Bytecode modifications at load time with Javassist Bytecode

[fabienbaligand]

Hi! I'm currently trying to perform some bytecode modifications on a class while loading this class in the JVM:

Usually, Javassist users are likely to inject source code into a CtClass but my concern is to inject code from a compiled class A into compiled class B. Plus I'd like to do so while loading Class B into the JVM (Class A doesn't require to be loaded into the JVM) through a ClassLoader of mine.

My first idea was to modify the findClass(..) method of my ClassLoader: I would intercept the moment class B is loaded and I could get its methods as CtMethods...Then I would use the Javassist Bytecode level API to obtain a ClassFile describing Class A. Thus I could use a CodeIterator to get some compiled code from Class A to finally inject it into some CtMethods of Class B. Eventually Class B would be loaded into the JVM and would behave with some code that belonged to A.

Unfortunately I don't know how to inject the byte[] obtained from Class A bytecode into any CtMethod of Class B. Maybe we just can't?!

I would really appreciate if you could tell me how to do such a thing and/or showing a piece of code would be great. If my approach is wrong, could you tell me how to proceed

Thank you very much!

-Fabien

[chiba]

If you want to copy a whole method body, you can simply add
a CtMethod from class A to class B.

If you want to insert only part of a method body, you must use
CodeIterator. For example,

CtClass b = ... ;
CtMethod m = b.getMethod(...);
MethodInfo minfo = m.getMethodInfo();
CodeAttribute ca = minfo.getCodeAttribute();
CodeIterator it = ca.iterator();
it.insert(position, code);

code specifies a byte array containing the bytecode that you want
to inject. For details, please look at the last section of the tutorial
of version 3.

Is this what you need?

[fabienbaligand]

Thank you Shiba,
I think I should be fine with your explanations.