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Java Language Modifying classes with agents


Firstly, make sure that the agent being used has the following attributes in the Manifest.mf:

Can-Redefine-Classes: true
Can-Retransform-Classes: true

Starting a java agent will let the agent access the class Instrumentation. With Instrumentation you can call addTransformer(ClassFileTransformer transformer). ClassFileTransformers will let you rewrite the bytes of classes. The class has only a single method which supplies the ClassLoader that loads the class, the class's name, a java.lang.Class instance of it, it's ProtectionDomain, and lastly the bytes of the class itself.

It looks like this:

byte[] transform(ClassLoader loader, String className, Class<?> classBeingRedefined, 
          ProtectionDomain protectionDomain, byte[] classfileBuffer)

Modifying a class purely from bytes can take ages. To remedy this there are libraries that can be used to convert the class bytes into something more usable.

In this example I'll be using ASM, but other alternatives like Javassist and BCEL have similar features.

ClassNode getNode(byte[] bytes) {
    // Create a ClassReader that will parse the byte array into a ClassNode
    ClassReader cr = new ClassReader(bytes);
    ClassNode cn = new ClassNode();
    try {
        // This populates the ClassNode
        cr.accept(cn, ClassReader.EXPAND_FRAMES);
        cr = null;
    } catch (Exception e) {
    return cn;

From here changes can be made to the ClassNode object. This makes changing field/method access incredibly easy. Plus with ASM's Tree API modifying the bytecode of methods is a breeze.

Once the edits are finished you can convert the ClassNode back into bytes with the following method and return them in the transform method:

public static byte[] getNodeBytes(ClassNode cn, boolean useMaxs) {
    ClassWriter cw = new ClassWriter(useMaxs ? ClassWriter.COMPUTE_MAXS : ClassWriter.COMPUTE_FRAMES);
    byte[] b = cw.toByteArray();
    return b;