Premain-Class: com.example.MyAgent

public class MyAgent {
    public static void premain(String agentArgs, Instrumentation inst) {
        System.out.println("Agent loaded!");
        inst.addTransformer(new MyTransformer());
    }
}

public class MyTransformer implements ClassFileTransformer {
    @Override
    public byte[] transform(
        ClassLoader loader, String className, Class<?> classBeingRedefined,
        ProtectionDomain protectionDomain, byte[] classfileBuffer
    ) {
        // 可以用 ASM/Javassist 修改字節(jié)碼
        if (className.equals("com/example/TargetClass")) {
            // 返回修改后的字節(jié)碼
        }
        return null; // 返回 null 表示不修改
    }
}

java -javaagent:myagent.jar -jar myapp.jar

VirtualMachine vm = VirtualMachine.attach("pid");
vm.loadAgent("myagent.jar");

import java.lang.instrument.Instrumentation;
public class PerformanceMonitorAgent {
    public static void premain(String agentArgs, Instrumentation inst) {
        System.out.println("PerformanceMonitorAgent loaded.");
        inst.addTransformer(new PerformanceTransformer());
    }
}

import java.lang.instrument.ClassFileTransformer;
import java.security.ProtectionDomain;
public class PerformanceTransformer implements ClassFileTransformer {
    @Override
    public byte[] transform(
        ClassLoader loader,
        String className,
        Class<?> classBeingRedefined,
        ProtectionDomain protectionDomain,
        byte[] classfileBuffer) {
        // 只監(jiān)控特定包下的類（比如com/example/）
        if (className != null && className.startsWith("com/example/")) {
            // 使用ASM或Javassist修改字節(jié)碼，在方法前后插入耗時(shí)統(tǒng)計(jì)代碼
            try {
                return MethodTimer.injectTimer(classfileBuffer);
            } catch (Exception e) {
                e.printStackTrace();
            }
        }
        return classfileBuffer;
    }
}

import javassist.*;
public class MethodTimer {
    public static byte[] injectTimer(byte[] classfileBuffer) throws Exception {
        ClassPool pool = ClassPool.getDefault();
        CtClass ctClass = pool.makeClass(new java.io.ByteArrayInputStream(classfileBuffer));
        for (CtMethod method : ctClass.getDeclaredMethods()) {
            if (!method.isEmpty() && !method.getName().equals("<init>")) {
                method.addLocalVariable("_startTime", CtClass.longType);
                method.insertBefore("_startTime = System.nanoTime();");
                method.insertAfter(
                    "System.out.println(\"[Performance] Method " + method.getName() +
                    " executed in \" + (System.nanoTime() - _startTime) + \" ns.\");"
                );
            }
        }
        byte[] byteCode = ctClass.toBytecode();
        ctClass.detach();
        return byteCode;
    }
}

package com.example;
public class Demo {
    public void test() {
        try { Thread.sleep(100); } catch (Exception e) {}
    }
    public static void main(String[] args) {
        new Demo().test();
    }
}

javac -cp javassist.jar:. PerformanceMonitorAgent.java PerformanceTransformer.java MethodTimer.java
jar cmf MANIFEST.MF perfagent.jar PerformanceMonitorAgent.class PerformanceTransformer.class MethodTimer.class

Premain-Class: PerformanceMonitorAgent
Can-Redefine-Classes: true
Can-Retransform-Classes: true

java -javaagent:perfagent.jar -cp javassist.jar:. com.example.Demo