Agent.java

package agent;

import org.objectweb.asm.ClassReader;
import org.objectweb.asm.ClassWriter;

import java.lang.instrument.ClassFileTransformer;
import java.lang.instrument.IllegalClassFormatException;
import java.lang.instrument.Instrumentation;
import java.security.ProtectionDomain;

public class Agent {

    public static void premain(String agentArgs, Instrumentation inst) {
        inst.addTransformer(new ClassFileTransformer() {
            @Override
            public byte[] transform(ClassLoader classLoader, String s, Class<?> aClass, ProtectionDomain protectionDomain, byte[] bytes) throws IllegalClassFormatException {

                if ("other/Stuff".equals(s)) {
                    // ASM Code
                    ClassReader reader = new ClassReader(bytes);
                    ClassWriter writer = new ClassWriter(reader, 0);
                    ClassPrinter visitor = new ClassPrinter(writer);
                    reader.accept(visitor, 0);
                    return writer.toByteArray();

                    // Javassist
//                    try {
//                        ClassPool cp = ClassPool.getDefault();
//                        CtClass cc = cp.get("other.Stuff");
//                        CtMethod m = cc.getDeclaredMethod("run");
//                        m.addLocalVariable("elapsedTime", CtClass.longType);
//                        m.insertBefore("elapsedTime = System.currentTimeMillis();");
//                        m.insertAfter("{elapsedTime = System.currentTimeMillis() - elapsedTime;"
//                                + "System.out.println(\"Method Executed in ms: \" + elapsedTime);}");
//                        byte[] byteCode = cc.toBytecode();
//                        cc.detach();
//                        return byteCode;
//                    } catch (Exception ex) {
//                        ex.printStackTrace();
//                    }
                }

                return null;
            }
        });
    }

}