Java Agent入门实战（三）-JVM Attach原理与使用

public class JpAgent {
    public static void premain(String agentArgs){
        System.out.println("我是一个参数的 Java Agent premain");
    }
    public static void agentmain (String agentArgs, Instrumentation inst) throws UnmodifiableClassException {
        inst.addTransformer(new JpClassFileTransformerDemo(), true);
        // retransformClasses 是 Java SE 6 里面的新方法，它跟 redefineClasses 一样，可以批量转换类定义
        inst.retransformClasses(Dog.class);
        System.out.println("我是两个参数的 Java Agent agentmain");
    }
    public static void agentmain (String agentArgs){
        System.out.println("我是一个参数的 Java Agent agentmain");
    }
}

package cn.jpsite.learning;
public class Dog {
}

public class WhileMain {
    public static void main(String[] args) throws InterruptedException {
        System.out.println(new Dog().say());
        int count = 0;
        while (true) {
            // 等待0.5秒
            Thread.sleep(500);
            count++;
            String say = new Dog().say();
            // 输出内容和次数
            System.out.println(say + count);
            // 内容不对或者次数达到1000次以上输出
            if (!"dog".equals(say) || count >= 1000) {
                System.out.println("有人偷了我的狗！");
                //break;
            }
        }
    }
}

// 这是一个修改后编译的.class文件，单独存放
public class Dog {
    public String say() {
        return "cat";
    }
}

Agent-Class: cn.jpsite.learning.javaagent01.JpAgent
Can-Retransform-Classes: true

java -javaagent:jpAgent.jar -cp example01-1.0-SNAPSHOT.jar cn.jpsite.learning.Main
java -javaagent:jpAgent.jar -cp example01-1.0-SNAPSHOT.jar cn.jpsite.learning.WhileMain
java -cp example01-1.0-SNAPSHOT.jar cn.jpsite.learning.WhileMain

import com.sun.tools.attach.AttachNotSupportedException;
import com.sun.tools.attach.VirtualMachine;
import com.sun.tools.attach.VirtualMachineDescriptor;
import java.io.IOException;
import java.util.List;
import java.util.Objects;
/**
 *
 * @author jiangpeng
 * @date 2019/12/1 0001
 */
public class AttachThread extends Thread {
    /**
     * 记录程序启动时的 VM 集合
     */
    private final List<VirtualMachineDescriptor> listBefore;
    /**
     要加载的agent.jar
     */
    private final String jar;
    private AttachThread(String attachJar, List<VirtualMachineDescriptor> vms) {
        listBefore = vms;
        jar = attachJar;
    }
    @Override
    public void run() {
        VirtualMachine vm;
        List<VirtualMachineDescriptor> listAfter;
        int count = 0;
        try {
            while (true) {
                listAfter = VirtualMachine.list();
                vm = hasNewVm(listAfter);
                if(vm == null){
                    System.out.println("没有新jvm程序，请手动指定java pid");
                    try{
                        vm = VirtualMachine.attach("7716");
                    }catch (AttachNotSupportedException e){
                        //System.out.println("拒绝访问 Disconnected from the target VM");
                    }
                }
                Thread.sleep(1000);
                System.out.println(count++);
                if (null != vm || count >= 100) {
                    break;
                }
            }
            Objects.requireNonNull(vm).loadAgent(jar);
            vm.detach();
        } catch (Exception e) {
            System.out.println("异常：" + e);
        }
    }
    /**
     * 判断是否有新的 JVM 程序运行
     */
    private VirtualMachine hasNewVm(List<VirtualMachineDescriptor> listAfter) throws IOException, AttachNotSupportedException {
        for (VirtualMachineDescriptor vmd : listAfter) {
            if (!listBefore.contains(vmd)) {
                // 如果 VM 有增加，，我们开始监控这个 VM
                System.out.println("有新的 vm 程序："+ vmd.displayName());
                return VirtualMachine.attach(vmd);
            }
        }
        return null;
    }
    public static void main(String[] args)  {
        new AttachThread("D:/learning/jpAgent.jar", VirtualMachine.list()).start();
    }
}

Can-Set-Native-Method-Prefix
System-Class-Path
Boot-Class-Path