java agent的使用【通俗易懂版】

一、静态代理Agent
1．生成Agent的jar包
（1）创建Agent项目，引入javassist.jar包
（2）编写premain方法

import java.lang.instrument.Instrumentation;

public class Agent1 {

		 
    public static void premain(String agentArgs, Instrumentation inst) {
        System.out.println("agentArgs : " + agentArgs);
        MyAgentTransfer trans = new MyAgentTransfer();
        MyAgentTransferboot trans_boot = new MyAgentTransferboot();
        inst.addTransformer(trans, true);
        inst.addTransformer(trans_boot, true);
    }
	
}

（3）创建MyAgentTransferboot类，实现ClassFileTransformer接口，重写transform方法

import java.lang.instrument.ClassFileTransformer;
import java.lang.instrument.IllegalClassFormatException;
import java.security.ProtectionDomain;
import javassist.CannotCompileException;
import javassist.ClassPool;
import javassist.CodeConverter;
import javassist.CtClass;
import javassist.CtMethod;
import javassist.NotFoundException;
import javassist.expr.ExprEditor;
import javassist.expr.MethodCall;

public class MyAgentTransferboot implements ClassFileTransformer{
	
	/** ClassLoader loader: 类加载器，用于加载当前类的类。 String className: 正在加载的类的名称（用斜杠分隔的形式，例如 java/lang/String）。
		Class<?> classBeingRedefined: 如果这个类正在被重新定义，则为其对应的 Class 对象；否则为 null。 ProtectionDomain protectionDomain: 类的保护域，提供关于类的安全权限的信息。
		byte[] classfileBuffer: 原始的字节码数组。 */
		@Override
		public byte[] transform(ClassLoader loader, String className,
            Class<?> classBeingRedefined, ProtectionDomain protectionDomain,
            byte[] classfileBuffer) throws IllegalClassFormatException {
        byte[] transformed = null;  
//        System.out.println("Transforming className::" + className);  
		if(!className.contains("TaxController") || !className.contains("")) {
        	return classfileBuffer;
        }
        ClassPool pool = null;  
        CtClass cl = null;  
        try {  
            pool = ClassPool.getDefault();
            cl = pool.makeClass(new java.io.ByteArrayInputStream(classfileBuffer));  
//            CtMethod aop_method = pool.get("com.jdktest.instrument.AopMethods").
//                    getDeclaredMethod("aopMethod");
//            System.out.println(aop_method.getLongName());
            CodeConverter convert = new CodeConverter();
             if (cl.isInterface() == false) {  
                CtMethod[] methods = cl.getDeclaredMethods();  
                for (int i = 0; i < methods.length; i++) {  
                    if (methods[i].isEmpty() == false) {  
                       // AOPInsertMethod(methods[i]);  
                        if(methods[i].getName().contains("getUrl") ) {
                                                	System.out.println("methods[i].getName::" + methods[i].getName());  
                        	String name =  methods[i].getName();
                        	methods[i].insertAt(92, "System.out.println(\"---------------Url:\" + Url);");
//                        	methods[i].insertBefore(" System.out.println(\"TTTTTTTTTTTTTTTTTT**********************************************" +name + "\");");
//                        	methods[i].insertBefore(" return \"100\";");
                        }
                    }  
                }  
                transformed = cl.toBytecode();  
            }  
        } catch (Exception e) {  
            System.err.println("Could not instrument  " + className  
                    + ",  exception : " + e.getMessage());  
        } finally {  
            if (cl != null) {  
                cl.detach();  
            }  
        }  
        return transformed;  
    }
    private void AOPInsertMethod(CtMethod method) throws NotFoundException,CannotCompileException {
        method.instrument(new ExprEditor() {  
            @Override
			public void edit(MethodCall m) throws CannotCompileException {  
                m.replace("{ long stime = System.currentTimeMillis(); $_ = $proceed($$);System.out.println(\""
                        + m.getClassName() + "." + m.getMethodName()
                        + " cost:\" + (System.currentTimeMillis() - stime) + \" ms\");}");
            }
        }); 
//      method.insertBefore("System.out.println(\"enter method\");");
//      method.insertAfter("System.out.println(\"leave method\");");
    }

（4）将Agent项目进行打包

（5）用压缩包方式打开生成的jar包，打开\META-INF\MANIFEST.MF文件，添加如下代码：

Can-Redefine-Classes: true
Can-Retransform-Classes: true
Premain-Class: com.javaAgent.Agent1

2．在项目中使用agent

命令：-javaagent:jar包地址=参数
如：-javaagent:D:\Work_Code\javaAgent\StringLenAgent.jar=‘1’
-javaagent:D:\Work_Code\javaAgent\StringLenAgent.jar

二、动态代理Attach
1．生成Attach的jar包
（1）创建Attach项目，引入javassist.jar包、com.sun.tools.attach-1.8.0_jdk8u172-b11.jar包
（2）编写agentmain方法

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

public class Attach1 {
	public static void agentmain(String attachArgs, Instrumentation instrumentation) {
		System.out.println("attach agentmain启动");
		System.out.println("attachArgs : " + attachArgs);
        Class<?>[] classes = instrumentation.getAllLoadedClasses();
        for (Class<?> cls : classes) {
            String className = cls.getName();
            if(className.contains("TaxController")) {
                CtClass ctClass;
                try {
                    ClassPool classPool = ClassPool.getDefault();
                    ctClass = classPool.get(className);
                    CtMethod[] methods = ctClass.getDeclaredMethods();  
                    for (int i = 0; i < methods.length; i++) {  
                        if (methods[i].isEmpty() == false) {  
                           // AOPInsertMethod(methods[i]);  
                            if(methods[i].getName().contains("getUrl") ) {
                            	System.out.println("methods[i].getName::" + methods[i].getName());  
                            	String name =  methods[i].getName();
                            	methods[i].insertAt(92, "System.out.println(\"---------------Url:\" + Url);");
//                            	methods[i].insertBefore(" System.out.println(\"TTTTTTTTTTTTTTTTTT**********************************************" +name + "\");");
//                            	methods[i].insertBefore(" return \"100\";");
                            }
                        }  
                    }  
                    ClassDefinition classDefinition = new ClassDefinition(cls, ctClass.toBytecode());
                    instrumentation.redefineClasses(classDefinition);
                } catch (Throwable e) {
                    System.out.println(e.getMessage());
                    e.printStackTrace();
                }
            }
          

（3）将Attach项目进行打包
（4）用压缩包方式打开生成的jar包，打开\META-INF\MANIFEST.MF文件，添加如下代码：
Agent-Class: com.javaAttach.Attach1
Can-Redefine-Classes: true
Can-Retransform-Classes: true
（5）启动需被代理的应用程序
编写main方法，给需被代理的应用程序添加代理

import java.util.concurrent.TimeUnit;
import com.sun.tools.attach.*;

public class AttachUseMain {
	public static void main(String[] args) throws Exception{
        System.out.println("AttachUseMain启动");
        long start = System.currentTimeMillis();
		System.out.println(start);
        for (VirtualMachineDescriptor vmd : VirtualMachine.list()) {
            String name = vmd.displayName();
            System.out.println(name + ":" + vmd.id());
            if (name.contains("Application")) {
                VirtualMachine vm = VirtualMachine.attach(vmd.id());
//                vm.loadAgent("D:\\Work_Code\\javaAgent\\StringLenAttach.jar=hello,ok,aa,bb,cc=3");
                vm.loadAgent("D:\\Work_Code\\javaAgent_1\\javaAttach\\StringLenAttach004.jar=22");
                TimeUnit.MINUTES.sleep(1);
                vm.detach();
                System.out.println("Agent 已成功附加到目标 JVM");
            }
        }
        
        long end = System.currentTimeMillis();
		System.out.println(end - start);
//        String targetPid = "25304"; //jps -l
//        // 指定要附加的 Agent JAR 路径
//        String agentPath = "D:\\Work_Code\\javaAgent\\StringLenAttach.jar";
//
//        // 附加到目标 JVM
//        VirtualMachine vm = VirtualMachine.attach(targetPid);
//        // 加载 Agent
//        vm.loadAgent(agentPath);
//        // 断开连接
//        vm.detach();
        System.out.println("over");
    }
}