简介：作者信息：张帅，花名洵澈，负责物流表达和履约相关研发工作。热衷于中间件相关技术。
原文链接
引言
在本篇文章中，我会通过几个简单的程序来说明agent的使用，最后在实战替换我会通过asm字节码框架来实现一个小工具，用于在程序运行中采集指定方法的参数和返回值。有关asm字节码的内容不是本文的重点，不会过多的分解，不明白的同学可以自己的Google下。
简介
Java代理提供了一种在加载字节码时，对字节码进行修改的方式。他共有两种方式执行，一种是在main方法执行之前，通过premain来实现，另一种是在程序运行中，通过attach api来实现。
在介绍agent之前，先给大家简单说下一个Instrumentation。它是JDK1.5提供的API，用于拦截类加载事件，通过字节码进行修改，它的主要方法如下：
public interface Instrumentation { //注册一个转换器，类加载事件会被注册的转换器所拦截 void addTransformer(ClassFileTransformer transformer, boolean canRetransform); //重新触发类加载 void retransformClasses(Class<?>... classes) throws UnmodifiableClassException; //直接替换类的定义 void redefineClasses(ClassDefinition... definitions) throws ClassNotFoundException, UnmodifiableClassException;}主要
premain是在main方法之前运行的方法，也是最常见的agent方式。运行时需要将agent程序打包成jar包，并在启动时添加命令来执行，如下文所示：
java -javaagent:agent.jar=xunche HelloWorld
premain共提供以下2种重载方法，Jvm启动时会先尝试使用第一种方法，若没有会使用第二种方法：
public static void premain(String agentArgs, Instrumentation inst);public static void premain(String agentArgs);一个简单的例子
下面我们通过一个程序来简单说明下premain的使用，首先我们准备下测试代码，测试代码比较简单，运行main方法并输出hello world。
package org.xunche.app;public class HelloWorld { public static void main(String[] args) { System.out.println("Hello World"); }}接下来我们看下agent的代码，运行premain方法并输出我们预期的参数。
package org.xunche.agent;public class HelloAgent { public static void premain(String args) { System.out.println("Hello Agent: " + args); }}为了能够使agent能够运行，我们需要将META-INF / MANIFEST.MF文件中的Premain- Class为我们编写的agent路径，然后通过以下方式将其打包成jar包，当然你也可以使用idea直接替换jar包。
echo 'Premain-Class: org.xunche.agent.HelloAgent' > manifest.mfjavac org/xunche/agent/HelloAgent.javajavac org/xunche/app/HelloWorld.javajar cvmf manifest.mf hello-agent.jar org/接下来，我们编译下并运行下测试代码，这里为了测试简单，我将编译后的类和agent的jar包放在同级目录下
java -javaagent:hello-agent.jar=xunche org/xunche/app/HelloWorld可以看到输出结果如下，agent中的premain方法有延续main方法执行
Hello Agent: xuncheHello World稍微复杂点的例子
通过上面的例子，是否对agent有个简单的了解呢？
下面我们来看一个稍微复杂点，我们通过agent来实现一个方法监控的功能。思路大致是这样的，若是非jdk的方法，我们通过asm在方法的执行入口和执行出口处，植入几行记录最佳的代码，当方法结束后，通过合并来获取方法的耗时。
首先还是看下测试代码，逻辑很简单，main方法执行时调用say Hi方法，输出hi，xunche，并随机睡眠中断。
package org.xunche.app;public class HelloXunChe { public static void main(String[] args) throws InterruptedException { HelloXunChe helloXunChe = new HelloXunChe(); helloXunChe.sayHi(); } public void sayHi() throws InterruptedException { System.out.println("hi, xunche"); sleep(); } public void sleep() throws InterruptedException { Thread.sleep((long) (Math.random() * 200)); }}接下来我们替换asm来植入我们自己的代码，在jvm加载类的时候，为类的每个方法加上统计方法调用耗时的代码，代码如下，这里的asm我使用了jdk自带的，当然你也可以使用官方的asm类库。
package org.xunche.agent;import jdk.internal.org.objectweb.asm.*;import jdk.internal.org.objectweb.asm.commons.AdviceAdapter;import java.lang.instrument.ClassFileTransformer;import java.lang.instrument.Instrumentation;import java.security.ProtectionDomain;public class TimeAgent { public static void premain(String args, Instrumentation instrumentation) { instrumentation.addTransformer(new TimeClassFileTransformer()); } private static class TimeClassFileTransformer implements ClassFileTransformer { @Override public byte[] transform(ClassLoader loader, String className, Class<?> classBeingRedefined, ProtectionDomain protectionDomain, byte[] classfileBuffer) { if (className.startsWith("java") || className.startsWith("jdk") || className.startsWith("javax") || className.startsWith("sun") || className.startsWith("com/sun")|| className.startsWith("org/xunche/agent")) { //return null或者执行异常会执行原来的字节码 return null; } System.out.println("loaded class: " + className); ClassReader reader = new ClassReader(classfileBuffer); ClassWriter writer = new ClassWriter(reader, ClassWriter.COMPUTE_FRAMES | ClassWriter.COMPUTE_MAXS); reader.accept(new TimeClassVisitor(writer), ClassReader.EXPAND_FRAMES); return writer.toByteArray(); } } public static class TimeClassVisitor extends ClassVisitor { public TimeClassVisitor(ClassVisitor classVisitor) { super(Opcodes.ASM5, classVisitor); } @Override public MethodVisitor visitMethod(int methodAccess, String methodName, String methodDesc, String signature, String[] exceptions) { MethodVisitor methodVisitor = cv.visitMethod(methodAccess, methodName, methodDesc, signature, exceptions); return new TimeAdviceAdapter(Opcodes.ASM5, methodVisitor, methodAccess, methodName, methodDesc); } } public static class TimeAdviceAdapter extends AdviceAdapter { private String methodName; protected TimeAdviceAdapter(int api, MethodVisitor methodVisitor, int methodAccess, String methodName, String methodDesc) { super(api, methodVisitor, methodAccess, methodName, methodDesc); this.methodName = methodName; } @Override protected void onMethodEnter() { //在方法入口处植入 if ("<init>".equals(methodName)|| "<clinit>".equals(methodName)) { return; } mv.visitTypeInsn(NEW, "java/lang/StringBuilder"); mv.visitInsn(DUP); mv.visitMethodInsn(INVOKESPECIAL, "java/lang/StringBuilder", "<init>", "()V", false); mv.visitVarInsn(ALOAD, 0); mv.visitMethodInsn(INVOKEVIRTUAL, "java/lang/Object", "getClass", "()Ljava/lang/Class;", false); mv.visitMethodInsn(INVOKEVIRTUAL, "java/lang/Class", "getName", "()Ljava/lang/String;", false); mv.visitMethodInsn(INVOKEVIRTUAL, "java/lang/StringBuilder", "append", "(Ljava/lang/String;)Ljava/lang/StringBuilder;", false); mv.visitLdcInsn("."); mv.visitMethodInsn(INVOKEVIRTUAL, "java/lang/StringBuilder", "append", "(Ljava/lang/String;)Ljava/lang/StringBuilder;", false); mv.visitLdcInsn(methodName); mv.visitMethodInsn(INVOKEVIRTUAL, "java/lang/StringBuilder", "append", "(Ljava/lang/String;)Ljava/lang/StringBuilder;", false); mv.visitMethodInsn(INVOKEVIRTUAL, "java/lang/StringBuilder", "toString", "()Ljava/lang/String;", false); mv.visitMethodInsn(INVOKESTATIC, "org/xunche/agent/TimeHolder", "start", "(Ljava/lang/String;)V", false); } @Override protected void onMethodExit(int i) { //在方法出口植入 if ("<init>".equals(methodName) || "<clinit>".equals(methodName)) { return; } mv.visitTypeInsn(NEW, "java/lang/StringBuilder"); mv.visitInsn(DUP); mv.visitMethodInsn(INVOKESPECIAL, "java/lang/StringBuilder", "<init>", "()V", false); mv.visitVarInsn(ALOAD, 0); mv.visitMethodInsn(INVOKEVIRTUAL, "java/lang/Object", "getClass", "()Ljava/lang/Class;", false); mv.visitMethodInsn(INVOKEVIRTUAL, "java/lang/Class", "getName", "()Ljava/lang/String;", false); mv.visitMethodInsn(INVOKEVIRTUAL, "java/lang/StringBuilder", "append", "(Ljava/lang/String;)Ljava/lang/StringBuilder;", false); mv.visitLdcInsn("."); mv.visitMethodInsn(INVOKEVIRTUAL, "java/lang/StringBuilder", "append", "(Ljava/lang/String;)Ljava/lang/StringBuilder;", false); mv.visitLdcInsn(methodName); mv.visitMethodInsn(INVOKEVIRTUAL, "java/lang/StringBuilder", "append", "(Ljava/lang/String;)Ljava/lang/StringBuilder;", false); mv.visitMethodInsn(INVOKEVIRTUAL, "java/lang/StringBuilder", "toString", "()Ljava/lang/String;", false); mv.visitVarInsn(ASTORE, 1); mv.visitFieldInsn(GETSTATIC, "java/lang/System", "out", "Ljava/io/PrintStream;"); mv.visitTypeInsn(NEW, "java/lang/StringBuilder"); mv.visitInsn(DUP); mv.visitMethodInsn(INVOKESPECIAL, "java/lang/StringBuilder", "<init>", "()V", false); mv.visitVarInsn(ALOAD, 1); mv.visitMethodInsn(INVOKEVIRTUAL, "java/lang/StringBuilder", "append", "(Ljava/lang/String;)Ljava/lang/StringBuilder;", false); mv.visitLdcInsn(": "); mv.visitMethodInsn(INVOKEVIRTUAL, "java/lang/StringBuilder", "append", "(Ljava/lang/String;)Ljava/lang/StringBuilder;", false); mv.visitVarInsn(ALOAD, 1); mv.visitMethodInsn(INVOKESTATIC, "org/xunche/agent/TimeHolder", "cost", "(Ljava/lang/String;)J", false); mv.visitMethodInsn(INVOKEVIRTUAL, "java/lang/StringBuilder", "append", "(J)Ljava/lang/StringBuilder;", false); mv.visitMethodInsn(INVOKEVIRTUAL, "java/lang/StringBuilder", "toString", "()Ljava/lang/String;", false); mv.visitMethodInsn(INVOKEVIRTUAL, "java/io/PrintStream", "println", "(Ljava/lang/String;)V", false); } }}的上述代码略长，ASM的部分可以略过。我们通过instrumentation.addTransformer注册一个转换器，转换器重写了变换方法，方法入参中的classfileBuffer表示的是原始的字节码，方法返回值表示的是真正要进行加载的字节码。
onMethodEnter方法中的代码含义是调用TimeHolder的start方法并初始化当前的方法名。
onMethodExit方法中的代码含义是调用TimeHolder的成本方法并合并当前的方法名，并打印成本方法的返回值。
下面来看下TimeHolder的代码：
package org.xunche.agent;import java.util.HashMap;import java.util.Map;public class TimeHolder { private static Map<String, Long> timeCache = new HashMap<>(); public static void start(String method) { timeCache.put(method, System.currentTimeMillis()); } public static long cost(String method) { return System.currentTimeMillis() - timeCache.get(method); }}至此之后，agent的代码编写完成，有关asm的部分不是本章的重点，日后再单独推出发表有关asm的文章。后的代码是怎样的。可以看到，与最开始的测试代码排序，每个方法都加入了我们统计方法耗时的代码。
package org.xunche.app;import org.xunche.agent.TimeHolder;public class HelloXunChe { public HelloXunChe() { } public static void main(String[] args) throws InterruptedException { TimeHolder.start(args.getClass().getName() + "." + "main"); HelloXunChe helloXunChe = new HelloXunChe(); helloXunChe.sayHi(); HelloXunChe helloXunChe = args.getClass().getName() + "." + "main"; System.out.println(helloXunChe + ": " + TimeHolder.cost(helloXunChe)); } public void sayHi() throws InterruptedException { TimeHolder.start(this.getClass().getName() + "." + "sayHi"); System.out.println("hi, xunche"); this.sleep(); String var1 = this.getClass().getName() + "." + "sayHi"; System.out.println(var1 + ": " + TimeHolder.cost(var1)); } public void sleep() throws InterruptedException { TimeHolder.start(this.getClass().getName() + "." + "sleep"); Thread.sleep((long)(Math.random() * 200.0D)); String var1 = this.getClass().getName() + "." + "sleep"; System.out.println(var1 + ": " + TimeHolder.cost(var1)); }}代理基础
上面的premain是通过agetn在应用启动前，对字节码进行修改，来实现我们想要的功能。实际上jdk提供了attach api，通过这个api，我们可以访问已经启动的Java进程。并通过agentmain方法来拦截类加载。下面我们来通过实战来具体说明下agentmain。
实战
本次实战的目标是实现一个小工具，其目标是能远程采集已经在运行中的Java进程的方法调用信息。听起来像不像BTrace，实际上 BTrace也是 这么实现的。只不过因为时间关系，本次的实战代码写的比较简陋，大家不必关注细节，看下实现的思路就好。
具体的实现思路如下：
agent对指定类的方法进行字节码的修改，采集方法的入参和返回值。并通过socket将请求和返回发送到服务端
服务端通过attach api访问运行中的Java进程，并加载agent，使agent程序能对目标进程实施
服务端加载agent时指定需要采集的类和方法
服务端开启一个端口，接受目标进程的请求信息
老规矩，先看测试代码，测试代码很简单，每隔100ms运行一次sayHi方法，并随机随身睡觉。
package org.xunche.app;public class HelloTraceAgent { public static void main(String[] args) throws InterruptedException { HelloTraceAgent helloTraceAgent = new HelloTraceAgent(); while (true) { helloTraceAgent.sayHi("xunche"); Thread.sleep(100); } } public String sayHi(String name) throws InterruptedException { sleep(); String hi = "hi, " + name + ", " + System.currentTimeMillis(); return hi; } public void sleep() throws InterruptedException { Thread.sleep((long) (Math.random() * 200)); }}接下看agent代码，思路同等监控方法耗时差不多，在方法出口处，通过asm植入采集方法入参和返回值的代码，并通过发件人将信息通过socket发送到服务端，代码如下：
package org.xunche.agent;import jdk.internal.org.objectweb.asm.*;import jdk.internal.org.objectweb.asm.commons.AdviceAdapter;import java.lang.instrument.ClassFileTransformer;import java.lang.instrument.Instrumentation;import java.lang.instrument.UnmodifiableClassException;import java.security.ProtectionDomain;public class TraceAgent { public static void agentmain(String args, Instrumentation instrumentation) throws ClassNotFoundException, UnmodifiableClassException { if (args == null) { return; } int index = args.lastIndexOf("."); if (index != -1) { String className = args.substring(0, index); String methodName = args.substring(index + 1); //目标代码已经加载，需要重新触发加载流程，才会通过注册的转换器进行转换 instrumentation.addTransformer(new TraceClassFileTransformer(className.replace(".", "/"), methodName), true); instrumentation.retransformClasses(Class.forName(className)); } } public static class TraceClassFileTransformer implements ClassFileTransformer { private String traceClassName; private String traceMethodName; public TraceClassFileTransformer(String traceClassName, String traceMethodName) { this.traceClassName = traceClassName; this.traceMethodName = traceMethodName; } @Override public byte[] transform(ClassLoader loader, String className, Class<?> classBeingRedefined, ProtectionDomain protectionDomain, byte[] classfileBuffer) { //过滤掉Jdk、agent、非指定类的方法 if (className.startsWith("java") || className.startsWith("jdk") || className.startsWith("javax") || className.startsWith("sun") || className.startsWith("com/sun") || className.startsWith("org/xunche/agent") || !className.equals(traceClassName)) { //return null会执行原来的字节码 return null; } ClassReader reader = new ClassReader(classfileBuffer); ClassWriter writer = new ClassWriter(reader, ClassWriter.COMPUTE_FRAMES | ClassWriter.COMPUTE_MAXS); reader.accept(new TraceVisitor(className, traceMethodName, writer), ClassReader.EXPAND_FRAMES); return writer.toByteArray(); } } public static class TraceVisitor extends ClassVisitor { private String className; private String traceMethodName; public TraceVisitor(String className, String traceMethodName, ClassVisitor classVisitor) { super(Opcodes.ASM5, classVisitor); this.className = className; this.traceMethodName = traceMethodName; } @Override public MethodVisitor visitMethod(int methodAccess, String methodName, String methodDesc, String signature, String[] exceptions) { MethodVisitor methodVisitor = cv.visitMethod(methodAccess, methodName, methodDesc, signature, exceptions); if (traceMethodName.equals(methodName)) { return new TraceAdviceAdapter(className, methodVisitor, methodAccess, methodName, methodDesc); } return methodVisitor; } } private static class TraceAdviceAdapter extends AdviceAdapter { private final String className; private final String methodName; private final Type[] methodArgs; private final String[] parameterNames; private final int[] lvtSlotIndex; protected TraceAdviceAdapter(String className, MethodVisitor methodVisitor, int methodAccess, String methodName, String methodDesc) { super(Opcodes.ASM5, methodVisitor, methodAccess, methodName, methodDesc); this.className = className; this.methodName = methodName; this.methodArgs = Type.getArgumentTypes(methodDesc); this.parameterNames = new String[this.methodArgs.length]; this.lvtSlotIndex = computeLvtSlotIndices(isStatic(methodAccess), this.methodArgs); } @Override public void visitLocalVariable(String name, String description, String signature, Label start, Label end, int index) { for (int i = 0; i < this.lvtSlotIndex.length; ++i) { if (this.lvtSlotIndex[i] == index) { this.parameterNames[i] = name; } } } @Override protected void onMethodExit(int opcode) { //排除构造方法和静态代码块 if ("<init>".equals(methodName) || "<clinit>".equals(methodName)) { return; } if (opcode == RETURN) { push((Type) null); } else if (opcode == LRETURN || opcode == DRETURN) { dup2(); box(Type.getReturnType(methodDesc)); } else { dup(); box(Type.getReturnType(methodDesc)); } Type objectType = Type.getObjectType("java/lang/Object"); push(lvtSlotIndex.length); newArray(objectType); for (int j = 0; j < lvtSlotIndex.length; j++) { int index = lvtSlotIndex[j]; Type type = methodArgs[j]; dup(); push(j); mv.visitVarInsn(ALOAD, index); box(type); arrayStore(objectType); } visitLdcInsn(className.replace("/", ".")); visitLdcInsn(methodName); mv.visitMethodInsn(INVOKESTATIC, "org/xunche/agent/Sender", "send", "(Ljava/lang/Object;[Ljava/lang/Object;Ljava/lang/String;Ljava/lang/String;)V", false); } private static int[] computeLvtSlotIndices(boolean isStatic, Type[] paramTypes) { int[] lvtIndex = new int[paramTypes.length]; int nextIndex = isStatic ? 0 : 1; for (int i = 0; i < paramTypes.length; ++i) { lvtIndex[i] = nextIndex; if (isWideType(paramTypes[i])) { nextIndex += 2; } else { ++nextIndex; } } return lvtIndex; } private static boolean isWideType(Type aType) { return aType == Type.LONG_TYPE || aType == Type.DOUBLE_TYPE; } private static boolean isStatic(int access) { return (access & 8) > 0; } }}SpringLocalVariableTableParameterNameNameDiscoverer，注意的同学可以自己研究下。接下来看下Sender中级代码：
public class Sender { private static final int SERVER_PORT = 9876; public static void send(Object response, Object[] request, String className, String methodName) { Message message = new Message(response, request, className, methodName); try { Socket socket = new Socket("localhost", SERVER_PORT); socket.getOutputStream().write(message.toString().getBytes()); socket.close(); } catch (IOException e) { e.printStackTrace(); } } private static class Message { private Object response; private Object[] request; private String className; private String methodName; public Message(Object response, Object[] request, String className, String methodName) { this.response = response; this.request = request; this.className = className; this.methodName = methodName; } @Override public String toString() { return "Message{" + "response=" + response + ", request=" + Arrays.toString(request) + ", className='" + className + '\'' + ", methodName='" + methodName + '\'' + '}'; } }}Sender中的代码不复杂，一看就懂，就不多说了。下面我们来看下服务端的代码，服务端要实现开启一个端口监听，接受请求信息，并使用attach api加载agent。
package org.xunche.app;import com.sun.tools.attach.AgentInitializationException;import com.sun.tools.attach.AgentLoadException;import com.sun.tools.attach.AttachNotSupportedException;import com.sun.tools.attach.VirtualMachine;import java.io.BufferedReader;import java.io.IOException;import java.io.InputStream;import java.io.InputStreamReader;import java.net.ServerSocket;import java.net.Socket;public class TraceAgentMain { private static final int SERVER_PORT = 9876; public static void main(String[] args) throws IOException, AttachNotSupportedException, AgentLoadException, AgentInitializationException { new Server().start(); //attach的进程 VirtualMachine vm = VirtualMachine.attach("85241"); //加载agent并指明需要采集信息的类和方法 vm.loadAgent("trace-agent.jar", "org.xunche.app.HelloTraceAgent.sayHi"); vm.detach(); } private static class Server implements Runnable { @Override public void run() { try { ServerSocket serverSocket = new ServerSocket(SERVER_PORT); while (true) { Socket socket = serverSocket.accept(); InputStream input = socket.getInputStream(); BufferedReader reader = new BufferedReader(new InputStreamReader(input)); System.out.println("receive message:" + reader.readLine()); } } catch (IOException e) { e.printStackTrace(); } } public void start() { Thread thread = new Thread(this); thread.start(); } }}运行上面的程序，可以看到服务端收到了org.xunche.app.HelloTraceAgent.sayHi的请求和返回信息。
receive message:Message{response=hi, xunche, 1581599464436, request=[xunche], className='org.xunche.app.HelloTraceAgent', methodName='sayHi'}小结
和通过agentmain实现了一个收集运行时方法调用信息的小工具，当然根据篇幅和时间问题，代码写的比较随意，大家多体会实际上，agent的作用远不止文章中介绍的这些，像 BTrace，arm，springloaded等中也都有用到agent。
转载自Hollis