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，最後在實戰替換我會通過asm字節碼框架來實現一個小工具，用於在程序運行中採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，用於攔截類加載事件，通過字節碼進行修改，它的主要方法如下：
public interface Instrumentation {
//註冊一個轉換器，類加載事件會被註冊的轉換器所攔截
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，並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啟動時會先嚐試使用第一種方法，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方法並輸出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方法並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，然後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，我們編譯下並運行下測試代碼，這裡為了測試簡單，我將編譯後的類和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
稍微複雜點的例子
通過上面的例子，是否對agent有個簡單的瞭解呢？
下面我們來看一個稍微複雜點，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，當方法結束後，通過合併來獲取方法的耗時。
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，並隨機睡眠中斷。
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，這裡的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 {
@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);
}
@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);
}
@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);
}
}
}的上述代碼略長，ASM的部分可以略過。我們通過instrumentation.addTransformer註冊一個轉換器，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方法並初始化當前的方法名。
onMethodExit方法中的代碼含義是調用TimeHolder的成本方法併合並當前的方法名，並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);
}
}至此之後，agent的代碼編寫完成，有關asm的部分不是本章的重點，日後再單獨推出發表有關asm的文章。後的代碼是怎樣的。可以看到，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。並通過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，其目標是能遠程採集已經在運行中的Java進程的方法調用信息。聽起來像不像BTrace，實際上 BTrace也是 這麼實現的。只不過因為時間關係，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，採集方法的入參和返回值。並通過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，並加載agent，使agent程序能對目標進程實施
服務端加載agent時指定需要採集的類和方法
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，並隨機隨身睡覺。
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植入採集方法入參和返回值的代碼，並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，才會通過註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;
}
@Override
public 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;
}
@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;
}
@Override
public 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，並使用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並指明需要採集信息的類和方法
vm.loadAgent("trace-agent.jar", "org.xunche.app.HelloTraceAgent.sayHi");
vm.detach();
}
private static class Server implements Runnable {
@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();
}
}
}
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