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上一篇文章不停机不更新代码线上调试BUG的工具
教会了大家如果使用arthas是定位系统线上问题
这篇文章教大家关于arthas的原理
arthas其实也是利用是了java agent。
这个是jvm提供出来的接口，在类加载之前会触发一个方法，让大家自定义自己想要切入的业务；

如何写一个自己的java agent

1、新建一个springboot的工程，用来模拟自己的app应用，应用每5秒会调用一个方法

package com.eujian.arthaslearn.controller;

public class MyService {

    public String send(){
        System.out.println("send被调用了");
        try {
            Thread.sleep(1000);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        return "send";
    }
}

github地址 https://github.com/hd-eujian/arthas-learn.git
码云地址 https://gitee.com/guoeryyj/arthas-learn.git

2、新建 agent工程，打包成jar包，去做class文件的aop
pom文件引入依赖

 <build>
        <plugins>
            <plugin>
                <groupId>org.apache.maven.plugins</groupId>
                <artifactId>maven-assembly-plugin</artifactId>
                <configuration>
                    <descriptorRefs>
                        <descriptorRef>jar-with-dependencies</descriptorRef>
                    </descriptorRefs>
                    <archive>
                        <manifestEntries>
                            <Premain-Class>com.eujian.agent.PreMainTraceAgent</Premain-Class>
                            <Agent-Class>com.eujian.agent.PreMainTraceAgent</Agent-Class>
                            <Can-Redefine-Classes>true</Can-Redefine-Classes>
                            <Can-Retransform-Classes>true</Can-Retransform-Classes>
                        </manifestEntries>
                    </archive>
                </configuration>

            </plugin>
        </plugins>
    </build>

新建文件PreMainTraceAgent

package com.eujian.agent;

import java.lang.instrument.Instrumentation;

public class PreMainTraceAgent {
    public static void agentmain (String agentArgs, Instrumentation inst) throws Exception {
        System.out.println("agent begin agentArgs="+agentArgs);
        MyClassFileTransformer myClassFileTransformer;

        //如果入参是1就清除aop
        if("1".equals(agentArgs)){
            myClassFileTransformer = new MyClassFileTransformer(true);
            inst.addTransformer(myClassFileTransformer,true);
        }else {
            myClassFileTransformer = new MyClassFileTransformer();
            inst.addTransformer(myClassFileTransformer,true);
        }
        System.out.println("agent end");

        Class[] allLoadedClasses = inst.getAllLoadedClasses();
        for (Class clazz : allLoadedClasses){
            if(clazz.getName().contains("com.eujian.arthaslearn.controller.MyService")){
                inst.retransformClasses(clazz);
                System.out.println("重新加载"+clazz);
            }
        }
    }
}

新建文件MyClassFileTransformer实现接口ClassFileTransformer

package com.eujian.agent;

import javassist.ClassPool;
import javassist.CtClass;
import javassist.CtMethod;

import java.io.ByteArrayOutputStream;
import java.io.IOException;
import java.io.InputStream;
import java.lang.instrument.ClassFileTransformer;
import java.lang.instrument.IllegalClassFormatException;
import java.security.ProtectionDomain;

public class MyClassFileTransformer implements ClassFileTransformer {

    private boolean isReLoad = false;

    public MyClassFileTransformer() {
    }

    public MyClassFileTransformer(boolean isReLoad) {
        this.isReLoad = isReLoad;
    }

    @Override
    public byte[] transform(ClassLoader loader, String className, Class<?> classBeingRedefined, ProtectionDomain protectionDomain, byte[] classfileBuffer) throws IllegalClassFormatException {

        if (!className.equals("com/eujian/arthaslearn/controller/MyService")) {
            return null;
        }
        try {

            System.out.println("进入 isReLoad:"+isReLoad);
            System.out.println("进入 className:"+className);
            System.out.println("进入 loader:"+loader);
            System.out.println("进入 classBeingRedefined:"+classBeingRedefined);
            CtClass cl = null;
            ClassPool classPool = ClassPool.getDefault();
            cl = classPool.getCtClass("com.eujian.arthaslearn.controller.MyService");

            System.out.println("cl.isFrozen()+"+cl.isFrozen());
            if(isReLoad){
                //重新加载本地class文件
                cl.defrost();
                return readStream(ClassLoader.getSystemResourceAsStream(className.replace('.', '/') + ".class"), true);
            }
            CtMethod method = cl.getDeclaredMethod("send");

            //插入你想要的代码
            method.insertBefore("System.out.println(\"send-begin\");");
            method.insertAfter("System.out.println(\"send-end\");");

            byte[] transformed = cl.toBytecode();

            cl.detach();
            return transformed;

        }catch (Exception e){
            e.printStackTrace();
        }
        return null;
    }
    private static byte[] readStream(InputStream inputStream, boolean close) throws IOException {
        if(inputStream == null) {
            throw new IOException("Class not found");
        } else {
            try {
                ByteArrayOutputStream outputStream = new ByteArrayOutputStream();
                byte[] data = new byte[4096];

                int bytesRead;
                while((bytesRead = inputStream.read(data, 0, data.length)) != -1) {
                    outputStream.write(data, 0, bytesRead);
                }

                outputStream.flush();
                byte[] var5 = outputStream.toByteArray();
                return var5;
            } finally {
                if(close) {
                    inputStream.close();
                }

            }
        }
    }
}

执行命令打包mvn clean assembly:assembly
github 地址：https://github.com/hd-eujian/agent.git
码云地址： https://gitee.com/guoeryyj/agent.git

新建一个main函数的工程，工程主要作用是把打包的jar包注入到运行的app中
代码如下

import com.sun.tools.attach.VirtualMachine;
import com.sun.tools.attach.VirtualMachineDescriptor;

import java.util.List;

public class Main {

    public static void main(String[] args) throws Exception{
        System.out.println("running JVM start ");
        String jarUrl = "/xxx/agent/target/agent-1.0-SNAPSHOT-jar-with-dependencies.jar";
        List<VirtualMachineDescriptor> list = VirtualMachine.list();
        for (VirtualMachineDescriptor vmd : list) {
            if (vmd.displayName().contains("ArthasLearnApplication")) {
                VirtualMachine virtualMachine = VirtualMachine.attach(vmd.id());

                //这个是加载jar包增强class文件
                virtualMachine.loadAgent(jarUrl);

                //这个是恢复class文件
//                virtualMachine.loadAgent(jarUrl,"1");
                virtualMachine.detach();
            }
        }
    }
}

实操环节

1、先启动arthas-learn工程

arthas-learn工程

2、agent工程打包
mvn clean assembly:assembly

3、执行main函数

注入jar包后的效果

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arthas最底层的java agent原理

arthas最底层的java agent原理

如何写一个自己的java agent

实操环节