簡介：?作者信息：張帥，花名洵澈，負責物流表達和履約相關研發工作。熱衷于中間件相關技術。

引言

在本篇文章中，我會通過幾個簡單的程序來說明agent的使用，最后在實戰替換我會通過asm字節碼框架來實現一個小工具，用于在程序運行中采集指定方法的參數和返回值。有關asm字節碼的內容不是本文的重點，不會過多的分解，不明白的同學可以自己的Google下。

簡介

Java代理提供了一種在加載字節碼時，對字節碼進行修改的方式。他共有兩種方式執行，一種是在main方法執行之前，通過premain來實現，另一種是在程序運行中，通過attach api來實現。

在介紹agent之前，先給大家簡單說下一個Instrumentation。它是JDK1.5提供的API，用于攔截類加載事件，通過字節碼進行修改，它的主要方法如下：

public interface Instrumentation {//注冊一個轉換器，類加載事件會被注冊的轉換器所攔截void addTransformer(ClassFileTransformer transformer, boolean canRetransform);//重新觸發類加載void retransformClasses(Class<?>... classes) throws UnmodifiableClassException;//直接替換類的定義void redefineClasses(ClassDefinition... definitions) throws ClassNotFoundException, UnmodifiableClassException; }

主要

premain是在main方法之前運行的方法，也是最常見的agent方式。運行時需要將agent程序打包成jar包，并在啟動時添加命令來執行，如下文所示：
java -javaagent:agent.jar=xunche HelloWorld

premain共提供以下2種重載方法，Jvm啟動時會先嘗試使用第一種方法，若沒有會使用第二種方法：

public static void premain(String agentArgs, Instrumentation inst); public static void premain(String agentArgs);

一個簡單的例子

下面我們通過一個程序來簡單說明下premain的使用，首先我們準備下測試代碼，測試代碼比較簡單，運行main方法并輸出hello world。

package org.xunche.app; public class HelloWorld {public static void main(String[] args) {System.out.println("Hello World");} }

接下來我們看下agent的代碼，運行premain方法并輸出我們預期的參數。

package org.xunche.agent; public class HelloAgent {public static void premain(String args) {System.out.println("Hello Agent: " + args);} }

為了能夠使agent能夠運行，我們需要將META-INF / MANIFEST.MF文件中的Premain- Class為我們編寫的agent路徑，然后通過以下方式將其打包成jar包，當然你也可以使用idea直接替換jar包。

echo 'Premain-Class: org.xunche.agent.HelloAgent' > manifest.mf javac org/xunche/agent/HelloAgent.java javac org/xunche/app/HelloWorld.java jar cvmf manifest.mf hello-agent.jar org/

接下來，我們編譯下并運行下測試代碼，這里為了測試簡單，我將編譯后的類和agent的jar包放在同級目錄下

java -javaagent:hello-agent.jar=xunche org/xunche/app/HelloWorld

可以看到輸出結果如下，agent中的premain方法有延續main方法執行

Hello Agent: xunche Hello World

稍微復雜點的例子

通過上面的例子，是否對agent有個簡單的了解呢？

下面我們來看一個稍微復雜點，我們通過agent來實現一個方法監控的功能。思路大致是這樣的，若是非jdk的方法，我們通過asm在方法的執行入口和執行出口處，植入幾行記錄最佳的代碼，當方法結束后，通過合并來獲取方法的耗時。

首先還是看下測試代碼，邏輯很簡單，main方法執行時調用say Hi方法，輸出hi，xunche，并隨機睡眠中斷。

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));} }

接下來我們替換asm來植入我們自己的代碼，在jvm加載類的時候，為類的每個方法加上統計方法調用耗時的代碼，代碼如下，這里的asm我使用了jdk自帶的，當然你也可以使用官方的asm類庫。

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 {@Overridepublic 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或者執行異常會執行原來的字節碼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);}@Overridepublic 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;}@Overrideprotected void onMethodEnter() {//在方法入口處植入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);}@Overrideprotected 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);}} }

的上述代碼略長，ASM的部分可以略過。我們通過instrumentation.addTransformer注冊一個轉換器，轉換器重寫了變換方法，方法入參中的classfileBuffer表示的是原始的字節碼，方法返回值表示的是真正要進行加載的字節碼。

onMethodEnter方法中的代碼含義是調用TimeHolder的start方法并初始化當前的方法名。

onMethodExit方法中的代碼含義是調用TimeHolder的成本方法并合并當前的方法名，并打印成本方法的返回值。

下面來看下TimeHolder的代碼：

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的代碼編寫完成，有關asm的部分不是本章的重點，日后再單獨推出發表有關asm的文章。后的代碼是怎樣的?？梢钥吹?#xff0c;與最開始的測試代碼排序，每個方法都加入了我們統計方法耗時的代碼。

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));} }

代理基礎

上面的premain是通過agetn在應用啟動前，對字節碼進行修改，來實現我們想要的功能。實際上jdk提供了attach api，通過這個api，我們可以訪問已經啟動的Java進程。并通過agentmain方法來攔截類加載。下面我們來通過實戰來具體說明下agentmain。

實戰

本次實戰的目標是實現一個小工具，其目標是能遠程采集已經在運行中的Java進程的方法調用信息。聽起來像不像BTrace，實際上 BTrace也是 這么實現的。只不過因為時間關系，本次的實戰代碼寫的比較簡陋，大家不必關注細節，看下實現的思路就好。

具體的實現思路如下：

agent對指定類的方法進行字節碼的修改，采集方法的入參和返回值。并通過socket將請求和返回發送到服務端
服務端通過attach api訪問運行中的Java進程，并加載agent，使agent程序能對目標進程實施
服務端加載agent時指定需要采集的類和方法
服務端開啟一個端口，接受目標進程的請求信息

老規矩，先看測試代碼，測試代碼很簡單，每隔100ms運行一次sayHi方法，并隨機隨身睡覺。

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代碼，思路同等監控方法耗時差不多，在方法出口處，通過asm植入采集方法入參和返回值的代碼，并通過發件人將信息通過socket發送到服務端，代碼如下：

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);//目標代碼已經加載，需要重新觸發加載流程，才會通過注冊的轉換器進行轉換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;}@Overridepublic byte[] transform(ClassLoader loader, String className, Class<?> classBeingRedefined, ProtectionDomain protectionDomain, byte[] classfileBuffer) {//過濾掉Jdk、agent、非指定類的方法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會執行原來的字節碼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;}@Overridepublic 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);}@Overridepublic 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;}}}@Overrideprotected void onMethodExit(int opcode) {//排除構造方法和靜態代碼塊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，注意的同學可以自己研究下。接下來看下Sender中級代碼：

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;}@Overridepublic String toString() {return "Message{" +"response=" + response +", request=" + Arrays.toString(request) +", className='" + className + '\'' +", methodName='" + methodName + '\'' +'}';}} }

Sender中的代碼不復雜，一看就懂，就不多說了。下面我們來看下服務端的代碼，服務端要實現開啟一個端口監聽，接受請求信息，并使用attach api加載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的進程VirtualMachine vm = VirtualMachine.attach("85241");//加載agent并指明需要采集信息的類和方法vm.loadAgent("trace-agent.jar", "org.xunche.app.HelloTraceAgent.sayHi");vm.detach();}private static class Server implements Runnable {@Overridepublic 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();}} }

運行上面的程序，可以看到服務端收到了org.xunche.app.HelloTraceAgent.sayHi的請求和返回信息。

receive message:Message{response=hi, xunche, 1581599464436, request=[xunche], className='org.xunche.app.HelloTraceAgent', methodName='sayHi'}

小結

和通過agentmain實現了一個收集運行時方法調用信息的小工具，當然根據篇幅和時間問題，代碼寫的比較隨意，大家多體會實際上，agent的作用遠不止文章中介紹的這些，像 BTrace，arm，springloaded等中也都有用到agent。

轉載自Hollis

與50位技術專家面對面20年技術見證，附贈技術全景圖

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