Summary
ClassFileApiBackend — introduced to handle class files with major version > 69 (Java 26+) using the JDK ClassFile API — silently skips probe handlers that use @Return, @TargetInstance, or @Duration parameters. This causes probes targeting Java 26+ classes to fire without those values, or to be dropped entirely, with only a debug-level log message.
The ASM backend (Instrumentor.java, used for class file versions ≤ 69) fully supports all three. The ClassFileApiBackend was introduced as a replacement for ASM on Java 26+ but was shipped without parity for these parameter types.
@Self was fixed in PR #836.
Affected parameter types
| Annotation |
getSelfParameter() |
Status |
@Self |
slot 0 (this) |
✅ Fixed in PR #836 |
@Return |
return value captured before return |
❌ Not implemented |
@TargetInstance |
target object of a call site |
❌ Not implemented (requires Kind.CALL support) |
@Duration |
elapsed nanos between ENTRY and RETURN |
❌ Not implemented |
Why each is non-trivial
@Return
Requires capturing the return value at the RETURN point before it leaves the operand stack:
dup (reference) or dup2 (long/double) to preserve the value- Store in a temp local, call the handler, reload — or call with value on stack depending on descriptor
- Must know the return type from the method descriptor at code-generation time
@Duration
Requires timing across two injection points:
- At ENTRY: inject
long __btrace_start = System.nanoTime(); stored in a new local variable slot
- At RETURN: inject
long __btrace_dur = System.nanoTime() - __btrace_start; and pass to handler
- Adding a local variable requires updating the
LocalVariableTable and stack map frames
@TargetInstance
Only relevant for Kind.CALL (call-site instrumentation), which ClassFileApiBackend does not support yet. Blocked on adding Kind.CALL support first.
Reproduction
Any @BTrace probe targeting a Java 26+ class (e.g. JDK internal classes when running on Java 27+) with:
@OnMethod(clazz = "some.Java27Class", method = "someMethod")
public static void handler(@Return Object ret) { ... } // silently droppedOn Java 25 (ASM backend): probe fires and ret is populated.
On Java 27 (ClassFileApiBackend): probe handler is silently skipped — no output, no error.
Location
btrace-agent/src/main/java24/io/btrace/instr/ClassFileApiBackend.java — emitProbeCall() method.
The @Self fix (PR #836) shows the pattern for adding a new parameter type: thread a context flag through buildClassTransform → buildMethodTransform → buildCodeTransform → emitProbeCall and emit the appropriate bytecode instruction.
Summary
ClassFileApiBackend— introduced to handle class files with major version > 69 (Java 26+) using the JDK ClassFile API — silently skips probe handlers that use@Return,@TargetInstance, or@Durationparameters. This causes probes targeting Java 26+ classes to fire without those values, or to be dropped entirely, with only a debug-level log message.The ASM backend (
Instrumentor.java, used for class file versions ≤ 69) fully supports all three. The ClassFileApiBackend was introduced as a replacement for ASM on Java 26+ but was shipped without parity for these parameter types.@Selfwas fixed in PR #836.Affected parameter types
getSelfParameter()@Selfthis)@Returnreturn@TargetInstanceKind.CALLsupport)@DurationWhy each is non-trivial
@ReturnRequires capturing the return value at the RETURN point before it leaves the operand stack:
dup(reference) ordup2(long/double) to preserve the value@DurationRequires timing across two injection points:
long __btrace_start = System.nanoTime();stored in a new local variable slotlong __btrace_dur = System.nanoTime() - __btrace_start;and pass to handlerLocalVariableTableand stack map frames@TargetInstanceOnly relevant for
Kind.CALL(call-site instrumentation), whichClassFileApiBackenddoes not support yet. Blocked on addingKind.CALLsupport first.Reproduction
Any
@BTraceprobe targeting a Java 26+ class (e.g. JDK internal classes when running on Java 27+) with:On Java 25 (ASM backend): probe fires and
retis populated.On Java 27 (ClassFileApiBackend): probe handler is silently skipped — no output, no error.
Location
btrace-agent/src/main/java24/io/btrace/instr/ClassFileApiBackend.java—emitProbeCall()method.The
@Selffix (PR #836) shows the pattern for adding a new parameter type: thread a context flag throughbuildClassTransform → buildMethodTransform → buildCodeTransform → emitProbeCalland emit the appropriate bytecode instruction.