Using Java Annotations - Generating Code

Question

I\'m using java 6 annotation processing api. I have followed the following excellent tutorial for creating an annotation processor that displays a message at build-time:

Answer 1

I don't believe Java supports runtime type mutation, meaning to modify the members on a given class you'd have to drop back to a compile time preprocessor or to a bytecode modification scheme.

I'd be able to point you in a better direction if I understood the "why" behind this question, but in the mean time, dynamic proxy classes might get you to where you want to be (JavaWorld article).

From the documentation:

A dynamic proxy class is a class that implements a list of interfaces specified at runtime such that a method invocation through one of the interfaces on an instance of the class will be encoded and dispatched to another object through a uniform interface. Thus, a dynamic proxy class can be used to create a type-safe proxy object for a list of interfaces without requiring pre-generation of the proxy class, such as with compile-time tools. Method invocations on an instance of a dynamic proxy class are dispatched to a single method in the instance's invocation handler, and they are encoded with a java.lang.reflect.Method object identifying the method that was invoked and an array of type Object containing the arguments.

Here's a decent tutorial on using Spring to inject dynamic proxies based on custom annotations. I think this is probably closest to the behavior you're after.

Answer 2

If you want runtime modification of you classes, you can use your own classloader and intercept loading of classes, introspect what you want and generate new bytecode using asm library instead of original classes. It is not very tricky, but you must be sure you need exactly that.