Writing a Patch

Let's assume that you've figured out all the boilerplate, or automated it with Lillero-processor. If you are wondering how to use that, refer to the project's README. I don't particularly wish to maintain a second independent copy of that information.

Take the following example:

private int counter = 0;
public void incrementCounter() {
    this.counter++;
}

Assume that counter is not directly incremented anywhere, and all calls pass through the method. Your task is to break the counter, and ensure it stays zero.

Again, you've already written your boilerplate: all that's left is the actual injection method. You have a ClassNode and a MethodNode; you probably don't need the ClassNode at all. How do you modify it, though? You should know that method.instructions is an InsnList, which means you can manipulate it freely. One such way to do it (and really the only one you need in almost all tasks) is to insert new instruction nodes.

Look at your code: in this case, with this assumption, the easiest way to achieve your task is obviously to return early.

public void inject(ClassNode clazz, MethodNode method) {
	method.instructions.insert(new InsnNode(RET));
}

The insert method added RET (which is equivalent to return without values) right at the start, not having specificed a position. While javac would refuse to compile a method like this one because it creates unreachable code, the bytecode sequence it would produce is actually perfectly valid; thus, using Lillero to create is perfectly valid. This is not the first discrepancy you will encounter between what javac wants you to do and what you actually can do.

Unfortunately, most patches are not as straightforward.