Customizable advice dispatcher

AFAIK, there are currently two ways advices are dispatched.

By default, advice methods are inlined in to the instrumented method.

public class InlinedAdvice {
    @Advice.OnMethodEnter
    public static void onFoo(@Advice.Argument(0) boolean bar) {
        System.out.println("foo: " + bar);
    }
}

public class InstrumentedClass {
    public void foo(boolean bar) {
        System.out.println("foo: " + bar);
    }
}

If the inline property is set to false, only a static method call is inserted into the instrumented method that calls the advice method.

public class DelegatedAdvice {
    @Advice.OnMethodEnter(inline = false)
    public static void onFoo(@Advice.Argument(0) boolean bar) {
        System.out.println("foo: " + bar);
    }
}


public class InstrumentedClass {
    public void foo(boolean bar) {
        InlinedAdvice.onFoo(bar);
    }
}

I’d like to have a way to customize the dispatching.

public class CustomDelegationAdvice {
    @Advice.OnMethodEnter(inline = false)
    @Advice.Dispatcher(CustomDispatcher.class)
    public static void onFoo(@Advice.Argument(0) boolean bar) {
        System.out.println("foo: " + bar);
    }
}


public class InstrumentedClass {
    public void foo(boolean bar) {
        MethodHandleDispatcher
            .getMethodHandle(getClass().getClassLoader(), "CustomDelegationAdvice#onFoo")
            .invoke(bar);
    }
}

MethodHandleDispatcher is a registry where I would make sure to add method handles to all advice methods. Currently, I do that style of dispatching manually by creating an inlined advice that contains the MethodHandle lookup and invocation. It would be great if I could just write the advice as it was a inline = true advice and if Byte Buddy could inject the method handle lookup and invocation in a similar way that it currently does for non-inlined advices via net.bytebuddy.asm.Advice.Dispatcher.Delegating.

This is in the context of this PR: https://github.com/elastic/apm-agent-java/pull/1090

Is that something that would be possible?

[raphw]

I’ll look into the possibility of adding additional semantics to a method return value.

As for returning null: the bootstrap mechanism does not support this. You can however bind a constant call site that drops all arguments and returns null, a primitive 0 or nothing depending on the advice methods return type. Have a look at the lookup API for creating such a constant handle.

[raphw]

Any constructor of an instance of CallSite can be used as a bootstrap method. You could for example subclass constant call site for doing so.

And yes, the bootstrap method needs to be visible (and exported) . Otherwise you could breach into protected code using invokedynamic.