Replace SimpleEvaluationProvider with ECJ compile+inject evaluator

[waltoss]

Why

The Java debugger's expression evaluator (SimpleEvaluationProvider) only supported simple variable names, field chains (a.b.c), and no-arg method calls (obj.method()). Any real debugging expression — arithmetic, method calls with arguments, ternary, constructors — returned:

"Expression evaluation not supported. For full expression evaluation, use dbg install java-full."

This made the Java debugger significantly less useful than the Node.js/Bun debugger for AI agents that need to inspect runtime state.

Before / After

# Before — SimpleEvaluationProvider (JDI only)

dbg> eval x
42

dbg> eval x + y
❌ Expression evaluation not supported. Only variable names, field access,
   and simple method calls are supported.

dbg> eval greeting.substring(1, 3)
❌ Method calls with arguments are not supported in lightweight mode.

dbg> eval obj.getName().length()
❌ Expression evaluation not supported.

dbg> eval a > b ? "yes" : "no"
❌ Expression evaluation not supported.

dbg> eval this.secret     (private field, from instance method)
❌ Variable not found: secret

dbg> eval new String("hi")
❌ Expression evaluation not supported.

# After — CompilingEvaluationProvider (ECJ + bytecode injection)

dbg> eval x
42

dbg> eval x + y
62

dbg> eval greeting.substring(1, 3)
"el"

dbg> eval obj.getName().length()
5

dbg> eval a > b ? "yes" : "no"
"no"

dbg> eval this.secret     (private field, from instance method)
"hidden"

dbg> eval new String("hi")
"hi"

dbg> eval names.stream().filter(n -> ((String)n).length() > 3).count()
2

How it works

1. Generate synthetic class with expression as a static method, locals as parameters
2. Compile with ECJ (or javac fallback) via temp files
3. Inject bytecode into debuggee via ClassLoader.defineClass() over JDWP
4. Force initialization via Class.forName(name, true, loader)
5. Invoke __eval(...) passing pre-captured local variable values
6. On "not visible" compile errors, automatically rewrite private field accesses to reflection (getDeclaredField + setAccessible) with proper type casting

Changes

• Replace SimpleEvaluationProvider with CompilingEvaluationProvider (561 lines)
• Add ECJ 3.40.0 dependency (3.3MB single JAR, zero transitive deps)
• Bump Java adapter requirement from 11 to 17 (ECJ needs JDK 17+)
• Add 38 new integration tests across 2 test files

Known limitations

Limitation	Cause	Workaround
Generic type erasure	JDI reports erased types — List<String> becomes List	Explicit cast: ((String)n).length()
Unimported types	Generated class has no imports	Use FQCN: java.util.stream.Collectors.toList()
Bare field access	fieldName without this. won't resolve	Use this.fieldName

Test plan

• 19 existing Java integration tests pass (no regressions)
• 11 new expression eval tests: arithmetic, method calls with args, chained calls, ternary, constructors, collection access, string concat
• 27 new edge case tests: primitives, casting, null handling, arrays, nested generics, lambdas, static fields, this context, private fields, instanceof, bitwise, compile errors
• 65 total tests, 0 failures

🤖 Generated with Claude Code