JAHA (JAva Hooking Agent) is a Java library for building Java agents that hook methods at runtime while preserving access to original method behavior.

• Java 8 (project targets Java 8 for compatibility)
• Zig installed (required for cross-platform building)

Below is an example of hooking Integer.toString().

import java.lang.instrument.Instrumentation;

import me.exeos.jaha.Jaha;

public final class AgentMain {

    public static void premain(String args, Instrumentation inst) {
        // Register hook classes
        Jaha.register(ToStringHook.class);

        // Apply hooks
        Jaha.load(inst);
    }
}

import me.exeos.jaha.Jaha;
import me.exeos.jaha.annotations.Apply;
import me.exeos.jaha.annotations.Hook;

@Hook(target = "java/lang/Integer")
public class ToStringHook {

    @Apply
    public String toString() {
        String original = (String) Jaha.callOriginalObjectMethod();
        return "This has been hooked: " + original;
    }
}

Let's say you attach the example to this Program:

import java.util.Random;

public class Main {

    public static void main(String[] args) {
        System.out.println(new Integer(new Random().nextInt()));
    }
}

The result would be:

This has been hooked: -1604796949

JAHA hooks methods by rewriting bytecode using Instrumentation.

You register classes annotated with @Hook(target = "...") using Jaha.register(...).

Each hook class provides replacement methods annotated with @Apply

Jaha.applyHooks(inst) installs a ClassFileTransformer.

When a target class is transformed, JAHA:

• parses the target bytecode
• finds matching @Apply methods in the hook source
• replaces target method bodies with hook method bodies

Before replacing a method, JAHA clones the original implementation into an internal runtime container Class.

This copy is what “original method” calls are routed to.

Inside hook methods, calls like:

• Jaha.callOriginalObjectMethod(...)
• Jaha.callOriginalIntMethod(...)
• etc.

are placeholders. During transformation, JAHA rewrites these calls to invoke the cloned original implementation.

• Jaha.applyHooks(Instrumentation inst)
  Applies all registered hooks

• register(Class<?> hookSource)
  Registers a hook class

• callOriginalVoidMethod(Object... params)
  Calls original method when return type is void.

• callOriginalObjectMethod(Object... params)
  Calls original method when return type is an object. (cast the result)

• callOriginalByteMethod(Object... params)
  Calls original method when return type is byte.

• callOriginalShortMethod(Object... params)
  Calls original method when return type is short.

• callOriginalIntMethod(Object... params)
  Calls original method when return type is int.

• callOriginalLongMethod(Object... params)
  Calls original method when return type is long.

• callOriginalFloatMethod(Object... params)
  Calls original method when return type is float.

• callOriginalDoubleMethod(Object... params)
  Calls original method when return type is double.

• callOriginalBooleanMethod(Object... params)
  Calls original method when return type is boolean.

• callOriginalCharMethod(Object... params)
  Calls original method when return type is char.

This project is LICENSED under the GPL-3.0.