public class MyAgent {/*** 该方法在main方法之前运行，与main方法运行在同一个JVM中* 并被同一个System ClassLoader装载* 被统一的安全策略(security policy)和上下文(context)管理*/public static void premain(String agentOps, Instrumentation inst) {System.out.println("=========premain方法执行========");System.out.println(agentOps);// 添加Transformerinst.addTransformer(new MyTransformer());}/*** 如果不存在 premain(String agentOps, Instrumentation inst) * 则会执行 premain(String agentOps)*/public static void premain(String agentOps) {System.out.println("=========premain方法执行2========");System.out.println(agentOps);}}

public class MyTransformer implements ClassFileTransformer {final static String prefix = "\nlong startTime = System.currentTimeMillis();\n";final static String postfix = "\nlong endTime = System.currentTimeMillis();\n";// 被处理的方法列表final static Map<String, List<String>> methodMap = new HashMap<String, List<String>>();public MyTransformer() {add("com.hwtest.demo.MyProgram.sayHello");add("com.hwtest.demo.MyProgram.sayHello2");}private void add(String methodString) {String className = methodString.substring(0, methodString.lastIndexOf("."));String methodName = methodString.substring(methodString.lastIndexOf(".") + 1);List<String> list = methodMap.get(className);if (list == null) {list = new ArrayList<String>();methodMap.put(className, list);}list.add(methodName);}@Overridepublic byte[] transform(ClassLoader loader, String className, Class<?> classBeingRedefined,ProtectionDomain protectionDomain, byte[] classfileBuffer) throws IllegalClassFormatException {className = className.replace("/", ".");if (methodMap.containsKey(className)) {// 判断加载的class的包路径是不是需要监控的类CtClass ctclass = null;try {ctclass = ClassPool.getDefault().get(className);// 使用全称,用于取得字节码类<使用javassist>for (String methodName : methodMap.get(className)) {String outputStr = "\nSystem.out.println(\"this method " + methodName+ " cost:\" +(endTime - startTime) +\"ms.\");";CtMethod ctmethod = ctclass.getDeclaredMethod(methodName);// 得到这方法实例String newMethodName = methodName + "$old";// 新定义一个方法叫做比如sayHello$oldctmethod.setName(newMethodName);// 将原来的方法名字修改// 创建新的方法，复制原来的方法，名字为原来的名字CtMethod newMethod = CtNewMethod.copy(ctmethod, methodName, ctclass, null);// 构建新的方法体StringBuilder bodyStr = new StringBuilder();bodyStr.append("{");bodyStr.append(prefix);bodyStr.append(newMethodName + "($$);\n");// 调用原有代码，类似于method();($$)表示所有的参数
                    bodyStr.append(postfix);bodyStr.append(outputStr);bodyStr.append("}");newMethod.setBody(bodyStr.toString());// 替换新方法ctclass.addMethod(newMethod);// 增加新方法
                }return ctclass.toBytecode();} catch (Exception e) {System.out.println(e.getMessage());e.printStackTrace();}}return null;}
}

public class MyProgram {public static void main(String[] args) {sayHello();sayHello2("hello world222222222");}public static void sayHello() {try {Thread.sleep(2000);System.out.println("hello world!!");} catch (InterruptedException e) {e.printStackTrace();}}public static void sayHello2(String hello) {try {Thread.sleep(1000);System.out.println(hello);} catch (InterruptedException e) {e.printStackTrace();}}
}

Manifest-Version: 1.0
Premain-Class: com.hwtest.MyAgent
Can-Redefine-Classes: true
Boot-Class-Path: javassist.jar

java -javaagent:MyAgent.jar  -jar MyProgram.jar=========premain方法执行========
null
hello world!!
this method sayHello cost:2000ms.
hello world222222222
this method sayHello2 cost:1000ms.