写那么多年Java还不知道啥是Java-agent的必须看一下

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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.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: xunche
Hello 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

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