AgentBuilder.RedefinitionStrategy.RETRANSFORMATION will cause method lost parameter names

[yangrubing]

Description

I am using ByteBuddy 1.12.13 to do bytecode enhancement , and it works well in most cases. Recently , our colleage report their app cann't get methods' parameter's name (they need parameter name to parse SpEL and already compiled their app with -parameters). Then, I checked our instrumentation code like below .

    protected ResettableClassFileTransformer buildClassFileTransformer(Instrumentation inst) {
        final ByteBuddy byteBuddy = new ByteBuddy().with(TypeValidation.of(false));
        InstrumentationListener instrumentationListener = new InstrumentationListener();
        AgentBuilder agentBuilder = new AgentBuilder.Default(byteBuddy)
                .ignore(getAgentBuilderIgnoreCondition())
                .with(new AgentBuilder.CircularityLock.Default())
                .with(AgentBuilder.RedefinitionStrategy.RETRANSFORMATION)
                .with(new AgentBuilder.InjectionStrategy.UsingUnsafe.OfFactory(ClassInjector.UsingUnsafe.Factory.resolve(inst)))
                .with(new AgentBuilder.InitializationStrategy.SelfInjection.Eager())
                .with(instrumentationListener)
                .disableClassFormatChanges();

        ElementMatcher.Junction<Object> typeMatcher = getTypeMatcher();
        if (typeMatcher == null) {
            logger.info("Element type matcher for {} is null, will not do the transform", this.getClass());
            return null;
        }
        ResettableClassFileTransformer transformer = agentBuilder.type(typeMatcher).transform(getTransformer()).installOn(inst);
        instrumentationListener.outputLog(true);
        return transformer;
    }

And , I tested in my laptop the parameter name seems did lost

    @GetMapping("/param")
    public String param(String test) {
        try {
            return HelloController.class.getMethod("param", String.class).getParameters()[0].getName(); 
        } catch (NoSuchMethodException e) {
            throw new RuntimeException(e);
        }
    }

// OUTPUT: arg0  , should return test

And above code return arg0 which means the parameter names are lost . And I tried to modify our instrumentation code , only change the .with(AgentBuilder.RedefinitionStrategy.RETRANSFORMATION) to .with(AgentBuilder.RedefinitionStrategy.REDEFINITION) make our code work and the test code return 'test' correctly.

Ask for help

• What's the difference between AgentBuilder.RedefinitionStrategy.RETRANSFORMATION and .with(AgentBuilder.RedefinitionStrategy.REDEFINITION?And why AgentBuilder.RedefinitionStrategy.RETRANSFORMATION behavior differenly ?
• Is it OK to use AgentBuilder.RedefinitionStrategy.REDEFINITION instead of AgentBuilder.RedefinitionStrategy.RETRANSFORMATION?

[yangrubing]

I compared the REDEFINITION and RETRANSFORMATION approaches and use jad to decopile the enhanced classes, and found the generated code are the same , even more confused now

[raphw]

This is unfortunately a bug in the JDK which forgets reconstruction of parameter names in some builds.

The difference is wether Instrumentation.redefineClasses or Instrumentation.retransformeClasses is used.

[raphw]

It's only the call to the VM. Since Java 6, retransformation is the preferred way.

[yangrubing]

Very helpful, thanks a lot, you are doing a great job👍

[yangrubing]

Sorry to reopen this again because I am more confused now , here is my code

The param method is my target method , and my ResettableClassFileTransformer

My interceptor class

And I dumped class generated by bytebuddy by using -Dnet.bytebuddy.dump , and I got two class in my directory

And then I executed javap -v for both classes, and both the class file don't have MethodParameters table in the output

Even the original class doesn't have MethodParameters

And I can confirm I compiled the class with -parameters as the class generated by my IDE indicated the MethodParameters exists as show below .

So could you please help make this clear, why even the origin class don;t have MethodParameters in the class file and how to fix that , thank you in advance. BTW . I wrote a small java agent use javaassit API and it works OK without lost the parameters with the same JDK build

[raphw]

This is a JVM bug. In Mockito, we work around it like this: https://github.com/mockito/mockito/blob/757b37a1445be3abd08238c0da62a9a9e635f1ba/src/main/java/org/mockito/internal/creation/bytebuddy/InlineBytecodeGenerator.java#L398

[yangrubing]

Thanks , tried the solution and it works now ,BTW , will this issue be fixed in later bytebuddy release ?

[raphw]

This is already fixed in newer JVM versions, I hope this just dies off.

[yangrubing]

Thanks.