Java Instrument动态修改字节码入门-添加方法耗时监控

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

六、执行结果