Java agent 还不了解的程序员该反省一下了 (1)
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 class 和 agent 的 jar 包放在了同级目录下
java?-javaagent:hello-agent.jar=xunche?org/xunche/app/HelloWorld
可以看到输出结果如下，agent 中的 premain 方法有限于 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，植入几行记录时间戳的代码，当方法结束后，通过时间戳来获取方法的耗时。
首先还是看下测试代码，逻辑很简单， main 方法执行时调用 sayHi 方法，输出 hi , xunche ，并随机睡眠一段时间。
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));
}
}
接下来我们借助 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);
}
}
}
上述的代码略长， 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 transform 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 方法并传入当前的方法名。
onMethodExit 方法中的代码含义是调用 TimeHolder 的 cost 方法并传入当前的方法名，并打印 cost 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，agent 的代码编写完成，有关 asm 的部分不是本章的重点，日后再单独推出一篇有关 asm 的文章。通过在类加载时植入我们监控的代码后，下面我们来看看，经过 asm 修改后的代码是怎样的。可以看到，与最开始的测试代码相比，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));
}
}
agentmain
=============
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处于运行中的 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 程序能对目标进程生效
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，并随机随眠一段时间。
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 Sender 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(class
Name.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) {
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