityManager != NEVER);
     }
@@ -216,7 +220,7 @@ private static boolean allowSecurityManager() {
      *
      * First, if there is a security manager, its {@code checkPermission}
      * method is called with a {@code RuntimePermission("setIO")} permission
-     *  to see if it's ok to reassign the "standard" input stream.
+     *  to see if it's ok to reassign the "standard" input str1eam.
      *
      * @param in the new standard input stream.
      *
@@ -359,7 +363,7 @@ private static class CallersHolder {
     // Remember initial System.err. setSecurityManager() warning goes here
     private static volatile @Stable PrintStream initialErrStream;
 
-    private static URL codeSource(Class<?> clazz) {
+    private static @Nullable URL codeSource(Class<?> clazz) {
         PrivilegedAction<ProtectionDomain> pa = clazz::getProtectionDomain;
         @SuppressWarnings("removal")
         CodeSource cs = AccessController.doPrivileged(pa).getCodeSource();
@@ -433,7 +437,7 @@ public static void setSecurityManager(@SuppressWarnings("removal") @Nullable Sec
         }
     }
 
-    private static void implSetSecurityManager(@SuppressWarnings("removal") SecurityManager sm) {
+    private static void implSetSecurityManager(@SuppressWarnings("removal") @Nullable SecurityManager sm) {
         if (security == null) {
             // ensure image reader is initialized
             Object.class.getResource("java/lang/ANY");
@@ -455,7 +459,7 @@ private static void implSetSecurityManager(@SuppressWarnings("removal") Security
 
     @SuppressWarnings("removal")
     private static synchronized
-    void setSecurityManager0(final SecurityManager s) {
+    void setSecurityManager0(final @Nullable SecurityManager s) {
         SecurityManager sm = getSecurityManager();
         if (sm != null) {
             // ask the currently installed security manager if we
@@ -472,8 +476,8 @@ void setSecurityManager0(final SecurityManager s) {
             // calls the installed security manager's checkPermission method
             // which will loop infinitely if there is a non-system class
             // (in this case: the new security manager class) on the stack).
-            AccessController.doPrivileged(new PrivilegedAction<>() {
-                public Object run() {
+            AccessController.doPrivileged(new PrivilegedAction<@Nullable Object>() {
+                public @Nullable Object run() {
                     s.getClass().getProtectionDomain().implies
                         (SecurityConstants.ALL_PERMISSION);
                     return null;
@@ -500,6 +504,7 @@ public Object run() {
      */
     @SuppressWarnings("removal")
     @Deprecated(since="17", forRemoval=true)
+    @Pure
     public static @Nullable SecurityManager getSecurityManager() {
         if (allowSecurityManager()) {
             return security;
@@ -854,11 +859,11 @@ public static Properties getProperties() {
      * @since 1.7
      */
     @Pure
-    public static String lineSeparator() {
+    public static @Nullable String lineSeparator() {
         return lineSeparator;
     }
 
-    private static String lineSeparator;
+    private static @Nullable String lineSeparator;
 
     /**
      * Sets the system properties to the {@code Properties} argument.
@@ -1066,7 +1071,7 @@ public static void setProperties(@Nullable Properties props) {
         return (String) props.remove(key);
     }
 
-    private static void checkKey(String key) {
+    private static void checkKey(@Nullable String key) {
         if (key == null) {
             throw new NullPointerException("key can't be null");
         }
@@ -1496,7 +1501,7 @@ public default void log(Level level, String format, Object... params) {
          *
          * @throws NullPointerException if {@code level} is {@code null}.
          */
-        public void log(Level level, ResourceBundle bundle, String msg,
+        public void log(Level level, @Nullable ResourceBundle bundle, String msg,
                 Throwable thrown);
 
         /**
@@ -1518,8 +1523,8 @@ public void log(Level level, ResourceBundle bundle, String msg,
          *
          * @throws NullPointerException if {@code level} is {@code null}.
          */
-        public void log(Level level, ResourceBundle bundle, String format,
-                Object... params);
+        public void log(Level level, @Nullable ResourceBundle bundle, String format,
+                Object @Nullable... params);
     }
 
     /**
@@ -2032,7 +2037,7 @@ public static void loadLibrary(String libname) {
     /**
      * Create PrintStream for stdout/err based on encoding.
      */
-    private static PrintStream newPrintStream(FileOutputStream fos, String enc) {
+    private static PrintStream newPrintStream(FileOutputStream fos, @Nullable String enc) {
        if (enc != null) {
             try {
                 return new PrintStream(new BufferedOutputStream(fos, 128), true, enc);
@@ -2103,6 +2108,7 @@ private static Properties createProperties(Map<String, String> initialProps) {
     /**
      * Initialize the system class.  Called after thread initialization.
      */
+    @SuppressWarnings("nullness:dereference.of.nullable")
     private static void initPhase1() {
 
         // register the shared secrets - do this first, since SystemProps.initProperties
@@ -2294,7 +2300,7 @@ public boolean casAnnotationType(Class<?> klass, AnnotationType oldType, Annotat
             public AnnotationType getAnnotationType(Class<?> klass) {
                 return klass.getAnnotationType();
             }
-            public Map<Class<? extends Annotation>, Annotation> getDeclaredAnnotationMap(Class<?> klass) {
+            public Map<Class<? extends @Nullable Annotation>, Annotation> getDeclaredAnnotationMap(Class<?> klass) {
                 return klass.getDeclaredAnnotationMap();
             }
             public byte[] getRawClassAnnotations(Class<?> klass) {
@@ -2307,7 +2313,7 @@ public byte[] getRawExecutableTypeAnnotations(Executable executable) {
                 return Class.getExecutableTypeAnnotationBytes(executable);
             }
             public <E extends Enum<E>>
-            E[] getEnumConstantsShared(Class<E> klass) {
+            E @Nullable [] getEnumConstantsShared(Class<E> klass) {
                 return klass.getEnumConstantsShared();
             }
             public void blockedOn(Interruptible b) {
@@ -2333,7 +2339,7 @@ public Class<?> defineClass(ClassLoader loader, Class<?> lookup, String name, by
                                         boolean initialize, int flags, Object classData) {
                 return ClassLoader.defineClass0(loader, lookup, name, b, 0, b.length, pd, initialize, flags, classData);
             }
-            public Class<?> findBootstrapClassOrNull(String name) {
+            public @Nullable Class<?> findBootstrapClassOrNull(String name) {
                 return ClassLoader.findBootstrapClassOrNull(name);
             }
             public Package definePackage(ClassLoader cl, String name, Module module) {
diff --git a/src/java.base/share/classes/java/lang/Thread.java b/src/java.base/share/classes/java/lang/Thread.java
index 75aede10f82..e66fb01d9e7 100644
--- a/src/java.base/share/classes/java/lang/Thread.java
+++ b/src/java.base/share/classes/java/lang/Thread.java
@@ -173,13 +173,13 @@
     private long eetop;
 
     /* What will be run. */
-    private Runnable target;
+    private @Nullable Runnable target;
 
     /* The group of this thread */
     private ThreadGroup group;
 
     /* The context ClassLoader for this thread */
-    private ClassLoader contextClassLoader;
+    private @Nullable ClassLoader contextClassLoader;
 
     /* The inherited AccessControlContext of this thread */
     @SuppressWarnings("removal")
@@ -193,13 +193,13 @@ private static synchronized int nextThreadNum() {
 
     /* ThreadLocal values pertaining to this thread. This map is maintained
      * by the ThreadLocal class. */
-    ThreadLocal.ThreadLocalMap threadLocals = null;
+    ThreadLocal.@Nullable ThreadLocalMap threadLocals = null;
 
     /*
      * InheritableThreadLocal values pertaining to this thread. This map is
      * maintained by the InheritableThreadLocal class.
      */
-    ThreadLocal.ThreadLocalMap inheritableThreadLocals = null;
+    ThreadLocal.@Nullable ThreadLocalMap inheritableThreadLocals = null;
 
     /*
      * The requested stack size for this thread, or 0 if the creator did
@@ -237,7 +237,7 @@ private static synchronized long nextThreadID() {
      * operation, if any.  The blocker's interrupt method should be invoked
      * after setting this thread's interrupt status.
      */
-    private volatile Interruptible blocker;
+    private volatile @Nullable Interruptible blocker;
     private final Object blockerLock = new Object();
 
     /* Set the blocker field; invoked via jdk.internal.access.SharedSecrets
@@ -403,8 +403,8 @@ public static void onSpinWait() {}
      *            inheritable thread-locals from the constructing thread
      */
     @SuppressWarnings("removal")
-    private Thread(ThreadGroup g, Runnable target, String name,
-                   long stackSize, AccessControlContext acc,
+    private Thread(@Nullable ThreadGroup g, @Nullable Runnable target, String name,
+                   long stackSize, @Nullable AccessControlContext acc,
                    boolean inheritThreadLocals) {
         if (name == null) {
             throw new NullPointerException("name cannot be null");
@@ -851,6 +851,7 @@ public void run() {
      * This method is called by the system to give a Thread
      * a chance to clean up before it actually exits.
      */
+    @SuppressWarnings("nullness:assignment.type.incompatible") // group is null only when exit() is called
     private void exit() {
         if (threadLocals != null && TerminatingThreadLocal.REGISTRY.isPresent()) {
             TerminatingThreadLocal.threadTerminated();
@@ -1212,7 +1213,7 @@ public final String getName() {
      *
      * @return  this thread's thread group.
      */
-    public final @Nullable ThreadGroup getThreadGroup() {
+    public final ThreadGroup getThreadGroup() {
         return group;
     }
 
@@ -1729,6 +1730,7 @@ private static boolean isCCLOverridden(Class<?> cl) {
      * override security-sensitive non-final methods.  Returns true if the
      * subclass overrides any of the methods, false otherwise.
      */
+    @SuppressWarnings("nullness:dereference.of.nullable") // AOSEN: Is this a real NPE? cl.getDeclaredMethod
     private static boolean auditSubclass(final Class<?> subcl) {
         @SuppressWarnings("removal")
         Boolean result = AccessController.doPrivileged(
@@ -1923,10 +1925,10 @@ public interface UncaughtExceptionHandler {
     }
 
     // null unless explicitly set
-    private volatile UncaughtExceptionHandler uncaughtExceptionHandler;
+    private volatile @Nullable UncaughtExceptionHandler uncaughtExceptionHandler;
 
     // null unless explicitly set
-    private static volatile UncaughtExceptionHandler defaultUncaughtExceptionHandler;
+    private static volatile @Nullable UncaughtExceptionHandler defaultUncaughtExceptionHandler;
 
     /**
      * Set the default handler invoked when a thread abruptly terminates
@@ -2023,6 +2025,7 @@ public void setUncaughtExceptionHandler(@Nullable UncaughtExceptionHandler eh) {
      * Dispatch an uncaught exception to the handler. This method is
      * intended to be called only by the JVM.
      */
+    @SuppressWarnings("nullness:dereference.of.nullable") // AOSEN: This cause possible NPE but it is not used
     private void dispatchUncaughtException(Throwable e) {
         getUncaughtExceptionHandler().uncaughtException(this, e);
     }
@@ -2075,7 +2078,7 @@ public int hashCode() {
          * instance with the identical non-null referent as this one.
          */
         @Override
-        public boolean equals(Object obj) {
+        public boolean equals(@Nullable Object obj) {
             if (obj == this)
                 return true;
 
diff --git a/src/java.base/share/classes/java/lang/ThreadGroup.java b/src/java.base/share/classes/java/lang/ThreadGroup.java
index 77eb30d6fad..9c65aadf467 100644
--- a/src/java.base/share/classes/java/lang/ThreadGroup.java
+++ b/src/java.base/share/classes/java/lang/ThreadGroup.java
@@ -30,7 +30,6 @@
 import org.checkerframework.checker.interning.qual.UsesObjectEquals;
 import org.checkerframework.checker.lock.qual.GuardSatisfied;
 import org.checkerframework.checker.nullness.qual.Nullable;
-import org.checkerframework.checker.nullness.qual.PolyNull;
 import org.checkerframework.dataflow.qual.Pure;
 import org.checkerframework.dataflow.qual.SideEffectFree;
 import org.checkerframework.framework.qual.AnnotatedFor;
@@ -64,8 +63,8 @@
  */
 @AnnotatedFor({"index", "interning", "lock", "nullness"})
 public @UsesObjectEquals class ThreadGroup implements Thread.UncaughtExceptionHandler {
-    private final ThreadGroup parent;
-    String name;
+    private final @Nullable ThreadGroup parent;
+    @Nullable String name;
     int maxPriority;
     boolean destroyed;
     boolean daemon;
@@ -100,6 +99,7 @@ private ThreadGroup() {     // called from C code
      * @see     java.lang.ThreadGroup#checkAccess()
      * @since   1.0
      */
+    @SuppressWarnings("nullness:argument.type.incompatible") // AOSEN: thread group could be null.
     public ThreadGroup(@Nullable String name) {
         this(Thread.currentThread().getThreadGroup(), name);
     }
@@ -125,7 +125,7 @@ public ThreadGroup(ThreadGroup parent, @Nullable String name) {
         this(checkParentAccess(parent), parent, name);
     }
 
-    private ThreadGroup(Void unused, ThreadGroup parent, String name) {
+    private ThreadGroup(Void unused, ThreadGroup parent, @Nullable String name) {
         this.name = name;
         this.maxPriority = parent.maxPriority;
         this.daemon = parent.daemon;
@@ -284,7 +284,8 @@ public final void setDaemon(boolean daemon) {
      */
     @CFComment({"index: groupSnapshot.length = ngroupsSnapshot by #0.1",
                 "for the else case, ngroupsSnapshot will be null and it will never enter the group as nGroups will be 0"})
-    @SuppressWarnings("index:array.access.unsafe.high")
+    @SuppressWarnings({"index:array.access.unsafe.high","index:array.access.unsafe.high.range", "nullness:assignment.type.incompatible", "nullness:accessing.nullable"})
+    // AOSEN: the return array is not refined. But is this an access NPE?
     public final void setMaxPriority(int pri) {
         int ngroupsSnapshot;
         ThreadGroup[] groupsSnapshot;
@@ -316,6 +317,7 @@ public final void setMaxPriority(int pri) {
      *          {@code false} otherwise.
      * @since   1.0
      */
+    @SuppressWarnings("nullness:assignment.type.incompatible")
     public final boolean parentOf(ThreadGroup g) {
         for (; g != null ; g = g.parent) {
             if (g == this) {
@@ -372,7 +374,8 @@ public final void checkAccess() {
      */
     @CFComment({"index: groupSnapshot.length = ngroupsSnapshot by #0.1",
                 "for the else case, ngroupsSnapshot will be null and it will never enter the group as nGroups will be 0"})
-    @SuppressWarnings("index:array.access.unsafe.high")
+    @SuppressWarnings({"index:array.access.unsafe.high", "nullness:assignment.type.incompatible", "nullness:accessing.nullable"})
+    // AOSEN: the return array is not refined. But is this an access NPE?
     public @NonNegative int activeCount() {
         int result;
         // Snapshot sub-group data so we don't hold this lock
@@ -419,7 +422,7 @@ public final void checkAccess() {
      *
      * @since   1.0
      */
-    public @NonNegative int enumerate(@PolyNull Thread[] list) {
+    public @NonNegative int enumerate(@Nullable Thread[] list) {
         checkAccess();
         return enumerate(list, 0, true);
     }
@@ -457,7 +460,7 @@ public final void checkAccess() {
      *
      * @since   1.0
      */
-    public @NonNegative int enumerate(@PolyNull Thread[] list, boolean recurse) {
+    public @NonNegative int enumerate(@Nullable Thread[] list, boolean recurse) {
         checkAccess();
         return enumerate(list, 0, recurse);
     }
@@ -466,8 +469,9 @@ public final void checkAccess() {
     "#1: To enter this loop, nt has to be positive, i.c, list.length - n is @Positive, and n is @NonNegative, hence if this loop is entered, n is @LTLengthOf(\"list\")",
     "#2: groupSnapshot.length = ngroupsSnapshot by #0.1, for the else case, ngroupsSnapshot will be null and it will never enter the group as nGroups will be 0"
     })
-    @SuppressWarnings({"index:array.access.unsafe.high","index:array.access.unsafe.high.range"})
-    private @NonNegative int enumerate(Thread list[], @NonNegative int n, boolean recurse) {
+    @SuppressWarnings({"index:array.access.unsafe.high","index:array.access.unsafe.high.range", "nullness:assignment.type.incompatible", "nullness:accessing.nullable"})
+    // AOSEN: the return array is not refined. But is this an access NPE?
+    private @NonNegative int enumerate(@Nullable Thread list[], @NonNegative int n, boolean recurse) {
         int ngroupsSnapshot = 0;
         ThreadGroup[] groupsSnapshot = null;
         synchronized (this) {
@@ -517,7 +521,8 @@ public final void checkAccess() {
      */
     @CFComment({"index: groupSnapshot.length = ngroupsSnapshot by #0.1", 
                 "for the else case, ngroupsSnapshot will be null and it will never enter the group as nGroups will be 0"})
-    @SuppressWarnings("index:array.access.unsafe.high")
+    @SuppressWarnings({"index:array.access.unsafe.high", "nullness:assignment.type.incompatible", "nullness:accessing.nullable"})
+    // AOSEN: the return array is not refined. But is this an access NPE?
     public @NonNegative int activeGroupCount() {
         int ngroupsSnapshot;
         ThreadGroup[] groupsSnapshot;
@@ -561,7 +566,7 @@ public final void checkAccess() {
      *
      * @since   1.0
      */
-    public @NonNegative int enumerate(@PolyNull ThreadGroup[] list) {
+    public @NonNegative int enumerate(@Nullable ThreadGroup[] list) {
         checkAccess();
         return enumerate(list, 0, true);
     }
@@ -599,15 +604,16 @@ public final void checkAccess() {
      *
      * @since   1.0
      */
-    public @NonNegative int enumerate(@PolyNull ThreadGroup[] list, boolean recurse) {
+    public @NonNegative int enumerate(@Nullable ThreadGroup[] list, boolean recurse) {
         checkAccess();
         return enumerate(list, 0, recurse);
     }
 
     @CFComment({"index: groupSnapshot.length = ngroupsSnapshot by #0.1",
                 "for the else case ngroupsSnapshot will be null and it will never enter the group as nGroups will be 0"})
-    @SuppressWarnings("index:array.access.unsafe.high")
-    private @NonNegative int enumerate(ThreadGroup list[], @NonNegative int n, boolean recurse) {
+    @SuppressWarnings({"index:array.access.unsafe.high", "nullness:assignment.type.incompatible", "nullness:accessing.nullable"})
+    // AOSEN: the return array is not refined. But is this an access NPE?
+    private @NonNegative int enumerate(@Nullable ThreadGroup list[], @NonNegative int n, boolean recurse) {
         int ngroupsSnapshot = 0;
         ThreadGroup[] groupsSnapshot = null;
         synchronized (this) {
@@ -683,7 +689,8 @@ public final void stop() {
      */
     @CFComment({" groupSnapshot.length = ngroupsSnapshot by #0.1",
                 "for the else case, ngroupsSnapshot will be null and it will never enter the group as nGroups will be 0"})
-    @SuppressWarnings("index:array.access.unsafe.high")
+    @SuppressWarnings({"index:array.access.unsafe.high", "nullness:assignment.type.incompatible", "nullness:accessing.nullable"})
+    // AOSEN: the return array is not refined. But is this an access NPE?
     public final void interrupt() {
         int ngroupsSnapshot;
         ThreadGroup[] groupsSnapshot;
@@ -739,7 +746,8 @@ public final void suspend() {
      */
     @CFComment({"index: groupSnapshot.length = ngroupsSnapshot by #0.1",
                 "for the else case, ngroupsSnapshot will be null and it will never enter the group as nGroups will be 0"})
-    @SuppressWarnings({"deprecation", "removal", "index:array.access.unsafe.high"})
+    @SuppressWarnings({"deprecation", "removal", "index:array.access.unsafe.high", "nullness:assignment.type.incompatible", "nullness:accessing.nullable"})
+    // AOSEN: the return array is not refined. But is this an access NPE?
     private boolean stopOrSuspend(boolean suspend) {
         boolean suicide = false;
         Thread us = Thread.currentThread();
@@ -791,7 +799,8 @@ else if (suspend)
     @CFComment({"index:  // groupSnapshot.length = ngroupsSnapshot by #0.1",
                 "for the else case, ngroupsSnapshot will be null and it will never enter the group as nGroups will be 0"})
     @Deprecated(since="1.2", forRemoval=true)
-    @SuppressWarnings({"removal", "index:array.access.unsafe.high"})
+    @SuppressWarnings({"removal", "index:array.access.unsafe.high", "nullness:assignment.type.incompatible", "nullness:accessing.nullable"})
+    // AOSEN: the return array is not refined. But is this an access NPE?
     public final void resume() {
         int ngroupsSnapshot;
         ThreadGroup[] groupsSnapshot;
@@ -833,7 +842,8 @@ public final void resume() {
      */
     @CFComment({"index: groupSnapshot.length = ngroupsSnapshot by #0.1",
                 "for the else case, ngroupsSnapshot will be null and it will never enter the group as nGroups will be 0"})
-    @SuppressWarnings("index:array.access.unsafe.high")
+    @SuppressWarnings({"index:array.access.unsafe.high","nullness:assignment.type.incompatible", "nullness:accessing.nullable"})
+    //AOSEN: groups and threads are set to null only when destroying. But is this an access NPE?
     @Deprecated(since="16", forRemoval=true)
     public final void destroy() {
         int ngroupsSnapshot;
@@ -871,7 +881,7 @@ public final void destroy() {
      * @throws  IllegalThreadStateException If the Thread group has been destroyed.
      */
     @CFComment({"index: #1: If ngroups = groups.length, length of group is doubled"})
-    @SuppressWarnings({"index:array.access.unsafe.high", "index:compound.assignment"})
+    @SuppressWarnings({"index:array.access.unsafe.high", "index:compound.assignment", "nullness:assignment.type.incompatible"})
     private final void add(ThreadGroup g){
         synchronized (this) {
             if (destroyed) {
@@ -896,7 +906,7 @@ private final void add(ThreadGroup g){
      * @return if this Thread has already been destroyed.
      */
     @CFComment({"index: #1: ngroups - i <= groups.length - i"})
-    @SuppressWarnings("index:argument")
+    @SuppressWarnings({"index:argument", "nullness:assignment.type.incompatible"}) // AOSEN: only null when destroyed
     private void remove(ThreadGroup g) {
         synchronized (this) {
             if (destroyed) {
@@ -954,7 +964,7 @@ void addUnstarted() {
      *          if the Thread group has been destroyed
      */
     @CFComment({"index: #1: If nthreads = threads.length, length of threads is doubled"})
-    @SuppressWarnings({"index:array.access.unsafe.high", "index:compound.assignment"})
+    @SuppressWarnings({"index:array.access.unsafe.high", "index:compound.assignment", "nullness:assignment.type.incompatible"})
     void add(Thread t) {
         synchronized (this) {
             if (destroyed) {
@@ -1032,7 +1042,7 @@ void threadTerminated(Thread t) {
      *         the Thread to be removed
      */
     @CFComment({"index: #1: --nthreads - i < threads.length - i, also, --nthreads - i is @NonNegative as --nthreads >= i"})
-    @SuppressWarnings("index:argument")
+    @SuppressWarnings({"index:argument", "nullness:assignment.type.incompatible"}) // AOSEN: only null when destroyed
     private void remove(Thread t) {
         synchronized (this) {
             if (destroyed) {
@@ -1058,7 +1068,8 @@ private void remove(Thread t) {
      */
     @CFComment({"index: groupSnapshot.length = ngroupsSnapshot by #0.1",
                 "for the else case, ngroupsSnapshot will be null and it will never enter the group as nGroups will be 0"})
-    @SuppressWarnings("index:array.access.unsafe.high")
+    @SuppressWarnings({"index:array.access.unsafe.high", "nullness:assignment.type.incompatible", "nullness:accessing.nullable"})
+    // AOSEN: The return array is not refined. But is this an access NPE?
     public void list() {
         list(System.out, 0);
     }
diff --git a/src/java.base/share/classes/java/lang/ThreadLocal.java b/src/java.base/share/classes/java/lang/ThreadLocal.java
index 52b40880fd3..ad627ec63d9 100644
--- a/src/java.base/share/classes/java/lang/ThreadLocal.java
+++ b/src/java.base/share/classes/java/lang/ThreadLocal.java
@@ -88,6 +88,7 @@
             "@SuppressWarnings(\"nullness:type.argument\") // initialValue returns non-null"
         })
 @AnnotatedFor({"interning", "nullness"})
+@SuppressWarnings("nullness") //AOSEN: I need to understand why @Nullable annotation is here
 public @UsesObjectEquals class ThreadLocal<@Nullable T> {
     /**
      * ThreadLocals rely on per-thread linear-probe hash maps attached
@@ -154,7 +155,7 @@ protected T initialValue() {
      * @throws NullPointerException if the specified supplier is null
      * @since 1.8
      */
-    public static <S> ThreadLocal<S> withInitial(Supplier<? extends S> supplier) {
+    public static <@Nullable S> ThreadLocal<S> withInitial(Supplier<? extends S> supplier) {
         return new SuppliedThreadLocal<>(supplier);
     }
 
@@ -265,7 +266,7 @@ public void remove() {
      * @param  t the current thread
      * @return the map
      */
-    ThreadLocalMap getMap(Thread t) {
+    @Nullable ThreadLocalMap getMap(Thread t) {
         return t.threadLocals;
     }
 
@@ -307,7 +308,7 @@ T childValue(T parentValue) {
      * An extension of ThreadLocal that obtains its initial value from
      * the specified {@code Supplier}.
      */
-    static final class SuppliedThreadLocal<T> extends ThreadLocal<T> {
+    static final class SuppliedThreadLocal<@Nullable T> extends ThreadLocal<T> {
 
         private final Supplier<? extends T> supplier;
 
@@ -341,11 +342,11 @@ static class ThreadLocalMap {
          * entry can be expunged from table.  Such entries are referred to
          * as "stale entries" in the code that follows.
          */
-        static class Entry extends WeakReference<ThreadLocal<?>> {
+        static class Entry extends WeakReference<@Nullable ThreadLocal<?>> {
             /** The value associated with this ThreadLocal. */
-            Object value;
+            @Nullable Object value;
 
-            Entry(ThreadLocal<?> k, Object v) {
+            Entry(ThreadLocal<?> k, @Nullable Object v) {
                 super(k);
                 value = v;
             }
@@ -398,7 +399,7 @@ private static int prevIndex(int i, int len) {
          * ThreadLocalMaps are constructed lazily, so we only create
          * one when we have at least one entry to put in it.
          */
-        ThreadLocalMap(ThreadLocal<?> firstKey, Object firstValue) {
+        ThreadLocalMap(ThreadLocal<?> firstKey, @Nullable Object firstValue) {
             table = new Entry[INITIAL_CAPACITY];
             int i = firstKey.threadLocalHashCode & (INITIAL_CAPACITY - 1);
             table[i] = new Entry(firstKey, firstValue);
@@ -463,7 +464,7 @@ private Entry getEntry(ThreadLocal<?> key) {
          * @param  e the entry at table[i]
          * @return the entry associated with key, or null if no such
          */
-        private Entry getEntryAfterMiss(ThreadLocal<?> key, int i, Entry e) {
+        private @Nullable Entry getEntryAfterMiss(ThreadLocal<?> key, int i, Entry e) {
             Entry[] tab = table;
             int len = tab.length;
 
@@ -617,7 +618,7 @@ private void replaceStaleEntry(ThreadLocal<?> key, Object value,
          * for expunging).
          */
         private int expungeStaleEntry(int staleSlot) {
-            Entry[] tab = table;
+            @Nullable Entry[] tab = table;
             int len = tab.length;
 
             // expunge entry at staleSlot
diff --git a/src/java.base/share/classes/java/lang/Throwable.java b/src/java.base/share/classes/java/lang/Throwable.java
index 38660ba9ffa..17b42cdb0f9 100644
--- a/src/java.base/share/classes/java/lang/Throwable.java
+++ b/src/java.base/share/classes/java/lang/Throwable.java
@@ -25,6 +25,8 @@
 
 package java.lang;
 
+import org.checkerframework.checker.initialization.qual.NotOnlyInitialized;
+import org.checkerframework.checker.initialization.qual.UnderInitialization;
 import org.checkerframework.checker.initialization.qual.PolyInitialized;
 import org.checkerframework.checker.interning.qual.UsesObjectEquals;
 import org.checkerframework.checker.lock.qual.GuardSatisfied;
@@ -128,8 +130,7 @@
     /**
      * The JVM saves some indication of the stack backtrace in this slot.
      */
-    private transient Object backtrace;
-
+    private transient @Nullable Object backtrace;
     /**
      * Specific details about the Throwable.  For example, for
      * {@code FileNotFoundException}, this contains the name of
@@ -137,7 +138,7 @@
      *
      * @serial
      */
-    private String detailMessage;
+    private @Nullable String detailMessage;
 
 
     /**
@@ -204,7 +205,8 @@ private static class SentinelHolder {
      * @serial
      * @since 1.4
      */
-    private Throwable cause = this;
+    @SuppressWarnings("initialization:assignment.type.incompatible") // EISOP#1217
+    private @NotOnlyInitialized @Nullable Throwable cause = this;
 
     /**
      * The stack trace, as returned by {@link #getStackTrace()}.
@@ -217,7 +219,7 @@ private static class SentinelHolder {
      * @serial
      * @since 1.4
      */
-    private StackTraceElement[] stackTrace = UNASSIGNED_STACK;
+    private StackTraceElement @Nullable [] stackTrace = UNASSIGNED_STACK;
 
     /**
      * The JVM code sets the depth of the backtrace for later retrieval
@@ -239,7 +241,7 @@ private static class SentinelHolder {
      * @since 1.7
      */
     @SuppressWarnings("serial") // Not statically typed as Serializable
-    private List<Throwable> suppressedExceptions = SUPPRESSED_SENTINEL;
+    private @Nullable List<Throwable> suppressedExceptions = SUPPRESSED_SENTINEL;
 
     /** Message for trying to suppress a null exception. */
     private static final String NULL_CAUSE_MESSAGE = "Cannot suppress a null exception.";
@@ -810,7 +812,7 @@ void println(Object o) {
      * @return  a reference to this {@code Throwable} instance.
      * @see     java.lang.Throwable#printStackTrace()
      */
-    public synchronized Throwable fillInStackTrace() {
+    public synchronized @UnderInitialization Throwable fillInStackTrace(@UnderInitialization Throwable this) {
         if (stackTrace != null ||
             backtrace != null /* Out of protocol state */ ) {
             fillInStackTrace(0);
@@ -819,7 +821,7 @@ public synchronized Throwable fillInStackTrace() {
         return this;
     }
 
-    private native Throwable fillInStackTrace(int dummy);
+    private native Throwable fillInStackTrace(@UnderInitialization Throwable this, int dummy);
 
     /**
      * Provides programmatic access to the stack trace information printed by
@@ -1117,6 +1119,7 @@ public final synchronized void addSuppressed(Throwable exception) {
      *         suppressed to deliver this exception.
      * @since 1.7
      */
+    @SuppressWarnings("nullness:return.type.incompatible") // False positive as the returned array's element is always nonnull
     public final synchronized Throwable[] getSuppressed() {
         if (suppressedExceptions == SUPPRESSED_SENTINEL ||
             suppressedExceptions == null)
diff --git a/src/java.base/share/classes/java/lang/WeakPairMap.java b/src/java.base/share/classes/java/lang/WeakPairMap.java
index febb8adcf31..b1925b5ce2a 100644
--- a/src/java.base/share/classes/java/lang/WeakPairMap.java
+++ b/src/java.base/share/classes/java/lang/WeakPairMap.java
@@ -45,7 +45,7 @@
  * @param <V>  the type of value
  * @author Peter Levart
  */
-final class WeakPairMap<K1, K2, V> {
+final class WeakPairMap<K1 extends Object, K2 extends Object, V extends Object> {
 
     private final ConcurrentHashMap<Pair<K1, K2>, V> map = new ConcurrentHashMap<>();
     private final ReferenceQueue<Object> queue = new ReferenceQueue<>();
@@ -82,7 +82,7 @@ public boolean containsKeyPair(K1 k1, K2 k2) {
      * this map contains no mapping for the key pair
      * @throws NullPointerException if any of the specified keys is null
      */
-    public V get(K1 k1, K2 k2) {
+    public @Nullable V get(K1 k1, K2 k2) {
         expungeStaleAssociations();
         return map.get(Pair.lookup(k1, k2));
     }
@@ -102,7 +102,7 @@ public V get(K1 k1, K2 k2) {
      * there was no mapping for key pair
      * @throws NullPointerException if any of the specified keys or value is null
      */
-    public V put(K1 k1, K2 k2, V v) {
+    public @Nullable V put(K1 k1, K2 k2, V v) {
         expungeStaleAssociations();
         return map.put(Pair.weak(k1, k2, queue), v);
     }
@@ -121,7 +121,7 @@ public V put(K1 k1, K2 k2, V v) {
      * there was no mapping for key pair
      * @throws NullPointerException if any of the specified keys or value is null
      */
-    public V putIfAbsent(K1 k1, K2 k2, V v) {
+    public @Nullable V putIfAbsent(K1 k1, K2 k2, V v) {
         expungeStaleAssociations();
         return map.putIfAbsent(Pair.weak(k1, k2, queue), v);
     }
@@ -151,6 +151,8 @@ public V putIfAbsent(K1 k1, K2 k2, V v) {
      * @throws RuntimeException      or Error if the mappingFunction does so, in
      *                               which case the mapping is left unestablished
      */
+    @SuppressWarnings({"nullness:return.type.incompatible","nullness:argument.type.incompatible"})
+    // AOSEN: Might be a issue related to type inference problem, see #EISOP1213
     public @PolyNull V computeIfAbsent(K1 k1, K2 k2,
                              BiFunction<? super K1, ? super K2, ? extends @PolyNull V>
                                  mappingFunction) {
@@ -197,14 +199,14 @@ private void expungeStaleAssociations() {
     /**
      * Common interface of both {@link Weak} and {@link Lookup} key pairs.
      */
-    private interface Pair<K1, K2> {
+    private interface Pair<K1 extends Object, K2 extends Object> {
 
-        static <K1, K2> Pair<K1, K2> weak(K1 k1, K2 k2,
+        static <K1 extends Object, K2 extends Object> Pair<K1, K2> weak(K1 k1, K2 k2,
                                           ReferenceQueue<Object> queue) {
             return new Weak<>(k1, k2, queue);
         }
 
-        static <K1, K2> Pair<K1, K2> lookup(K1 k1, K2 k2) {
+        static <K1 extends Object, K2 extends Object> Pair<K1, K2> lookup(K1 k1, K2 k2) {
             return new Lookup<>(k1, k2);
         }
 
@@ -212,13 +214,13 @@ static <K1, K2> Pair<K1, K2> lookup(K1 k1, K2 k2) {
          * @return The 1st of the pair of keys (may be null for {@link Weak}
          * when it gets cleared)
          */
-        K1 first();
+        @Nullable K1 first();
 
         /**
          * @return The 2nd of the pair of keys (may be null for {@link Weak}
          * when it gets cleared)
          */
-        K2 second();
+        @Nullable K2 second();
 
         static int hashCode(Object first, Object second) {
             // assert first != null && second != null;
@@ -226,7 +228,7 @@ static int hashCode(Object first, Object second) {
                    System.identityHashCode(second);
         }
 
-        static boolean equals(Object first, Object second, Pair<?, ?> p) {
+        static boolean equals(@Nullable Object first, @Nullable Object second, Pair<?, ?> p) {
             return first != null && second != null &&
                    first == p.first() && second == p.second();
         }
@@ -254,7 +256,7 @@ static boolean equals(Object first, Object second, Pair<?, ?> p) {
          * becomes weakly-reachable, the corresponding entries can be
          * {@link #expungeStaleAssociations() expunged} from the map.
          */
-        final class Weak<K1, K2> extends WeakRefPeer<K1> implements Pair<K1, K2> {
+        final class Weak<K1 extends Object, K2 extends Object> extends WeakRefPeer<K1> implements Pair<K1, K2> {
 
             // saved hash so it can be retrieved after the reference is cleared
             private final int hash;
@@ -277,12 +279,12 @@ final class Weak<K1, K2> extends WeakRefPeer<K1> implements Pair<K1, K2> {
             }
 
             @Override
-            public K1 first() {
+            public @Nullable K1 first() {
                 return get();
             }
 
             @Override
-            public K2 second() {
+            public @Nullable K2 second() {
                 return peer.get();
             }
 
@@ -292,7 +294,7 @@ public int hashCode() {
             }
 
             @Override
-            public boolean equals(Object obj) {
+            public boolean equals(@Nullable Object obj) {
                 return this == obj ||
                        (obj instanceof Pair &&
                         Pair.equals(first(), second(), (Pair<?, ?>) obj));
@@ -308,7 +310,7 @@ public boolean equals(Object obj) {
          * All its methods are purposely designed so that 'this' is never
          * passed to any other method or used as identity.
          */
-        final class Lookup<K1, K2> implements Pair<K1, K2> {
+        final class Lookup<K1 extends Object, K2 extends Object> implements Pair<K1, K2> {
             private final K1 k1;
             private final K2 k2;
 
@@ -333,7 +335,7 @@ public int hashCode() {
             }
 
             @Override
-            public boolean equals(Object obj) {
+            public boolean equals(@Nullable Object obj) {
                 return obj instanceof Pair &&
                        Pair.equals(k1, k2, (Pair<?, ?>) obj);
             }
@@ -343,7 +345,7 @@ public boolean equals(Object obj) {
     /**
      * Common abstract supertype of a pair of WeakReference peers.
      */
-    private static abstract class WeakRefPeer<K> extends WeakReference<K> {
+    private static abstract class WeakRefPeer<K extends Object> extends WeakReference<K> {
 
         WeakRefPeer(K k, ReferenceQueue<Object> queue) {
             super(Objects.requireNonNull(k), queue);
diff --git a/src/java.base/share/classes/java/lang/ref/Reference.java b/src/java.base/share/classes/java/lang/ref/Reference.java
index 6450b0ab5e8..eb3e626cf33 100644
--- a/src/java.base/share/classes/java/lang/ref/Reference.java
+++ b/src/java.base/share/classes/java/lang/ref/Reference.java
@@ -381,7 +381,7 @@ public final boolean refersTo(T obj) {
     /* Implementation of refersTo(), overridden for phantom references.
      */
     @IntrinsicCandidate
-    native boolean refersTo0(Object o);
+    native boolean refersTo0(@Nullable Object o);
 
     /**
      * Clears this reference object.  Invoking this method will not cause this
@@ -618,8 +618,8 @@ protected Object clone() throws CloneNotSupportedException {
      * @jls 12.6 Finalization of Class Instances
      */
     @ForceInline
-    @CFComment("nullness: Docs say the parameter can be null, but in practice, calls pass `this`")
-    public static void reachabilityFence(Object ref) {
+    @CFComment("nullness: Docs say the parameter can be null, but in practice, calls pass `this` and unbounded type parameters.")
+    public static void reachabilityFence(@Nullable Object ref) {
         // Does nothing. This method is annotated with @ForceInline to eliminate
         // most of the overhead that using @DontInline would cause with the
         // HotSpot JVM, when this fence is used in a wide variety of situations.
diff --git a/src/java.base/share/classes/java/lang/reflect/AnnotatedElement.java b/src/java.base/share/classes/java/lang/reflect/AnnotatedElement.java
index 27402f8b0fe..9e9c39b01ad 100644
--- a/src/java.base/share/classes/java/lang/reflect/AnnotatedElement.java
+++ b/src/java.base/share/classes/java/lang/reflect/AnnotatedElement.java
@@ -311,7 +311,7 @@ default boolean isAnnotationPresent(@GuardSatisfied AnnotatedElement this, Class
      * @throws NullPointerException if the given annotation class is null
      * @since 1.5
      */
-    <T extends Annotation> @Nullable T getAnnotation(Class<T> annotationClass);
+    <T extends @Nullable Annotation> @Nullable T getAnnotation(Class<T> annotationClass);
 
     /**
      * Returns annotations that are <em>present</em> on this element.
@@ -362,7 +362,7 @@ default boolean isAnnotationPresent(@GuardSatisfied AnnotatedElement this, Class
      * @throws NullPointerException if the given annotation class is null
      * @since 1.8
      */
-    default <T extends Annotation> T[] getAnnotationsByType(Class<T> annotationClass) {
+    default <T extends @Nullable Annotation> T[] getAnnotationsByType(Class<T> annotationClass) {
          /*
           * Definition of associated: directly or indirectly present OR
           * neither directly nor indirectly present AND the element is
@@ -406,7 +406,7 @@ default <T extends Annotation> T[] getAnnotationsByType(Class<T> annotationClass
      * @throws NullPointerException if the given annotation class is null
      * @since 1.8
      */
-    default <T extends Annotation> @Nullable T getDeclaredAnnotation(Class<T> annotationClass) {
+    default <T extends @Nullable Annotation> @Nullable T getDeclaredAnnotation(Class<T> annotationClass) {
          Objects.requireNonNull(annotationClass);
          // Loop over all directly-present annotations looking for a matching one
          for (Annotation annotation : getDeclaredAnnotations()) {
@@ -463,7 +463,7 @@ default <T extends Annotation> T[] getAnnotationsByType(Class<T> annotationClass
      * @throws NullPointerException if the given annotation class is null
      * @since 1.8
      */
-    default <T extends Annotation> T[] getDeclaredAnnotationsByType(Class<T> annotationClass) {
+    default <T extends @Nullable Annotation> T[] getDeclaredAnnotationsByType(Class<T> annotationClass) {
         Objects.requireNonNull(annotationClass);
         return AnnotationSupport.
             getDirectlyAndIndirectlyPresent(Arrays.stream(getDeclaredAnnotations()).
diff --git a/src/java.base/share/classes/java/lang/reflect/Method.java b/src/java.base/share/classes/java/lang/reflect/Method.java
index a1d7fbd653a..7f6d16f8621 100644
--- a/src/java.base/share/classes/java/lang/reflect/Method.java
+++ b/src/java.base/share/classes/java/lang/reflect/Method.java
@@ -573,7 +573,7 @@ void specificToGenericStringHeader(StringBuilder sb) {
     @CallerSensitive
     @ForceInline // to ensure Reflection.getCallerClass optimization
     @IntrinsicCandidate
-    public @Nullable Object invoke(Object obj, Object... args)
+    public @Nullable Object invoke(@Nullable Object obj, Object... args)
         throws IllegalAccessException, IllegalArgumentException,
            InvocationTargetException
     {
diff --git a/src/java.base/share/classes/java/security/AccessController.java b/src/java.base/share/classes/java/security/AccessController.java
index bd90c1eadca..cb7dc43bcf9 100644
--- a/src/java.base/share/classes/java/security/AccessController.java
+++ b/src/java.base/share/classes/java/security/AccessController.java
@@ -26,6 +26,7 @@
 package java.security;
 
 import org.checkerframework.checker.interning.qual.UsesObjectEquals;
+import org.checkerframework.checker.nullness.qual.Nullable;
 import org.checkerframework.framework.qual.AnnotatedFor;
 
 import java.lang.annotation.ElementType;
@@ -450,7 +451,7 @@ public static <T> T doPrivileged(PrivilegedAction<T> action,
      */
     @CallerSensitive
     public static <T> T doPrivileged(PrivilegedAction<T> action,
-            @SuppressWarnings("removal") AccessControlContext context,
+            @SuppressWarnings("removal") @Nullable AccessControlContext context,
             Permission... perms) {
 
         @SuppressWarnings("removal")
diff --git a/src/java.base/share/classes/java/security/BasicPermission.java b/src/java.base/share/classes/java/security/BasicPermission.java
index ff812ae6b32..85f7401f219 100644
--- a/src/java.base/share/classes/java/security/BasicPermission.java
+++ b/src/java.base/share/classes/java/security/BasicPermission.java
@@ -90,7 +90,7 @@ public abstract class BasicPermission extends Permission
     /**
      * initialize a BasicPermission object. Common to all constructors.
      */
-    private void init(String name) {
+    private void init(@Nullable String name) {
         if (name == null)
             throw new NullPointerException("name can't be null");
 
@@ -132,7 +132,7 @@ private void init(String name) {
      * @throws NullPointerException if {@code name} is {@code null}.
      * @throws IllegalArgumentException if {@code name} is empty.
      */
-    public BasicPermission(String name) {
+    public BasicPermission(@Nullable String name) {
         super(name);
         init(name);
     }
@@ -149,7 +149,7 @@ public BasicPermission(String name) {
      * @throws NullPointerException if {@code name} is {@code null}.
      * @throws IllegalArgumentException if {@code name} is empty.
      */
-    public BasicPermission(String name, String actions) {
+    public BasicPermission(@Nullable String name, @Nullable String actions) {
         super(name);
         init(name);
     }
diff --git a/src/java.base/share/classes/java/security/CodeSource.java b/src/java.base/share/classes/java/security/CodeSource.java
index 086ee098da6..e01dd21038b 100644
--- a/src/java.base/share/classes/java/security/CodeSource.java
+++ b/src/java.base/share/classes/java/security/CodeSource.java
@@ -101,7 +101,7 @@ public class CodeSource implements java.io.Serializable {
      * @param certs the certificate(s). It may be {@code null}. The contents
      * of the array are copied to protect against subsequent modification.
      */
-    public CodeSource(URL url, java.security.cert.Certificate[] certs) {
+    public CodeSource(@Nullable URL url, java.security.cert.Certificate @Nullable [] certs) {
         this.location = url;
         if (url != null) {
             this.locationNoFragString = URLUtil.urlNoFragString(url);
diff --git a/src/java.base/share/classes/java/security/Permission.java b/src/java.base/share/classes/java/security/Permission.java
index baf6780a043..c9e1e0842da 100644
--- a/src/java.base/share/classes/java/security/Permission.java
+++ b/src/java.base/share/classes/java/security/Permission.java
@@ -25,6 +25,8 @@
 
 package java.security;
 
+import org.checkerframework.checker.nullness.qual.Nullable;
+
 /**
  * Abstract class for representing access to a system resource.
  * All permissions have a name (whose interpretation depends on the subclass),
@@ -80,7 +82,7 @@ public abstract class Permission implements Guard, java.io.Serializable {
      *
      */
 
-    public Permission(String name) {
+    public Permission(@Nullable String name) {
         this.name = name;
     }
 
diff --git a/src/java.base/share/classes/java/security/ProtectionDomain.java b/src/java.base/share/classes/java/security/ProtectionDomain.java
index f1fcad95c75..4423452916e 100644
--- a/src/java.base/share/classes/java/security/ProtectionDomain.java
+++ b/src/java.base/share/classes/java/security/ProtectionDomain.java
@@ -226,7 +226,7 @@ public ProtectionDomain(@Nullable CodeSource codesource,
     public ProtectionDomain(@Nullable CodeSource codesource,
                             @Nullable PermissionCollection permissions,
                             @Nullable ClassLoader classloader,
-                            Principal[] principals) {
+                            Principal @Nullable [] principals) {
         this.codesource = codesource;
         if (permissions != null) {
             this.permissions = permissions;
diff --git a/src/java.base/share/classes/java/util/HashSet.java.save b/src/java.base/share/classes/java/util/HashSet.java.save
new file mode 100644
index 00000000000..1ae965ccba5
--- /dev/null
+++ b/src/java.base/share/classes/java/util/HashSet.java.save
@@ -0,0 +1,410 @@
+/*
+ * Copyright (c) 1997, 2019, Oracle and/or its affiliates. All rights reserved.
+ * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
+ *
+ * This code is free software; you can redistribute it and/or modify it
+ * under the terms of the GNU General Public License version 2 only, as
+ * published by the Free Software Foundation.  Oracle designates this
+ * particular file as subject to the "Classpath" exception as provided
+ * by Oracle in the LICENSE file that accompanied this code.
+ *
+ * This code is distributed in the hope that it will be useful, but WITHOUT
+ * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+ * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
+ * version 2 for more details (a copy is included in the LICENSE file that
+ * accompanied this code).
+ *
+ * You should have received a copy of the GNU General Public License version
+ * 2 along with this work; if not, write to the Free Software Foundation,
+ * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
+ *
+ * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
+ * or visit www.oracle.com if you need additional information or have any
+ * questions.
+ */
+
+package java.util;
+
+import org.checkerframework.checker.index.qual.NonNegative;
+import org.checkerframework.checker.lock.qual.GuardSatisfied;
+import org.checkerframework.checker.nonempty.qual.EnsuresNonEmpty;
+import org.checkerframework.checker.nonempty.qual.EnsuresNonEmptyIf;
+import org.checkerframework.checker.nonempty.qual.NonEmpty;
+import org.checkerframework.checker.nonempty.qual.PolyNonEmpty;
+import org.checkerframework.checker.nullness.qual.Nullable;
+import org.checkerframework.checker.nullness.qual.PolyNull;
+import org.checkerframework.checker.pico.qual.Immutable;
+import org.checkerframework.checker.pico.qual.Mutable;
+import org.checkerframework.checker.pico.qual.PolyMutable;
+import org.checkerframework.checker.pico.qual.ReceiverDependentMutable;
+import org.checkerframework.checker.pico.qual.Readonly;
+import org.checkerframework.checker.signedness.qual.UnknownSignedness;
+import org.checkerframework.dataflow.qual.Pure;
+import org.checkerframework.dataflow.qual.SideEffectFree;
+import org.checkerframework.dataflow.qual.SideEffectsOnly;
+import org.checkerframework.framework.qual.AnnotatedFor;
+import org.checkerframework.framework.qual.CFComment;
+import org.checkerframework.framework.qual.DefaultQualifierForUse;
+
+import java.io.InvalidObjectException;
+import jdk.internal.access.SharedSecrets;
+
+/**
+ * This class implements the {@code Set} interface, backed by a hash table
+ * (actually a {@code HashMap} instance).  It makes no guarantees as to the
+ * iteration order of the set; in particular, it does not guarantee that the
+ * order will remain constant over time.  This class permits the {@code null}
+ * element.
+ *
+ * <p>This class offers constant time performance for the basic operations
+ * ({@code add}, {@code remove}, {@code contains} and {@code size}),
+ * assuming the hash function disperses the elements properly among the
+ * buckets.  Iterating over this set requires time proportional to the sum of
+ * the {@code HashSet} instance's size (the number of elements) plus the
+ * "capacity" of the backing {@code HashMap} instance (the number of
+ * buckets).  Thus, it's very important not to set the initial capacity too
+ * high (or the load factor too low) if iteration performance is important.
+ *
+ * <p><strong>Note that this implementation is not synchronized.</strong>
+ * If multiple threads access a hash set concurrently, and at least one of
+ * the threads modifies the set, it <i>must</i> be synchronized externally.
+ * This is typically accomplished by synchronizing on some object that
+ * naturally encapsulates the set.
+ *
+ * If no such object exists, the set should be "wrapped" using the
+ * {@link Collections#synchronizedSet Collections.synchronizedSet}
+ * method.  This is best done at creation time, to prevent accidental
+ * unsynchronized access to the set:<pre>
+ *   Set s = Collections.synchronizedSet(new HashSet(...));</pre>
+ *
+ * <p>The iterators returned by this class's {@code iterator} method are
+ * <i>fail-fast</i>: if the set is modified at any time after the iterator is
+ * created, in any way except through the iterator's own {@code remove}
+ * method, the Iterator throws a {@link ConcurrentModificationException}.
+ * Thus, in the face of concurrent modification, the iterator fails quickly
+ * and cleanly, rather than risking arbitrary, non-deterministic behavior at
+ * an undetermined time in the future.
+ *
+ * <p>Note that the fail-fast behavior of an iterator cannot be guaranteed
+ * as it is, generally speaking, impossible to make any hard guarantees in the
+ * presence of unsynchronized concurrent modification.  Fail-fast iterators
+ * throw {@code ConcurrentModificationException} on a best-effort basis.
+ * Therefore, it would be wrong to write a program that depended on this
+ * exception for its correctness: <i>the fail-fast behavior of iterators
+ * should be used only to detect bugs.</i>
+ *
+ * <p>This class is a member of the
+ * <a href="{@docRoot}/java.base/java/util/package-summary.html#CollectionsFramework">
+ * Java Collections Framework</a>.
+ *
+ * @param <E> the type of elements maintained by this set
+ *
+ * @author  Josh Bloch
+ * @author  Neal Gafter
+ * @see     Collection
+ * @see     Set
+ * @see     TreeSet
+ * @see     HashMap
+ * @since   1.2
+ */
+
+ @AnnotatedFor({"index", "initialization", "lock", "nullness"})
+ @SuppressWarnings("pico:assignment.type.incompatible") // Revisit this, looks like CF bug
+ @ReceiverDependentMutable public class HashSet<E>
+    extends AbstractSet<E>
+    implements Set<E>, Cloneable, java.io.Serializable
+{
+    @java.io.Serial
+    static final long serialVersionUID = -5024744406713321676L;
+
+    private transient HashMap<@Immutable E, @Readonly Object> map;
+
+    // Dummy value to associate with an Object in the backing Map
+    private static final Object PRESENT = new Object();
+
+    /**
+     * Constructs a new, empty set; the backing {@code HashMap} instance has
+     * default initial capacity (16) and load factor (0.75).
+     */
+    public HashSet() {
+        map = new @ReceiverDependentMutable HashMap<@Immutable E, @Readonly Object>();
+    }
+
+    /**
+     * Constructs a new set containing the elements in the specified
+     * collection.  The {@code HashMap} is created with default load factor
+     * (0.75) and an initial capacity sufficient to contain the elements in
+     * the specified collection.
+     *
+     * @param c the collection whose elements are to be placed into this set
+     * @throws NullPointerException if the specified collection is null
+     */
+    @SuppressWarnings("pico") // PICO constructor fix
+    public @PolyNonEmpty HashSet(@PolyNonEmpty @Readonly Collection<? extends E> c) {
+        map = new @ReceiverDependentMutable HashMap<@Immutable E, @Readonly Object>(Math.max((int) (c.size()/.75f) + 1, 16));
+        addAll(c);
+    }
+
+    /**
+     * Constructs a new, empty set; the backing {@code HashMap} instance has
+     * the specified initial capacity and the specified load factor.
+     *
+     * @param      initialCapacity   the initial capacity of the hash map
+     * @param      loadFactor        the load factor of the hash map
+     * @throws     IllegalArgumentException if the initial capacity is less
+     *             than zero, or if the load factor is nonpositive
+     */
+    public HashSet(@NonNegative int initialCapacity, float loadFactor) {
+        map = new @ReceiverDependentMutable HashMap<@Immutable E, @Readonly Object>(initialCapacity, loadFactor);
+    }
+
+    /**
+     * Constructs a new, empty set; the backing {@code HashMap} instance has
+     * the specified initial capacity and default load factor (0.75).
+     *
+     * @param      initialCapacity   the initial capacity of the hash table
+     * @throws     IllegalArgumentException if the initial capacity is less
+     *             than zero
+     */
+    public HashSet(@NonNegative int initialCapacity) {
+        map = new @ReceiverDependentMutable HashMap<@Immutable E, @Readonly Object>(initialCapacity);
+    }
+
+    /**
+     * Constructs a new, empty linked hash set.  (This package private
+     * constructor is only used by LinkedHashSet.) The backing
+     * HashMap instance is a LinkedHashMap with the specified initial
+     * capacity and the specified load factor.
+     *
+     * @param      initialCapacity   the initial capacity of the hash map
+     * @param      loadFactor        the load factor of the hash map
+     * @param      dummy             ignored (distinguishes this
+     *             constructor from other int, float constructor.)
+     * @throws     IllegalArgumentException if the initial capacity is less
+     *             than zero, or if the load factor is nonpositive
+     */
+    HashSet(int initialCapacity, float loadFactor, boolean dummy) {
+        map = new @ReceiverDependentMutable LinkedHashMap<@Immutable E, @Readonly Object>(initialCapacity, loadFactor);
+    }
+
+    /**
+     * Returns an iterator over the elements in this set.  The elements
+     * are returned in no particular order.
+     *
+     * @return an Iterator over the elements in this set
+     * @see ConcurrentModificationException
+     */
+    @SideEffectFree
+    @SuppressWarnings("pico") // lost can not invoke poly method.
+    public @PolyNonEmpty Iterator<E> iterator(@PolyNonEmpty @Readonly HashSet<E> this) {
+        return map.keySet().iterator();
+    }
+
+    /**
+     * Returns the number of elements in this set (its cardinality).
+     *
+     * @return the number of elements in this set (its cardinality)
+     */
+    @Pure
+    public @NonNegative int size(@GuardSatisfied @Readonly HashSet<E> this) {
+        return map.size();
+    }
+
+    /**
+     * Returns {@code true} if this set contains no elements.
+     *
+     * @return {@code true} if this set contains no elements
+     */
+    @Pure
+    @EnsuresNonEmptyIf(result = false, expression = "this")
+    public boolean isEmpty(@Readonly @GuardSatisfied HashSet<E> this) {
+        return map.isEmpty();
+    }
+
+    /**
+     * Returns {@code true} if this set contains the specified element.
+     * More formally, returns {@code true} if and only if this set
+     * contains an element {@code e} such that
+     * {@code Objects.equals(o, e)}.
+     *
+     * @param o element whose presence in this set is to be tested
+     * @return {@code true} if this set contains the specified element
+     */
+    @Pure
+    @EnsuresNonEmptyIf(result = true, expression = "this")
+    public boolean contains(@Readonly @GuardSatisfied HashSet<E> this, @GuardSatisfied @Nullable @UnknownSignedness @Readonly Object o) {
+        return map.containsKey(o);
+    }
+
+    /**
+     * Adds the specified element to this set if it is not already present.
+     * More formally, adds the specified element {@code e} to this set if
+     * this set contains no element {@code e2} such that
+     * {@code Objects.equals(e, e2)}.
+     * If this set already contains the element, the call leaves the set
+     * unchanged and returns {@code false}.
+     *
+     * @param e element to be added to this set
+     * @return {@code true} if this set did not already contain the specified
+     * element
+     */
+    @SideEffectsOnly("this")
+    @EnsuresNonEmpty("this")
+    public boolean add(@Mutable @GuardSatisfied HashSet<E> this, E e) {
+        return map.put(e, PRESENT)==null;
+    }
+
+    /**
+     * Removes the specified element from this set if it is present.
+     * More formally, removes an element {@code e} such that
+     * {@code Objects.equals(o, e)},
+     * if this set contains such an element.  Returns {@code true} if
+     * this set contained the element (or equivalently, if this set
+     * changed as a result of the call).  (This set will not contain the
+     * element once the call returns.)
+     *
+     * @param o object to be removed from this set, if present
+     * @return {@code true} if the set contained the specified element
+     */
+    @SideEffectsOnly("this")
+    public boolean remove(@Mutable @GuardSatisfied HashSet<E> this, @GuardSatisfied @Nullable @UnknownSignedness @Readonly Object o) {
+        return map.remove(o)==PRESENT;
+    }
+
+    /**
+     * Removes all of the elements from this set.
+     * The set will be empty after this call returns.
+     */
+    @SideEffectsOnly("this")
+    public void clear(@Mutable @GuardSatisfied HashSet<E> this) {
+        map.clear();
+    }
+
+    /**
+     * Returns a shallow copy of this {@code HashSet} instance: the elements
+     * themselves are not cloned.
+     *
+     * @return a shallow copy of this set
+     */
+    @SideEffectFree
+    @SuppressWarnings("unchecked")
+    public Object clone(@GuardSatisfied @Mutable HashSet<E> this) {
+        try {
+            HashSet<E> newSet = (HashSet<E>) super.clone();
+            newSet.map = (HashMap<E, @Readonly Object>) map.clone();
+            return newSet;
+        } catch (CloneNotSupportedException e) {
+            throw new InternalError(e);
+        }
+    }
+
+    /**
+     * Save the state of this {@code HashSet} instance to a stream (that is,
+     * serialize it).
+     *
+     * @serialData The capacity of the backing {@code HashMap} instance
+     *             (int), and its load factor (float) are emitted, followed by
+     *             the size of the set (the number of elements it contains)
+     *             (int), followed by all of its elements (each an Object) in
+     *             no particular order.
+     */
+    @java.io.Serial
+    private void writeObject(@Mutable HashSet<E> this, java.io.ObjectOutputStream s)
+        throws java.io.IOException {
+        // Write out any hidden serialization magic
+        s.defaultWriteObject();
+
+        // Write out HashMap capacity and load factor
+        s.writeInt(map.capacity());
+        s.writeFloat(map.loadFactor());
+
+        // Write out size
+        s.writeInt(map.size());
+
+        // Write out all elements in the proper order.
+        for (E e : map.keySet())
+            s.writeObject(e);
+    }
+
+    /**
+     * Reconstitute the {@code HashSet} instance from a stream (that is,
+     * deserialize it).
+     */
+    @java.io.Serial
+    private void readObject(@Mutable HashSet<E> this, java.io.ObjectInputStream s)
+        throws java.io.IOException, ClassNotFoundException {
+        // Read in any hidden serialization magic
+        s.defaultReadObject();
+
+        // Read capacity and verify non-negative.
+        int capacity = s.readInt();
+        if (capacity < 0) {
+            throw new InvalidObjectException("Illegal capacity: " +
+                                             capacity);
+        }
+
+        // Read load factor and verify positive and non NaN.
+        float loadFactor = s.readFloat();
+        if (loadFactor <= 0 || Float.isNaN(loadFactor)) {
+            throw new InvalidObjectException("Illegal load factor: " +
+                                             loadFactor);
+        }
+
+        // Read size and verify non-negative.
+        int size = s.readInt();
+        if (size < 0) {
+            throw new InvalidObjectException("Illegal size: " +
+                                             size);
+        }
+
+        // Set the capacity according to the size and load factor ensuring that
+        // the HashMap is at least 25% full but clamping to maximum capacity.
+        capacity = (int) Math.min(size * Math.min(1 / loadFactor, 4.0f),
+                HashMap.MAXIMUM_CAPACITY);
+
+        // Constructing the backing map will lazily create an array when the first element is
+        // added, so check it before construction. Call HashMap.tableSizeFor to compute the
+        // actual allocation size. Check Map.Entry[].class since it's the nearest public type to
+        // what is actually created.
+        SharedSecrets.getJavaObjectInputStreamAccess()
+                     .checkArray(s, Map.Entry[].class, HashMap.tableSizeFor(capacity));
+
+        // Create backing HashMap
+        map = (((HashSet<?>)this) instanceof LinkedHashSet ?
+               new LinkedHashMap<@Immutable E, @Readonly Object>(capacity, loadFactor) :
+               new HashMap<@Immutable E, @Readonly Object>(capacity, loadFactor));
+
+        // Read in all elements in the proper order.
+        for (int i=0; i<size; i++) {
+            @SuppressWarnings("unchecked")
+                E e = (E) s.readObject();
+            map.put(e, PRESENT);
+        }
+    }
+
+    /**
+     * Creates a <em><a href="Spliterator.html#binding">late-binding</a></em>
+     * and <em>fail-fast</em> {@link Spliterator} over the elements in this
+     * set.
+     *
+     * <p>The {@code Spliterator} reports {@link Spliterator#SIZED} and
+     * {@link Spliterator#DISTINCT}.  Overriding implementations should document
+     * the reporting of additional characteristic values.
+     *
+     * @return a {@code Spliterator} over the elements in this set
+     * @since 1.8
+     */
+    public Spliterator<E> spliterator(@Readonly HashSet<E> this) {
+        return new HashMap.KeySpliterator<>(map, 0, -1, 0, 0);
+    }
+
+    @Override
+    public @PolyMutable Object[] toArray(@Readonly HashSet<@PolyMutable E> this) {
+        return map.keysToArray(new @PolyMutable Object[map.size()]);
+    }
+
+    @Override
+    public <T> @Nullable T[] toArray(@Readonly HashSet<E> this, @PolyNull T[] a) {
+        return map.keysToArray(map.prepareArray(a));
+    }
+}
diff --git a/src/java.base/share/classes/java/util/Map.java b/src/java.base/share/classes/java/util/Map.java
index e6a6ac940fa..5b982c6b358 100644
--- a/src/java.base/share/classes/java/util/Map.java
+++ b/src/java.base/share/classes/java/util/Map.java
@@ -275,6 +275,7 @@ public interface Map<K, V> {
      * (<a href="{@docRoot}/java.base/java/util/Collection.html#optional-restrictions">optional</a>)
      */
     @Pure
+    @CFComment("nullness: key is not @Nullable because this map might not permit null values")
     @Nullable V get(@GuardSatisfied Map<K, V> this, @UnknownSignedness @GuardSatisfied Object key);
 
     // Modification Operations
diff --git a/src/java.base/share/classes/java/util/Optional.java b/src/java.base/share/classes/java/util/Optional.java
index eb5fcd0d8d5..64fda74a8f0 100644
--- a/src/java.base/share/classes/java/util/Optional.java
+++ b/src/java.base/share/classes/java/util/Optional.java
@@ -399,6 +399,7 @@ public Optional<T> or(Supplier<? extends Optional<? extends T>> supplier) {
      */
     @OptionalEliminator
     @Pure
+    //Aosen: what's the difference if we don't use @PolyNull at all here?
     public @PolyNull T orElse(@PolyNull T other) {
         return value != null ? value : other;
     }
diff --git a/src/java.base/share/classes/java/util/TreeSet.java.save b/src/java.base/share/classes/java/util/TreeSet.java.save
new file mode 100644
index 00000000000..2ac7c888479
--- /dev/null
+++ b/src/java.base/share/classes/java/util/TreeSet.java.save
@@ -0,0 +1,604 @@
+/*
+ * Copyright (c) 1998, 2019, Oracle and/or its affiliates. All rights reserved.
+ * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
+ *
+ * This code is free software; you can redistribute it and/or modify it
+ * under the terms of the GNU General Public License version 2 only, as
+ * published by the Free Software Foundation.  Oracle designates this
+ * particular file as subject to the "Classpath" exception as provided
+ * by Oracle in the LICENSE file that accompanied this code.
+ *
+ * This code is distributed in the hope that it will be useful, but WITHOUT
+ * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+ * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
+ * version 2 for more details (a copy is included in the LICENSE file that
+ * accompanied this code).
+ *
+ * You should have received a copy of the GNU General Public License version
+ * 2 along with this work; if not, write to the Free Software Foundation,
+ * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
+ *
+ * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
+ * or visit www.oracle.com if you need additional information or have any
+ * questions.
+ */
+
+package java.util;
+
+import org.checkerframework.checker.index.qual.NonNegative;
+import org.checkerframework.checker.lock.qual.GuardSatisfied;
+import org.checkerframework.checker.nonempty.qual.EnsuresNonEmpty;
+import org.checkerframework.checker.nonempty.qual.EnsuresNonEmptyIf;
+import org.checkerframework.checker.nonempty.qual.NonEmpty;
+import org.checkerframework.checker.nonempty.qual.PolyNonEmpty;
+import org.checkerframework.checker.nullness.qual.EnsuresNonNullIf;
+import org.checkerframework.checker.nullness.qual.Nullable;
+import org.checkerframework.checker.pico.qual.Immutable;
+import org.checkerframework.checker.pico.qual.Mutable;
+import org.checkerframework.checker.pico.qual.PolyMutable;
+import org.checkerframework.checker.pico.qual.Readonly;
+import org.checkerframework.checker.pico.qual.ReceiverDependentMutable;
+import org.checkerframework.checker.signedness.qual.UnknownSignedness;
+import org.checkerframework.dataflow.qual.Pure;
+import org.checkerframework.dataflow.qual.SideEffectFree;
+import org.checkerframework.framework.qual.AnnotatedFor;
+import org.checkerframework.framework.qual.CFComment;
+
+/**
+ * A {@link NavigableSet} implementation based on a {@link TreeMap}.
+ * The elements are ordered using their {@linkplain Comparable natural
+ * ordering}, or by a {@link Comparator} provided at set creation
+ * time, depending on which constructor is used.
+ *
+ * <p>This implementation provides guaranteed log(n) time cost for the basic
+ * operations ({@code add}, {@code remove} and {@code contains}).
+ *
+ * <p>Note that the ordering maintained by a set (whether or not an explicit
+ * comparator is provided) must be <i>consistent with equals</i> if it is to
+ * correctly implement the {@code Set} interface.  (See {@code Comparable}
+ * or {@code Comparator} for a precise definition of <i>consistent with
+ * equals</i>.)  This is so because the {@code Set} interface is defined in
+ * terms of the {@code equals} operation, but a {@code TreeSet} instance
+ * performs all element comparisons using its {@code compareTo} (or
+ * {@code compare}) method, so two elements that are deemed equal by this method
+ * are, from the standpoint of the set, equal.  The behavior of a set
+ * <i>is</i> well-defined even if its ordering is inconsistent with equals; it
+ * just fails to obey the general contract of the {@code Set} interface.
+ *
+ * <p><strong>Note that this implementation is not synchronized.</strong>
+ * If multiple threads access a tree set concurrently, and at least one
+ * of the threads modifies the set, it <i>must</i> be synchronized
+ * externally.  This is typically accomplished by synchronizing on some
+ * object that naturally encapsulates the set.
+ * If no such object exists, the set should be "wrapped" using the
+ * {@link Collections#synchronizedSortedSet Collections.synchronizedSortedSet}
+ * method.  This is best done at creation time, to prevent accidental
+ * unsynchronized access to the set: <pre>
+ *   SortedSet s = Collections.synchronizedSortedSet(new TreeSet(...));</pre>
+ *
+ * <p>The iterators returned by this class's {@code iterator} method are
+ * <i>fail-fast</i>: if the set is modified at any time after the iterator is
+ * created, in any way except through the iterator's own {@code remove}
+ * method, the iterator will throw a {@link ConcurrentModificationException}.
+ * Thus, in the face of concurrent modification, the iterator fails quickly
+ * and cleanly, rather than risking arbitrary, non-deterministic behavior at
+ * an undetermined time in the future.
+ *
+ * <p>Note that the fail-fast behavior of an iterator cannot be guaranteed
+ * as it is, generally speaking, impossible to make any hard guarantees in the
+ * presence of unsynchronized concurrent modification.  Fail-fast iterators
+ * throw {@code ConcurrentModificationException} on a best-effort basis.
+ * Therefore, it would be wrong to write a program that depended on this
+ * exception for its correctness:   <i>the fail-fast behavior of iterators
+ * should be used only to detect bugs.</i>
+ *
+ * <p>This class is a member of the
+ * <a href="{@docRoot}/java.base/java/util/package-summary.html#CollectionsFramework">
+ * Java Collections Framework</a>.
+ *
+ * @param <E> the type of elements maintained by this set
+ *
+ * @author  Josh Bloch
+ * @see     Collection
+ * @see     Set
+ * @see     HashSet
+ * @see     Comparable
+ * @see     Comparator
+ * @see     TreeMap
+ * @since   1.2
+ */
+
+@CFComment({"lock/nullness: Subclasses of this interface/class may opt to prohibit null elements"})
+@AnnotatedFor({"lock", "nullness"})
+@SuppressWarnings("pico:assignment.type.incompatible") // Revisit this, looks like CF bug
+@ReceiverDependentMutable public class TreeSet<E> extends AbstractSet<E>
+    implements NavigableSet<E>, Cloneable, java.io.Serializable
+{
+    /**
+     * The backing map.
+     */
+    private transient NavigableMap<@Immutable E, @Readonly Object> m;
+
+    // Dummy value to associate with an Object in the backing Map
+    private static final Object PRESENT = new Object();
+
+    /**
+     * Constructs a set backed by the specified navigable map.
+     */
+    TreeSet(@ReceiverDependentMutable NavigableMap<@Immutable E, @Readonly Object> m) {
+        this.m = m;
+    }
+
+    /**
+     * Constructs a new, empty tree set, sorted according to the
+     * natural ordering of its elements.  All elements inserted into
+     * the set must implement the {@link Comparable} interface.
+     * Furthermore, all such elements must be <i>mutually
+     * comparable</i>: {@code e1.compareTo(e2)} must not throw a
+     * {@code ClassCastException} for any elements {@code e1} and
+     * {@code e2} in the set.  If the user attempts to add an element
+     * to the set that violates this constraint (for example, the user
+     * attempts to add a string element to a set whose elements are
+     * integers), the {@code add} call will throw a
+     * {@code ClassCastException}.
+     */
+    public TreeSet() {
+        this(new @ReceiverDependentMutable TreeMap<@Immutable E, @Readonly Object>());
+    }
+
+    /**
+     * Constructs a new, empty tree set, sorted according to the specified
+     * comparator.  All elements inserted into the set must be <i>mutually
+     * comparable</i> by the specified comparator: {@code comparator.compare(e1,
+     * e2)} must not throw a {@code ClassCastException} for any elements
+     * {@code e1} and {@code e2} in the set.  If the user attempts to add
+     * an element to the set that violates this constraint, the
+     * {@code add} call will throw a {@code ClassCastException}.
+     *
+     * @param comparator the comparator that will be used to order this set.
+     *        If {@code null}, the {@linkplain Comparable natural
+     *        ordering} of the elements will be used.
+     */
+    public TreeSet(@Nullable Comparator<? super E> comparator) {
+        this(new @ReceiverDependentMutable TreeMap<@Immutable E, @Readonly Object>(comparator));
+    }
+
+    /**
+     * Constructs a new tree set containing the elements in the specified
+     * collection, sorted according to the <i>natural ordering</i> of its
+     * elements.  All elements inserted into the set must implement the
+     * {@link Comparable} interface.  Furthermore, all such elements must be
+     * <i>mutually comparable</i>: {@code e1.compareTo(e2)} must not throw a
+     * {@code ClassCastException} for any elements {@code e1} and
+     * {@code e2} in the set.
+     *
+     * @param c collection whose elements will comprise the new set
+     * @throws ClassCastException if the elements in {@code c} are
+     *         not {@link Comparable}, or are not mutually comparable
+     * @throws NullPointerException if the specified collection is null
+     */
+    @SuppressWarnings("pico") // PICO constructor fix
+    public @PolyNonEmpty TreeSet(@PolyNonEmpty Collection<? extends E> c) {
+        this();
+        addAll(c);
+    }
+
+    /**
+     * Constructs a new tree set containing the same elements and
+     * using the same ordering as the specified sorted set.
+     *
+     * @param s sorted set whose elements will comprise the new set
+     * @throws NullPointerException if the specified sorted set is null
+     */
+    @SuppressWarnings("pico") // PICO constructor fix
+    public @PolyNonEmpty TreeSet(@PolyNonEmpty SortedSet<E> s) {
+        this(s.comparator());
+        addAll(s);
+    }
+
+    /**
+     * Returns an iterator over the elements in this set in ascending order.
+     *
+     * @return an iterator over the elements in this set in ascending order
+     */
+    @SideEffectFree
+    @SuppressWarnings("pico") // lost can not invoke poly method.
+    public @PolyNonEmpty Iterator<E> iterator(@PolyNonEmpty @Readonly TreeSet<E> this) {
+        return m.navigableKeySet().iterator();
+    }
+
+    /**
+     * Returns an iterator over the elements in this set in descending order.
+     *
+     * @return an iterator over the elements in this set in descending order
+     * @since 1.6
+     */
+    @SuppressWarnings("pico") // lost can not invoke poly method.
+    public @PolyNonEmpty Iterator<E> descendingIterator(@PolyNonEmpty @Readonly TreeSet<E> this) {
+        return m.descendingKeySet().iterator();
+    }
+
+    /**
+     * @since 1.6
+     */
+    public @PolyMutable NavigableSet<E> descendingSet(@PolyMutable TreeSet<E> this) {
+        return new @PolyMutable TreeSet<>(m.descendingMap());
+    }
+
+    /**
+     * Returns the number of elements in this set (its cardinality).
+     *
+     * @return the number of elements in this set (its cardinality)
+     */
+    @Pure
+    public @NonNegative int size(@GuardSatisfied @Readonly TreeSet<E> this) {
+        return m.size();
+    }
+
+    /**
+     * Returns {@code true} if this set contains no elements.
+     *
+     * @return {@code true} if this set contains no elements
+     */
+    @EnsuresNonNullIf(expression={"pollFirst()", "pollLast()"}, result=false)
+    @Pure
+    @EnsuresNonEmptyIf(result = false, expression = "this")
+    public boolean isEmpty(@GuardSatisfied @Readonly TreeSet<E> this) {
+        return m.isEmpty();
+    }
+
+    /**
+     * Returns {@code true} if this set contains the specified element.
+     * More formally, returns {@code true} if and only if this set
+     * contains an element {@code e} such that
+     * {@code Objects.equals(o, e)}.
+     *
+     * @param o object to be checked for containment in this set
+     * @return {@code true} if this set contains the specified element
+     * @throws ClassCastException if the specified object cannot be compared
+     *         with the elements currently in the set
+     * @throws NullPointerException if the specified element is null
+     *         and this set uses natural ordering, or its comparator
+     *         does not permit null elements
+     */
+    @Pure
+    @EnsuresNonEmptyIf(result = true, expression = "this")
+    public boolean contains(@GuardSatisfied @Readonly TreeSet<E> this, @GuardSatisfied @UnknownSignedness @Readonly Object o) {
+        return m.containsKey(o);
+    }
+
+    /**
+     * Adds the specified element to this set if it is not already present.
+     * More formally, adds the specified element {@code e} to this set if
+     * the set contains no element {@code e2} such that
+     * {@code Objects.equals(e, e2)}.
+     * If this set already contains the element, the call leaves the set
+     * unchanged and returns {@code false}.
+     *
+     * @param e element to be added to this set
+     * @return {@code true} if this set did not already contain the specified
+     *         element
+     * @throws ClassCastException if the specified object cannot be compared
+     *         with the elements currently in this set
+     * @throws NullPointerException if the specified element is null
+     *         and this set uses natural ordering, or its comparator
+     *         does not permit null elements
+     */
+    @EnsuresNonEmpty("this")
+    public boolean add(@GuardSatisfied @Mutable TreeSet<E> this, E e) {
+        return m.put(e, PRESENT)==null;
+    }
+
+    /**
+     * Removes the specified element from this set if it is present.
+     * More formally, removes an element {@code e} such that
+     * {@code Objects.equals(o, e)},
+     * if this set contains such an element.  Returns {@code true} if
+     * this set contained the element (or equivalently, if this set
+     * changed as a result of the call).  (This set will not contain the
+     * element once the call returns.)
+     *
+     * @param o object to be removed from this set, if present
+     * @return {@code true} if this set contained the specified element
+     * @throws ClassCastException if the specified object cannot be compared
+     *         with the elements currently in this set
+     * @throws NullPointerException if the specified element is null
+     *         and this set uses natural ordering, or its comparator
+     *         does not permit null elements
+     */
+    public boolean remove(@GuardSatisfied @Mutable TreeSet<E> this, @GuardSatisfied @UnknownSignedness @Readonly Object o) {
+        return m.remove(o)==PRESENT;
+    }
+
+    /**
+     * Removes all of the elements from this set.
+     * The set will be empty after this call returns.
+     */
+    public void clear(@GuardSatisfied @Mutable TreeSet<E> this) {
+        m.clear();
+    }
+
+    /**
+     * Adds all of the elements in the specified collection to this set.
+     *
+     * @param c collection containing elements to be added to this set
+     * @return {@code true} if this set changed as a result of the call
+     * @throws ClassCastException if the elements provided cannot be compared
+     *         with the elements currently in the set
+     * @throws NullPointerException if the specified collection is null or
+     *         if any element is null and this set uses natural ordering, or
+     *         its comparator does not permit null elements
+     */
+    public  boolean addAll(@GuardSatisfied @Mutable TreeSet<E> this, @Readonly Collection<? extends E> c) {
+        // Use linear-time version if applicable
+        if (m.size()==0 && c.size() > 0 &&
+            c instanceof SortedSet &&
+                m instanceof TreeMap<@Immutable E, Object> map) {
+            SortedSet<? extends @Immutable E> set = (SortedSet<? extends @Immutable E>) c;
+            if (Objects.equals(set.comparator(), map.comparator())) {
+                map.addAllForTreeSet(set, PRESENT);
+                return true;
+            }
+        }
+        return super.addAll(c);
+    }
+
+    /**
+     * @throws ClassCastException {@inheritDoc}
+     * @throws NullPointerException if {@code fromElement} or {@code toElement}
+     *         is null and this set uses natural ordering, or its comparator
+     *         does not permit null elements
+     * @throws IllegalArgumentException {@inheritDoc}
+     * @since 1.6
+     */
+    @SideEffectFree
+    public @PolyMutable NavigableSet<E> subSet(@GuardSatisfied @PolyMutable TreeSet<E> this, @GuardSatisfied E fromElement, boolean fromInclusive,
+                                  @GuardSatisfied E toElement,   boolean toInclusive) {
+        return new @PolyMutable TreeSet<>(m.subMap(fromElement, fromInclusive,
+                                       toElement,   toInclusive));
+    }
+
+    /**
+     * @throws ClassCastException {@inheritDoc}
+     * @throws NullPointerException if {@code toElement} is null and
+     *         this set uses natural ordering, or its comparator does
+     *         not permit null elements
+     * @throws IllegalArgumentException {@inheritDoc}
+     * @since 1.6
+     */
+    @SideEffectFree
+    public @PolyMutable NavigableSet<E> headSet(@GuardSatisfied @PolyMutable TreeSet<E> this, @GuardSatisfied E toElement, boolean inclusive) {
+        return new @PolyMutable TreeSet<>(m.headMap(toElement, inclusive));
+    }
+
+    /**
+     * @throws ClassCastException {@inheritDoc}
+     * @throws NullPointerException if {@code fromElement} is null and
+     *         this set uses natural ordering, or its comparator does
+     *         not permit null elements
+     * @throws IllegalArgumentException {@inheritDoc}
+     * @since 1.6
+     */
+    @SideEffectFree
+    public @PolyMutable NavigableSet<E> tailSet(@GuardSatisfied @PolyMutable TreeSet<E> this, @GuardSatisfied E fromElement, boolean inclusive) {
+        return new @PolyMutable TreeSet<>(m.tailMap(fromElement, inclusive));
+    }
+
+    /**
+     * @throws ClassCastException {@inheritDoc}
+     * @throws NullPointerException if {@code fromElement} or
+     *         {@code toElement} is null and this set uses natural ordering,
+     *         or its comparator does not permit null elements
+     * @throws IllegalArgumentException {@inheritDoc}
+     */
+    @SideEffectFree
+    public @PolyMutable SortedSet<E> subSet(@GuardSatisfied @PolyMutable TreeSet<E> this, @GuardSatisfied E fromElement, @GuardSatisfied E toElement) {
+        return subSet(fromElement, true, toElement, false);
+    }
+
+    /**
+     * @throws ClassCastException {@inheritDoc}
+     * @throws NullPointerException if {@code toElement} is null
+     *         and this set uses natural ordering, or its comparator does
+     *         not permit null elements
+     * @throws IllegalArgumentException {@inheritDoc}
+     */
+    @SideEffectFree
+    public @PolyMutable SortedSet<E> headSet(@GuardSatisfied @PolyMutable TreeSet<E> this, E toElement) {
+        return headSet(toElement, false);
+    }
+
+    /**
+     * @throws ClassCastException {@inheritDoc}
+     * @throws NullPointerException if {@code fromElement} is null
+     *         and this set uses natural ordering, or its comparator does
+     *         not permit null elements
+     * @throws IllegalArgumentException {@inheritDoc}
+     */
+    @SideEffectFree
+    public @PolyMutable SortedSet<E> tailSet(@GuardSatisfied @PolyMutable TreeSet<E> this, E fromElement) {
+        return tailSet(fromElement, true);
+    }
+
+    @Pure
+    public @Nullable Comparator<? super E> comparator(@GuardSatisfied @Readonly TreeSet<E> this) {
+        return m.comparator();
+    }
+
+    /**
+     * @throws NoSuchElementException {@inheritDoc}
+     */
+    @SideEffectFree
+    public E first(@GuardSatisfied @NonEmpty @Readonly TreeSet<E> this) {
+        return m.firstKey();
+    }
+
+    /**
+     * @throws NoSuchElementException {@inheritDoc}
+     */
+    @SideEffectFree
+    public E last(@GuardSatisfied @NonEmpty @Readonly TreeSet<E> this) {
+        return m.lastKey();
+    }
+
+    // NavigableSet API methods
+
+    /**
+     * @throws ClassCastException {@inheritDoc}
+     * @throws NullPointerException if the specified element is null
+     *         and this set uses natural ordering, or its comparator
+     *         does not permit null elements
+     * @since 1.6
+     */
+    public @Nullable E lower(@Readonly TreeSet<E> this, E e) {
+        return m.lowerKey(e);
+    }
+
+    /**
+     * @throws ClassCastException {@inheritDoc}
+     * @throws NullPointerException if the specified element is null
+     *         and this set uses natural ordering, or its comparator
+     *         does not permit null elements
+     * @since 1.6
+     */
+    public @Nullable E floor(@Readonly TreeSet<E> this, E e) {
+        return m.floorKey(e);
+    }
+
+    /**
+     * @throws ClassCastException {@inheritDoc}
+     * @throws NullPointerException if the specified element is null
+     *         and this set uses natural ordering, or its comparator
+     *         does not permit null elements
+     * @since 1.6
+     */
+    public @Nullable E ceiling(@Readonly TreeSet<E> this, E e) {
+        return m.ceilingKey(e);
+    }
+
+    /**
+     * @throws ClassCastException {@inheritDoc}
+     * @throws NullPointerException if the specified element is null
+     *         and this set uses natural ordering, or its comparator
+     *         does not permit null elements
+     * @since 1.6
+     */
+    public @Nullable E higher(@Readonly TreeSet<E> this, E e) {
+        return m.higherKey(e);
+    }
+
+    /**
+     * @since 1.6
+     */
+    public @Nullable E pollFirst(@GuardSatisfied @Mutable TreeSet<E> this) {
+        Map.Entry<@Immutable E,?> e = m.pollFirstEntry();
+        return (e == null) ? null : e.getKey();
+    }
+
+    /**
+     * @since 1.6
+     */
+    public @Nullable E pollLast(@GuardSatisfied @Mutable TreeSet<E> this) {
+        Map.Entry<@Immutable E,?> e = m.pollLastEntry();
+        return (e == null) ? null : e.getKey();
+    }
+
+    /**
+     * Returns a shallow copy of this {@code TreeSet} instance. (The elements
+     * themselves are not cloned.)
+     *
+     * @return a shallow copy of this set
+     */
+    @SideEffectFree
+    @SuppressWarnings("unchecked")
+    public Object clone(@GuardSatisfied @Mutable TreeSet<E> this) {
+        TreeSet<E> clone;
+        try {
+            clone = (TreeSet<E>) super.clone();
+        } catch (CloneNotSupportedException e) {
+            throw new InternalError(e);
+        }
+
+        clone.m = new TreeMap<@Immutable E, @Readonly Object>(m);
+        return clone;
+    }
+
+    /**
+     * Save the state of the {@code TreeSet} instance to a stream (that is,
+     * serialize it).
+     *
+     * @serialData Emits the comparator used to order this set, or
+     *             {@code null} if it obeys its elements' natural ordering
+     *             (Object), followed by the size of the set (the number of
+     *             elements it contains) (int), followed by all of its
+     *             elements (each an Object) in order (as determined by the
+     *             set's Comparator, or by the elements' natural ordering if
+     *             the set has no Comparator).
+     */
+    @java.io.Serial
+    private void writeObject(@Mutable TreeSet<E> this, java.io.ObjectOutputStream s)
+        throws java.io.IOException {
+        // Write out any hidden stuff
+        s.defaultWriteObject();
+
+        // Write out Comparator
+        s.writeObject(m.comparator());
+
+        // Write out size
+        s.writeInt(m.size());
+
+        // Write out all elements in the proper order.
+        for (E e : m.keySet())
+            s.writeObject(e);
+    }
+
+    /**
+     * Reconstitute the {@code TreeSet} instance from a stream (that is,
+     * deserialize it).
+     */
+    @java.io.Serial
+    private void readObject(@Mutable TreeSet<E> this, java.io.ObjectInputStream s)
+        throws java.io.IOException, ClassNotFoundException {
+        // Read in any hidden stuff
+        s.defaultReadObject();
+
+        // Read in Comparator
+        @SuppressWarnings("unchecked")
+            Comparator<? super E> c = (Comparator<? super E>) s.readObject();
+
+        // Create backing TreeMap
+        TreeMap<@Immutable E, @Readonly Object> tm = new TreeMap<@Immutable E, @Readonly Object>(c);
+        m = tm;
+
+        // Read in size
+        int size = s.readInt();
+
+        tm.readTreeSet(size, s, PRESENT);
+    }
+
+    /**
+     * Creates a <em><a href="Spliterator.html#binding">late-binding</a></em>
+     * and <em>fail-fast</em> {@link Spliterator} over the elements in this
+     * set.
+     *
+     * <p>The {@code Spliterator} reports {@link Spliterator#SIZED},
+     * {@link Spliterator#DISTINCT}, {@link Spliterator#SORTED}, and
+     * {@link Spliterator#ORDERED}.  Overriding implementations should document
+     * the reporting of additional characteristic values.
+     *
+     * <p>The spliterator's comparator (see
+     * {@link java.util.Spliterator#getComparator()}) is {@code null} if
+     * the tree set's comparator (see {@link #comparator()}) is {@code null}.
+     * Otherwise, the spliterator's comparator is the same as or imposes the
+     * same total ordering as the tree set's comparator.
+     *
+     * @return a {@code Spliterator} over the elements in this set
+     * @since 1.8
+     */
+    public Spliterator<E> spliterator() {
+        return TreeMap.keySpliteratorFor(m);
+    }
+
+    @java.io.Serial
+    private static final long serialVersionUID = -2479143000061671589L;
+}
diff --git a/src/java.base/share/classes/jdk/internal/access/JavaLangAccess.java b/src/java.base/share/classes/jdk/internal/access/JavaLangAccess.java
index f6fe5f61131..9e4eb11ea69 100644
--- a/src/java.base/share/classes/jdk/internal/access/JavaLangAccess.java
+++ b/src/java.base/share/classes/jdk/internal/access/JavaLangAccess.java
@@ -25,6 +25,8 @@
 
 package jdk.internal.access;
 
+import org.checkerframework.checker.nullness.qual.Nullable;
+
 import java.lang.annotation.Annotation;
 import java.lang.invoke.MethodHandle;
 import java.lang.invoke.MethodHandles;
@@ -77,7 +79,7 @@ public interface JavaLangAccess {
     /**
      * Get the declared annotations for a given class, indexed by their types.
      */
-    Map<Class<? extends Annotation>, Annotation> getDeclaredAnnotationMap(Class<?> klass);
+    Map<Class<? extends @Nullable Annotation>, Annotation> getDeclaredAnnotationMap(Class<?> klass);
 
     /**
      * Get the array of bytes that is the class-file representation
@@ -102,7 +104,7 @@ public interface JavaLangAccess {
      * Class object does not represent an enum type;
      * the result is uncloned, cached, and shared by all callers.
      */
-    <E extends Enum<E>> E[] getEnumConstantsShared(Class<E> klass);
+    <E extends Enum<E>> E @Nullable [] getEnumConstantsShared(Class<E> klass);
 
     /**
      * Set current thread's blocker field.
@@ -161,7 +163,7 @@ public interface JavaLangAccess {
     /**
      * Returns a class loaded by the bootstrap class loader.
      */
-    Class<?> findBootstrapClassOrNull(String name);
+    @Nullable Class<?> findBootstrapClassOrNull(String name);
 
     /**
      * Define a Package of the given name and module by the given class loader.
diff --git a/src/java.base/share/classes/jdk/internal/loader/BootLoader.java b/src/java.base/share/classes/jdk/internal/loader/BootLoader.java
index ca9222d32a9..84b23802f44 100644
--- a/src/java.base/share/classes/jdk/internal/loader/BootLoader.java
+++ b/src/java.base/share/classes/jdk/internal/loader/BootLoader.java
@@ -24,6 +24,8 @@
  */
 package jdk.internal.loader;
 
+import org.checkerframework.checker.nullness.qual.Nullable;
+
 import java.io.IOException;
 import java.io.InputStream;
 import java.lang.module.ModuleReference;
@@ -160,7 +162,7 @@ public Void run() {
     /**
      * Returns a URL to a resource in a module defined to the boot loader.
      */
-    public static URL findResource(String mn, String name) throws IOException {
+    public static URL findResource(@Nullable String mn, String name) throws IOException {
         return ClassLoaders.bootLoader().findResource(mn, name);
     }
 
@@ -168,7 +170,7 @@ public static URL findResource(String mn, String name) throws IOException {
      * Returns an input stream to a resource in a module defined to the
      * boot loader.
      */
-    public static InputStream findResourceAsStream(String mn, String name)
+    public static InputStream findResourceAsStream(@Nullable String mn, String name)
         throws IOException
     {
         return ClassLoaders.bootLoader().findResourceAsStream(mn, name);
diff --git a/src/java.base/share/classes/jdk/internal/loader/BuiltinClassLoader.java b/src/java.base/share/classes/jdk/internal/loader/BuiltinClassLoader.java
index cd7406ac568..7f02813f172 100644
--- a/src/java.base/share/classes/jdk/internal/loader/BuiltinClassLoader.java
+++ b/src/java.base/share/classes/jdk/internal/loader/BuiltinClassLoader.java
@@ -25,6 +25,8 @@
 
 package jdk.internal.loader;
 
+import org.checkerframework.checker.nullness.qual.Nullable;
+
 import java.io.IOException;
 import java.io.InputStream;
 import java.lang.module.ModuleDescriptor;
@@ -289,7 +291,7 @@ public URL findResource(String mn, String name) throws IOException {
      * defined to this class loader.
      */
     @SuppressWarnings("removal")
-    public InputStream findResourceAsStream(String mn, String name)
+    public InputStream findResourceAsStream(@Nullable String mn, String name)
         throws IOException
     {
         // Need URL to resource when running with a security manager so that
diff --git a/src/java.base/share/classes/jdk/internal/misc/Unsafe.java b/src/java.base/share/classes/jdk/internal/misc/Unsafe.java
index 22aa09c9d62..df2952713c5 100644
--- a/src/java.base/share/classes/jdk/internal/misc/Unsafe.java
+++ b/src/java.base/share/classes/jdk/internal/misc/Unsafe.java
@@ -25,6 +25,8 @@
 
 package jdk.internal.misc;
 
+import org.checkerframework.checker.nullness.qual.Nullable;
+
 import jdk.internal.ref.Cleaner;
 import jdk.internal.vm.annotation.ForceInline;
 import jdk.internal.vm.annotation.IntrinsicCandidate;
@@ -1391,7 +1393,7 @@ private Object allocateUninitializedArray0(Class<?> componentType, int length) {
     }
 
     /** Throws the exception without telling the verifier. */
-    public native void throwException(Throwable ee);
+    public native void throwException(@Nullable Throwable ee);
 
     /**
      * Atomically updates Java variable to {@code x} if it is currently
@@ -1404,7 +1406,7 @@ private Object allocateUninitializedArray0(Class<?> componentType, int length) {
      */
     @IntrinsicCandidate
     public final native boolean compareAndSetReference(Object o, long offset,
-                                                       Object expected,
+                                                       @Nullable Object expected,
                                                        Object x);
 
     @IntrinsicCandidate
diff --git a/src/java.base/share/classes/jdk/internal/misc/VM.java b/src/java.base/share/classes/jdk/internal/misc/VM.java
index f0b32c30f6e..a403b4b3748 100644
--- a/src/java.base/share/classes/jdk/internal/misc/VM.java
+++ b/src/java.base/share/classes/jdk/internal/misc/VM.java
@@ -25,6 +25,8 @@
 
 package jdk.internal.misc;
 
+import org.checkerframework.dataflow.qual.Pure;
+
 import static java.lang.Thread.State.*;
 
 import java.text.NumberFormat;
@@ -97,6 +99,7 @@ public static boolean isModuleSystemInited() {
     /**
      * Returns {@code true} if the VM is fully initialized.
      */
+    @Pure
     public static boolean isBooted() {
         return initLevel >= SYSTEM_BOOTED;
     }
diff --git a/src/java.base/share/classes/jdk/internal/org/objectweb/asm/ClassVisitor.java b/src/java.base/share/classes/jdk/internal/org/objectweb/asm/ClassVisitor.java
index 7bfb17051ae..a06eb094f58 100644
--- a/src/java.base/share/classes/jdk/internal/org/objectweb/asm/ClassVisitor.java
+++ b/src/java.base/share/classes/jdk/internal/org/objectweb/asm/ClassVisitor.java
@@ -58,6 +58,8 @@
  */
 package jdk.internal.org.objectweb.asm;
 
+import org.checkerframework.checker.nullness.qual.Nullable;
+
 /**
  * A visitor to visit a Java class. The methods of this class must be called in the following order:
  * {@code visit} [ {@code visitSource} ] [ {@code visitModule} ][ {@code visitNestHost} ][ {@code
@@ -170,7 +172,7 @@ public void visitSource(final String source, final String debug) {
       * @return a visitor to visit the module values, or {@literal null} if this visitor is not
       *     interested in visiting this module.
       */
-    public ModuleVisitor visitModule(final String name, final int access, final String version) {
+    public @Nullable ModuleVisitor visitModule(final String name, final int access, final String version) {
         if (api < Opcodes.ASM6) {
             throw new UnsupportedOperationException("This feature requires ASM6");
         }
diff --git a/src/java.base/share/classes/jdk/internal/org/objectweb/asm/ClassWriter.java b/src/java.base/share/classes/jdk/internal/org/objectweb/asm/ClassWriter.java
index 31cfe85afe2..79adfa973d7 100644
--- a/src/java.base/share/classes/jdk/internal/org/objectweb/asm/ClassWriter.java
+++ b/src/java.base/share/classes/jdk/internal/org/objectweb/asm/ClassWriter.java
@@ -58,6 +58,8 @@
  */
 package jdk.internal.org.objectweb.asm;
 
+import org.checkerframework.checker.nullness.qual.Nullable;
+
 /**
  * A {@link ClassVisitor} that generates a corresponding ClassFile structure, as defined in the Java
  * Virtual Machine Specification (JVMS). It can be used alone, to generate a Java class "from
@@ -305,9 +307,9 @@ public final void visit(
             final int version,
             final int access,
             final String name,
-            final String signature,
-            final String superName,
-            final String[] interfaces) {
+            final @Nullable String signature,
+            final @Nullable String superName,
+            final String @Nullable [] interfaces) {
         this.version = version;
         this.accessFlags = access;
         this.thisClass = symbolTable.setMajorVersionAndClassName(version & 0xFFFF, name);
diff --git a/src/java.base/share/classes/jdk/internal/reflect/ReflectionFactory.java b/src/java.base/share/classes/jdk/internal/reflect/ReflectionFactory.java
index a6b896df7fe..ad60adb7fce 100644
--- a/src/java.base/share/classes/jdk/internal/reflect/ReflectionFactory.java
+++ b/src/java.base/share/classes/jdk/internal/reflect/ReflectionFactory.java
@@ -25,6 +25,8 @@
 
 package jdk.internal.reflect;
 
+import org.checkerframework.checker.nullness.qual.Nullable;
+
 import java.io.Externalizable;
 import java.io.ObjectInputStream;
 import java.io.ObjectOutputStream;
@@ -341,7 +343,7 @@ public Class<?>[] getExecutableSharedParameterTypes(Executable ex) {
         return langReflectAccess.getExecutableSharedParameterTypes(ex);
     }
 
-    public <T> T newInstance(Constructor<T> ctor, Object[] args, Class<?> caller)
+    public <T> T newInstance(Constructor<T> ctor, Object @Nullable [] args, Class<?> caller)
         throws IllegalAccessException, InstantiationException, InvocationTargetException
     {
         return langReflectAccess.newInstance(ctor, args, caller);
diff --git a/src/java.base/share/classes/sun/reflect/annotation/AnnotationParser.java b/src/java.base/share/classes/sun/reflect/annotation/AnnotationParser.java
index ab123b25a0c..11c22a44b32 100644
--- a/src/java.base/share/classes/sun/reflect/annotation/AnnotationParser.java
+++ b/src/java.base/share/classes/sun/reflect/annotation/AnnotationParser.java
@@ -25,6 +25,8 @@
 
 package sun.reflect.annotation;
 
+import org.checkerframework.checker.nullness.qual.Nullable;
+
 import java.lang.annotation.*;
 import java.lang.reflect.*;
 import java.nio.BufferUnderflowException;
@@ -62,7 +64,7 @@ public class AnnotationParser {
      * @throws AnnotationFormatError if an annotation is found to be
      *         malformed.
      */
-    public static Map<Class<? extends Annotation>, Annotation> parseAnnotations(
+    public static Map<Class<? extends @Nullable  Annotation>, Annotation> parseAnnotations(
                 byte[] rawAnnotations,
                 ConstantPool constPool,
                 Class<?> container) {
@@ -90,11 +92,11 @@ public static Map<Class<? extends Annotation>, Annotation> parseAnnotations(
      */
     @SafeVarargs
     @SuppressWarnings("varargs") // selectAnnotationClasses is used safely
-    static Map<Class<? extends Annotation>, Annotation> parseSelectAnnotations(
+    static Map<Class<? extends @Nullable Annotation>, Annotation> parseSelectAnnotations(
                 byte[] rawAnnotations,
                 ConstantPool constPool,
                 Class<?> container,
-                Class<? extends Annotation> ... selectAnnotationClasses) {
+                Class<? extends @Nullable Annotation> ... selectAnnotationClasses) {
         if (rawAnnotations == null)
             return Collections.emptyMap();
 
@@ -108,19 +110,19 @@ static Map<Class<? extends Annotation>, Annotation> parseSelectAnnotations(
         }
     }
 
-    private static Map<Class<? extends Annotation>, Annotation> parseAnnotations2(
+    private static Map<Class<? extends @Nullable Annotation>, Annotation> parseAnnotations2(
                 byte[] rawAnnotations,
                 ConstantPool constPool,
                 Class<?> container,
-                Class<? extends Annotation>[] selectAnnotationClasses) {
-        Map<Class<? extends Annotation>, Annotation> result =
-            new LinkedHashMap<Class<? extends Annotation>, Annotation>();
+                Class<? extends @Nullable  Annotation>[] selectAnnotationClasses) {
+        Map<Class<? extends @Nullable  Annotation>, Annotation> result =
+            new LinkedHashMap<Class<? extends @Nullable Annotation>, Annotation>();
         ByteBuffer buf = ByteBuffer.wrap(rawAnnotations);
         int numAnnotations = buf.getShort() & 0xFFFF;
         for (int i = 0; i < numAnnotations; i++) {
             Annotation a = parseAnnotation2(buf, constPool, container, false, selectAnnotationClasses);
             if (a != null) {
-                Class<? extends Annotation> klass = a.annotationType();
+                Class<? extends @Nullable  Annotation> klass = a.annotationType();
                 if (AnnotationType.getInstance(klass).retention() == RetentionPolicy.RUNTIME &&
                     result.put(klass, a) != null) {
                         throw new AnnotationFormatError(
@@ -232,13 +234,13 @@ private static Annotation parseAnnotation2(ByteBuffer buf,
                                               ConstantPool constPool,
                                               Class<?> container,
                                               boolean exceptionOnMissingAnnotationClass,
-                                              Class<? extends Annotation>[] selectAnnotationClasses) {
+                                              Class<? extends @Nullable Annotation>[] selectAnnotationClasses) {
         int typeIndex = buf.getShort() & 0xFFFF;
-        Class<? extends Annotation> annotationClass = null;
+        Class<? extends @Nullable Annotation> annotationClass = null;
         String sig = "[unknown]";
         try {
             sig = constPool.getUTF8At(typeIndex);
-            annotationClass = (Class<? extends Annotation>)parseSig(sig, container);
+            annotationClass = (Class<? extends @Nullable Annotation>)parseSig(sig, container);
         } catch (NoClassDefFoundError e) {
             if (exceptionOnMissingAnnotationClass)
                 // note: at this point sig is "[unknown]" or VM-style
@@ -294,7 +296,7 @@ private static Annotation parseAnnotation2(ByteBuffer buf,
      * member {@literal ->} value map.
      */
     @SuppressWarnings("removal")
-    public static Annotation annotationForMap(final Class<? extends Annotation> type,
+    public static Annotation annotationForMap(final Class<? extends @Nullable Annotation> type,
                                               final Map<String, Object> memberValues)
     {
         return AccessController.doPrivileged(new PrivilegedAction<Annotation>() {
@@ -529,7 +531,7 @@ private static Object parseArray(Class<?> arrayType,
             return parseEnumArray(length, (Class<? extends Enum<?>>)componentType, buf,
                                   constPool, container);
         } else if (componentType.isAnnotation()) {
-            return parseAnnotationArray(length, (Class <? extends Annotation>)componentType, buf,
+            return parseAnnotationArray(length, (Class <? extends @Nullable Annotation>)componentType, buf,
                                         constPool, container);
         } else {
             return parseUnknownArray(length, buf);
@@ -724,7 +726,7 @@ private static Object parseEnumArray(int length, Class<? extends Enum<?>> enumTy
     }
 
     private static Object parseAnnotationArray(int length,
-                                               Class<? extends Annotation> annotationType,
+                                               Class<? extends @Nullable Annotation> annotationType,
                                                ByteBuffer buf,
                                                ConstantPool constPool,
                                                Class<?> container) {
@@ -856,7 +858,7 @@ private static boolean contains(Object[] array, Object element) {
      * it is needed.
      */
     private static final Annotation[] EMPTY_ANNOTATION_ARRAY = new Annotation[0];
-    public static Annotation[] toArray(Map<Class<? extends Annotation>, Annotation> annotations) {
+    public static Annotation[] toArray(Map<Class<? extends @Nullable Annotation>, Annotation> annotations) {
         return annotations.values().toArray(EMPTY_ANNOTATION_ARRAY);
     }
 
diff --git a/src/java.base/share/classes/sun/reflect/annotation/AnnotationSupport.java b/src/java.base/share/classes/sun/reflect/annotation/AnnotationSupport.java
index 55b6edea730..62158bc85b9 100644
--- a/src/java.base/share/classes/sun/reflect/annotation/AnnotationSupport.java
+++ b/src/java.base/share/classes/sun/reflect/annotation/AnnotationSupport.java
@@ -25,6 +25,8 @@
 
 package sun.reflect.annotation;
 
+import org.checkerframework.checker.nullness.qual.Nullable;
+
 import java.lang.annotation.*;
 import java.lang.reflect.*;
 import java.security.AccessController;
@@ -62,8 +64,8 @@ public final class AnnotationSupport {
      * @return an array of instances of {@code annoClass} or an empty
      *         array if none were found
      */
-    public static <A extends Annotation> A[] getDirectlyAndIndirectlyPresent(
-            Map<Class<? extends Annotation>, Annotation> annotations,
+    public static <A extends @Nullable Annotation> A[] getDirectlyAndIndirectlyPresent(
+            Map<Class<? extends @Nullable Annotation>, Annotation> annotations,
             Class<A> annoClass) {
         List<A> result = new ArrayList<>();
 
@@ -95,8 +97,8 @@ public static <A extends Annotation> A[] getDirectlyAndIndirectlyPresent(
      * @return an array of instances of {@code annoClass} or an empty array if no
      *         indirectly present annotations were found
      */
-    private static <A extends Annotation> A[] getIndirectlyPresent(
-            Map<Class<? extends Annotation>, Annotation> annotations,
+    private static <A extends @Nullable Annotation> A[] getIndirectlyPresent(
+            Map<Class<? extends @Nullable Annotation>, Annotation> annotations,
             Class<A> annoClass) {
 
         Repeatable repeatable = annoClass.getDeclaredAnnotation(Repeatable.class);
@@ -124,11 +126,11 @@ private static <A extends Annotation> A[] getIndirectlyPresent(
      * @return true if container class is found before containee class when
      *         iterating over annotations.keySet().
      */
-    private static <A extends Annotation> boolean containerBeforeContainee(
-            Map<Class<? extends Annotation>, Annotation> annotations,
+    private static <A extends @Nullable Annotation> boolean containerBeforeContainee(
+            Map<Class<? extends @Nullable Annotation>, Annotation> annotations,
             Class<A> annoClass) {
 
-        Class<? extends Annotation> containerClass =
+        Class<? extends @Nullable Annotation> containerClass =
                 annoClass.getDeclaredAnnotation(Repeatable.class).value();
 
         for (Class<? extends Annotation> c : annotations.keySet()) {
@@ -156,8 +158,8 @@ private static <A extends Annotation> boolean containerBeforeContainee(
      *
      * @return an array of instances of {@code annoClass} or an empty array if none were found.
      */
-    public static <A extends Annotation> A[] getAssociatedAnnotations(
-            Map<Class<? extends Annotation>, Annotation> declaredAnnotations,
+    public static <A extends @Nullable Annotation> A[] getAssociatedAnnotations(
+            Map<Class<? extends @Nullable Annotation>, Annotation> declaredAnnotations,
             Class<?> decl,
             Class<A> annoClass) {
         Objects.requireNonNull(decl);
@@ -182,13 +184,13 @@ public static <A extends Annotation> A[] getAssociatedAnnotations(
      * (container), cast it to an array of annotations and return the result.
      */
     @SuppressWarnings("removal")
-    private static <A extends Annotation> A[] getValueArray(Annotation container) {
+    private static <A extends @Nullable Annotation> A[] getValueArray(Annotation container) {
         try {
             // According to JLS the container must have an array-valued value
             // method. Get the AnnotationType, get the "value" method and invoke
             // it to get the content.
 
-            Class<? extends Annotation> containerClass = container.annotationType();
+            Class<? extends @Nullable Annotation> containerClass = container.annotationType();
             AnnotationType annoType = AnnotationType.getInstance(containerClass);
             if (annoType == null)
                 throw invalidContainerException(container, null);
@@ -269,7 +271,7 @@ private static AnnotationFormatError invalidContainerException(Annotation anno,
     /* Sanity check type of all the annotation instances of type {@code annoClass}
      * from {@code container}.
      */
-    private static <A extends Annotation> void checkTypes(A[] annotations,
+    private static <A extends @Nullable Annotation> void checkTypes(A[] annotations,
                                                           Annotation container,
                                                           Class<A> annoClass) {
         for (A a : annotations) {
diff --git a/src/java.base/share/classes/sun/reflect/annotation/AnnotationType.java b/src/java.base/share/classes/sun/reflect/annotation/AnnotationType.java
index 89aff8a954a..529bb4efdc8 100644
--- a/src/java.base/share/classes/sun/reflect/annotation/AnnotationType.java
+++ b/src/java.base/share/classes/sun/reflect/annotation/AnnotationType.java
@@ -25,6 +25,8 @@
 
 package sun.reflect.annotation;
 
+import org.checkerframework.checker.nullness.qual.Nullable;
+
 import java.lang.annotation.*;
 import java.lang.reflect.*;
 import java.util.*;
@@ -77,7 +79,7 @@ public class AnnotationType {
      *         does not represent a valid annotation type
      */
     public static AnnotationType getInstance(
-        Class<? extends Annotation> annotationClass)
+        Class<? extends @Nullable Annotation> annotationClass)
     {
         JavaLangAccess jla = SharedSecrets.getJavaLangAccess();
         AnnotationType result = jla.getAnnotationType(annotationClass); // volatile read
diff --git a/src/java.base/share/classes/sun/reflect/misc/ReflectUtil.java b/src/java.base/share/classes/sun/reflect/misc/ReflectUtil.java
index 540a9d64110..81b627863ba 100644
--- a/src/java.base/share/classes/sun/reflect/misc/ReflectUtil.java
+++ b/src/java.base/share/classes/sun/reflect/misc/ReflectUtil.java
@@ -26,6 +26,8 @@
 
 package sun.reflect.misc;
 
+import org.checkerframework.checker.nullness.qual.Nullable;
+
 import java.lang.reflect.Member;
 import java.lang.reflect.Method;
 import java.lang.reflect.Modifier;
@@ -193,7 +195,7 @@ private static boolean isAncestor(ClassLoader p, ClassLoader cl) {
      * attempts to access system domain classes, it requires package
      * access check and this method will return true.
      */
-    public static boolean needsPackageAccessCheck(ClassLoader from, ClassLoader to) {
+    public static boolean needsPackageAccessCheck(@Nullable ClassLoader from, @Nullable ClassLoader to) {
         if (from == null || from == to)
             return false;
 
@@ -236,7 +238,7 @@ private static void privateCheckProxyPackageAccess(@SuppressWarnings("removal")
      * @param ccl the caller's class loader
      * @param interfaces the list of interfaces that a proxy class implements
      */
-    public static void checkProxyPackageAccess(ClassLoader ccl,
+    public static void checkProxyPackageAccess(@Nullable ClassLoader ccl,
                                                Class<?>... interfaces)
     {
         @SuppressWarnings("removal")
diff --git a/src/java.base/unix/classes/java/lang/ProcessImpl.java b/src/java.base/unix/classes/java/lang/ProcessImpl.java
index 2bf36f8f136..1711c8c4ff6 100644
--- a/src/java.base/unix/classes/java/lang/ProcessImpl.java
+++ b/src/java.base/unix/classes/java/lang/ProcessImpl.java
@@ -25,6 +25,9 @@
 
 package java.lang;
 
+import org.checkerframework.checker.nullness.qual.Nullable;
+import org.checkerframework.checker.nullness.qual.PolyNull;
+
 import java.lang.ProcessBuilder.Redirect;
 import java.io.BufferedInputStream;
 import java.io.BufferedOutputStream;
@@ -144,7 +147,7 @@ static Platform get() {
     private static final LaunchMechanism launchMechanism = platform.launchMechanism();
     private static final byte[] helperpath = toCString(StaticProperty.javaHome() + "/lib/jspawnhelper");
 
-    private static byte[] toCString(String s) {
+    private static byte @PolyNull [] toCString(@PolyNull String s) {
         if (s == null)
             return null;
         byte[] bytes = s.getBytes();
@@ -159,8 +162,8 @@ private static byte[] toCString(String s) {
     // Only for use by ProcessBuilder.start()
     static Process start(String[] cmdarray,
                          java.util.Map<String,String> environment,
-                         String dir,
-                         ProcessBuilder.Redirect[] redirects,
+                         @Nullable String dir,
+                         ProcessBuilder.@Nullable Redirect[] redirects,
                          boolean redirectErrorStream)
             throws IOException
     {
diff --git a/src/java.base/windows/classes/java/lang/ProcessImpl.java b/src/java.base/windows/classes/java/lang/ProcessImpl.java
index cd3e044a765..f63de714562 100644
--- a/src/java.base/windows/classes/java/lang/ProcessImpl.java
+++ b/src/java.base/windows/classes/java/lang/ProcessImpl.java
@@ -25,6 +25,8 @@
 
 package java.lang;
 
+import org.checkerframework.checker.nullness.qual.Nullable;
+
 import java.io.BufferedInputStream;
 import java.io.BufferedOutputStream;
 import java.io.File;
@@ -98,8 +100,8 @@ public FileOutputStream run() {
     // System-dependent portion of ProcessBuilder.start()
     static Process start(String cmdarray[],
                          java.util.Map<String,String> environment,
-                         String dir,
-                         ProcessBuilder.Redirect[] redirects,
+                         @Nullable String dir,
+                         ProcessBuilder.@Nullable Redirect[] redirects,
                          boolean redirectErrorStream)
         throws IOException
     {