fix(cli): validate Java before downloading server JAR (#258)

cli/cmd/server.go

@@ -42,6 +42,7 @@ var (
 	serverFindLocalJAR        = findLocalJAR
 	serverCheckJava           = checkJava
 	serverCheckAIProviderKeys = checkAIProviderKeys
+	serverLaunch              = launchServer
 )
 
 var serverCmd = &cobra.Command{
@@ -101,6 +102,22 @@ func logFile() string {
 
 func runServerStart(cmd *cobra.Command, args []string) error {
 	cmd.SilenceUsage = true
+
+	// Validate JDK before doing anything
+	javaOk, javaVersion := serverCheckJava()
+	if !javaOk {
+		if javaVersion != "" {
+			return fmt.Errorf(
+				"Java %s detected but Java 21+ is required.\n"+
+					"  Install Java 21+: https://adoptium.net/\n"+
+					"  Run 'agentspan doctor' for full diagnostics.", javaVersion)
+		}
+		return fmt.Errorf(
+			"Java is not installed. The Agentspan server requires Java 21+.\n" +
+				"  Install: https://adoptium.net/\n" +
+				"  Run 'agentspan doctor' for full diagnostics.")
+	}
+
 	dir := serverDir()
 	if err := os.MkdirAll(dir, 0o755); err != nil {
 		return fmt.Errorf("create server dir: %w", err)
@@ -160,21 +177,10 @@ func runServerStart(cmd *cobra.Command, args []string) error {
 
 	serverCheckAIProviderKeys()
 
-	// Validate JDK before launching java
-	javaOk, javaVersion := serverCheckJava()
-	if !javaOk {
-		if javaVersion != "" {
-			return fmt.Errorf(
-				"Java %s detected but Java 21+ is required.\n"+
-					"  Install Java 21+: https://adoptium.net/\n"+
-					"  Run 'agentspan doctor' for full diagnostics.", javaVersion)
-		}
-		return fmt.Errorf(
-			"Java is not installed. The Agentspan server requires Java 21+.\n" +
-				"  Install: https://adoptium.net/\n" +
-				"  Run 'agentspan doctor' for full diagnostics.")
-	}
+	return serverLaunch(jarPath, dir)
+}
 
+func launchServer(jarPath, dir string) error {
 	bold := color.New(color.Bold)
 	bold.Printf("Starting agent runtime on port %s...\n", serverPort)
 

cli/cmd/server_test.go

@@ -111,6 +111,7 @@ func TestServerStartUsesRequestedVersion(t *testing.T) {
 	prevCheckJava := serverCheckJava
 	prevCheckAI := serverCheckAIProviderKeys
 	prevProcessRunning := serverProcessRunning
+	prevLaunch := serverLaunch
 	prevServerJar := serverJar
 	prevServerLocal := serverLocal
 	prevServerVersion := serverVersion
@@ -123,6 +124,7 @@ func TestServerStartUsesRequestedVersion(t *testing.T) {
 		serverCheckJava = prevCheckJava
 		serverCheckAIProviderKeys = prevCheckAI
 		serverProcessRunning = prevProcessRunning
+		serverLaunch = prevLaunch
 		serverJar = prevServerJar
 		serverLocal = prevServerLocal
 		serverVersion = prevServerVersion
@@ -154,14 +156,11 @@ func TestServerStartUsesRequestedVersion(t *testing.T) {
 		return "", nil
 	}
 	serverCheckAIProviderKeys = func() {}
-	serverCheckJava = func() (bool, string) { return false, "" }
+	serverCheckJava = func() (bool, string) { return true, "21.0.1" }
+	serverLaunch = func(jarPath, dir string) error { return nil }
 
-	err := runServerStart(serverStartCmd, nil)
-	if err == nil {
-		t.Fatal("expected runServerStart to stop at Java validation in test")
-	}
-	if !strings.Contains(err.Error(), "Java is not installed") {
-		t.Fatalf("unexpected error: %v", err)
+	if err := runServerStart(serverStartCmd, nil); err != nil {
+		t.Fatalf("runServerStart returned error: %v", err)
 	}
 	if !versionedCalled {
 		t.Fatal("expected versioned JAR downloader to be called")
@@ -175,6 +174,50 @@ func TestServerStartUsesRequestedVersion(t *testing.T) {
 	}
 }
 
+func TestServerStartFailsWhenJavaMissing(t *testing.T) {
+	newTempHome(t)
+
+	prevCheckJava := serverCheckJava
+	prevLaunch := serverLaunch
+	prevEnsureLatest := serverEnsureLatestJAR
+	prevEnsureVersioned := serverEnsureVersionedJAR
+	prevFindLocal := serverFindLocalJAR
+	t.Cleanup(func() {
+		serverCheckJava = prevCheckJava
+		serverLaunch = prevLaunch
+		serverEnsureLatestJAR = prevEnsureLatest
+		serverEnsureVersionedJAR = prevEnsureVersioned
+		serverFindLocalJAR = prevFindLocal
+	})
+
+	serverCheckJava = func() (bool, string) { return false, "" }
+	// Nothing below the Java check should be reached.
+	serverEnsureLatestJAR = func(string) error {
+		t.Fatal("ensureLatestJAR should not be called when Java is missing")
+		return nil
+	}
+	serverEnsureVersionedJAR = func(string, string) error {
+		t.Fatal("ensureVersionedJAR should not be called when Java is missing")
+		return nil
+	}
+	serverFindLocalJAR = func() (string, error) {
+		t.Fatal("findLocalJAR should not be called when Java is missing")
+		return "", nil
+	}
+	serverLaunch = func(jarPath, dir string) error {
+		t.Fatal("launchServer should not be called when Java is missing")
+		return nil
+	}
+
+	err := runServerStart(serverStartCmd, nil)
+	if err == nil {
+		t.Fatal("expected an error when Java is missing")
+	}
+	if !strings.Contains(err.Error(), "Java is not installed") {
+		t.Fatalf("unexpected error: %v", err)
+	}
+}
+
 func freeTCPPort(t *testing.T) string {
 	t.Helper()
 	ln, err := net.Listen("tcp", "127.0.0.1:0")