Java Instrument动态修改字节码入门-添加方法耗时监控

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平常在统计方法执行的耗时时长时，一般都是在方法的开头和结尾通过System.currentTimeMillis()拿到时间，然后做差值，计算耗时，这样不得不在每个方法中都重复这样的操作，现在使用Instrument，可以优雅的实现该功能。

一、编写Agent类

package com.jdktest.instrument;import java.lang.instrument.ClassFileTransformer;
import java.lang.instrument.Instrumentation;public class AopAgentTest {static private Instrumentation _inst = null;  /** * The agent class must implement a public static premain method similar in principle to the main application entry point. * After the Java Virtual Machine (JVM) has initialized, * each premain method will be called in the order the agents were specified, * then the real application main method will be called.**/  public static void premain(String agentArgs, Instrumentation inst) {  System.out.println("AopAgentTest.premain() was called.");  /* Provides services that allow Java programming language agents to instrument programs running on the JVM.*/  _inst = inst; /* ClassFileTransformer : An agent provides an implementation of this interface in order to transform class files.*/  ClassFileTransformer trans = new AopAgentTransformer(); System.out.println("Adding a AopAgentTest instance to the JVM.");  /*Registers the supplied transformer.*/_inst.addTransformer(trans);  }  
}

代码功能详见注释。

二、编写ClassFileTransformer

ClassFileTransformer需要实现transform方法，根据自己的功能需要修改class字节码，在修改字节码过程中需要借助javassist进行字节码编辑。

javassist是一个开源的分析、编辑和创建java字节码的类库。通过使用javassist对字节码操作可以实现动态”AOP”框架。

关于java字节码的处理，目前有很多工具，如bcel，asm(cglib只是对asm又封装了一层)。不过这些都需要直接跟虚拟机指令打交道。javassist的主要的优点，在于简单，而且快速，直接使用java编码的形式，而不需要了解虚拟机指令，就能动态改变类的结构，或者动态生成类。

package com.jdktest.instrument;import java.lang.instrument.ClassFileTransformer;
import java.lang.instrument.IllegalClassFormatException;
import java.security.ProtectionDomain;import javassist.CannotCompileException;
import javassist.ClassPool;
import javassist.CodeConverter;
import javassist.CtClass;
import javassist.CtMethod;
import javassist.NotFoundException;
import javassist.expr.ExprEditor;
import javassist.expr.MethodCall;public class AopAgentTransformer implements ClassFileTransformer{public byte[] transform(ClassLoader loader, String className,Class<?> classBeingRedefined, ProtectionDomain protectionDomain,byte[] classfileBuffer) throws IllegalClassFormatException {byte[] transformed = null;  System.out.println("Transforming " + className);  ClassPool pool = null;  CtClass cl = null;  try {  pool = ClassPool.getDefault();cl = pool.makeClass(new java.io.ByteArrayInputStream(  classfileBuffer));  //            CtMethod aop_method = pool.get("com.jdktest.instrument.AopMethods").
//                    getDeclaredMethod("aopMethod");
//            System.out.println(aop_method.getLongName());CodeConverter convert = new CodeConverter();if (cl.isInterface() == false) {  CtMethod[] methods = cl.getDeclaredMethods();  for (int i = 0; i < methods.length; i++) {  if (methods[i].isEmpty() == false) {  AOPInsertMethod(methods[i]);  }  }  transformed = cl.toBytecode();  }  } catch (Exception e) {  System.err.println("Could not instrument  " + className  + ",  exception : " + e.getMessage());  } finally {  if (cl != null) {  cl.detach();  }  }  return transformed;  }private void AOPInsertMethod(CtMethod method) throws NotFoundException,CannotCompileException {//situation 1:添加监控时间method.instrument(new ExprEditor() {  public void edit(MethodCall m) throws CannotCompileException {  m.replace("{ long stime = System.currentTimeMillis(); $_ = $proceed($$);System.out.println(\""+ m.getClassName() + "." + m.getMethodName()+ " cost:\" + (System.currentTimeMillis() - stime) + \" ms\");}");}}); //situation 2:在方法体前后语句
//      method.insertBefore("System.out.println(\"enter method\");");
//      method.insertAfter("System.out.println(\"leave method\");");}}

三、打包Agent jar包

因为agent依赖javassist，在build时需要加入<Boot-Class-Path>，pom文件如下：

<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd"><modelVersion>4.0.0</modelVersion><groupId>com.jdktest</groupId><artifactId>MyInstrument</artifactId><version>0.0.1-SNAPSHOT</version><packaging>jar</packaging><name>instrument</name><url>http://maven.apache.org</url><properties><project.build.sourceEncoding>UTF-8</project.build.sourceEncoding></properties><dependencies><dependency><groupId>javassist</groupId><artifactId>javassist</artifactId><version>3.8.0.GA</version></dependency><dependency><groupId>junit</groupId><artifactId>junit</artifactId><version>3.8.1</version><scope>test</scope></dependency></dependencies><build>  <plugins>  <plugin>  <groupId>org.apache.maven.plugins</groupId>  <artifactId>maven-jar-plugin</artifactId>  <version>2.2</version>  <configuration>  <archive>  <manifestEntries>  <Premain-Class>com.jdktest.instrument.AopAgentTest</Premain-Class>  <Boot-Class-Path>E:/maven-lib/javassist/javassist/3.8.0.GA/javassist-3.8.0.GA.jar</Boot-Class-Path>  </manifestEntries>  </archive>  </configuration>  </plugin>  <plugin>  <artifactId>maven-compiler-plugin </artifactId >  <configuration>  <source> 1.6 </source >  <target> 1.6 </target>  </configuration>  </plugin>  </plugins>   </build> 
</project>

四、需要添加耗时监控的client

随意编写需要添加耗时监控的代码并打jar包。

package com.jdktest.SayHello;public class Target {public static void main(String[] args) {new Target().sayHello();}public void sayHello(){System.out.println("Hello, guys!");}
}

如果sayHello方法中又调用了同类中的方法或者别的类中的方法，这些被调用的方法的耗时仍可被监控到。

五、执行

在cmd下执行如下命令：

java -javaagent:E:/workspace/MyInstrument/target/MyInstrument-0.0.1-SNAPSHOT.jar -cp E:/workspace/SayHello/target/SayHello-0.0.1-SNAPSHOT.jar com.jdktest.SayHello.Target