intercept field access to log with bytebuddy

Question

I am trying to log field writes with bytebuddy. After reading some earlier posts, I started using MemberSubstitution and got something going using the following code:

private static Method FIELD_INTERCEPTOR = // reflective reference to interceptFieldWrite
AsmVisitorWrapper VISITOR = MemberSubstitution.relaxed()
  .field(ElementMatchers.any())
  .onWrite()
  .replaceWith(FIELD_INTERCEPTOR)
  .on(ElementMatchers.isMethod());
..
public static void interceptFieldWrite(Object object,Object value) {
   System.out.println("intercepted field write in object " + object + " , attempt to set value to " + value);
}
..

The part I am struggling with is how to pass a reference to the field for which the access is intercepted to interceptFieldWrite (as string or instance of Field). If possible I would of course like to avoid reflection completely. I don't actually want to completely substitute field access, but just want to add a method with some checks just before it takes place. Is there a feature in bytebuddy to do this, or do I have to use something more low-level than ASM to achieve this ?

Answer 1

Byte Buddy offers this but you will have to compose your own StackManipulations to do so. The mechanism in MemberSubstitution is called replaceWithChain. Here you specify Steps where each step can do what you intend:

• invoke a method via MethodInvocation.
• write a field via FieldAccessor.

You will have to load the arguments to the method call and the field access prior to using the above stack manipulations via the MethodVariableAccess where the targeted element's offsets are represented by offsets.

In your case, this would require to read the target instance via

MethodVaribaleAccess.of(parameters.get(0)).loadFrom(offsets.get(0));
MethodVaribaleAccess.of(parameters.get(1)).loadFrom(offsets.get(1));

and the to execute the method or field write in question. The targeted field will be passed as target, you can cast it to FieldDescription if you only ever intercept fields.

Make sure you only intercept non-static fields where the this instance will not be passed.