Java字节码3-使用ByteBuddy实现一个Java-Agent

1、概述
在前面两节中，咱们实现了Agent，可是其不管在使用方式和功能上面都有必定的局限性。本文咱们借助字节码工具ByteBuddy，写出高级的Agent。

ByteBuddy不单单是为了生成Java-Agent，它提供的API甚至能够改变重写一个Java类，本文咱们使用其API实现和第二节同样的功能，给目标类中的函数统计其调用耗时。

2、实现
一、修改pom.xml
本节和上节的不一样点，主要有两个。一个是引入ByteBuddy的依赖，另外一个是须要将ByteBuddy的包经过shade打入到Agent中。下面只截取关键代码：

<dependency>
    <groupId>net.bytebuddy</groupId>
    <artifactId>byte-buddy</artifactId>
    <version>1.5.7</version>
</dependency>

<dependency>
    <groupId>net.bytebuddy</groupId>
    <artifactId>byte-buddy-agent</artifactId>
    <version>1.5.7</version>
</dependency>


<plugin> 
  <groupId>org.apache.maven.plugins</groupId>  
  <artifactId>maven-shade-plugin</artifactId>  
  <executions> 
    <execution> 
      <phase>package</phase>  
      <goals> 
        <goal>shade</goal> 
      </goals> 
    </execution> 
  </executions>  
  <configuration> 
    <artifactSet> 
      <includes> 
        <include>javassist:javassist:jar:</include>  
        <include>net.bytebuddy:byte-buddy:jar:</include>  
        <include>net.bytebuddy:byte-buddy-agent:jar:</include> 
      </includes> 
    </artifactSet> 
  </configuration> 
</plugin>

二、实现一个Agent
与以前相同的是，这里仍然是在premain处进行处理。经过AgentBuilder方法，生成一个Agent。这里有两点须要特别说明：其一是在AgentBuilder.type处，这里能够指定须要拦截的类；其二是在builder.method处，这里能够指定须要拦截的方法。固然其API支持各类isStatic、isPublic等等一系列方式。

public class MyAgent {

    public static void premain(String agentArgs, Instrumentation inst) {
        System.out.println("this is an perform monitor agent.");

        AgentBuilder.Transformer transformer = new AgentBuilder.Transformer() {
            @Override
            public DynamicType.Builder<?> transform(DynamicType.Builder<?> builder,
                                                    TypeDescription typeDescription,
                                                    ClassLoader classLoader) {
                return builder
                        .method(ElementMatchers.<MethodDescription>any()) // 拦截任意方法
                        .intercept(MethodDelegation.to(TimeInterceptor.class)); // 委托
            }
        };

        AgentBuilder.Listener listener = new AgentBuilder.Listener() {
            @Override
            public void onTransformation(TypeDescription typeDescription, ClassLoader classLoader, JavaModule module, DynamicType dynamicType) {}

            @Override
            public void onIgnored(TypeDescription typeDescription, ClassLoader classLoader, JavaModule module) { }

            @Override
            public void onError(String typeName, ClassLoader classLoader, JavaModule module, Throwable throwable) { }

            @Override
            public void onComplete(String typeName, ClassLoader classLoader, JavaModule module) { }
        };

        new AgentBuilder
                .Default()
                .type(ElementMatchers.nameStartsWith("com.example.demo")) // 指定须要拦截的类
                .transform(transformer)
                .with(listener)
                .installOn(inst);
    }
}

三、实现一个用来委托的Interceptor

在上一步实现Transformer的过程当中，委托了一个TimeInterceptor.class。下面是其实现方式，整个的try语句是原有的代码执行，咱们在以前打了时间戳，并在其结束后，计算并打印了其调用耗时。

public class TimeInterceptor  {
    @RuntimeType
    public static Object intercept(@Origin Method method,
                                   @SuperCall Callable<?> callable) throws Exception {
        long start = System.currentTimeMillis();
        try {
            // 原有函数执行
            return callable.call();
        } finally {
            System.out.println(method + ": took " + (System.currentTimeMillis() - start) + "ms");
        }
    }
}

3、运行

这里须要注意的是，咱们定义的包路径要和Agent中定义的相同，不然Agent没法Hook到这个类及其方法。

package com.example.demo;

public class AgentTest {

    private void fun1() throws Exception {
        System.out.println("this is fun 1.");
        Thread.sleep(500);
    }

    private void fun2() throws Exception {
        System.out.println("this is fun 2.");
        Thread.sleep(500);
    }

    public static void main(String[] args) throws Exception {
        AgentTest test = new AgentTest();
        test.fun1();
        test.fun2();

    }
}

结果：

this is an perform monitor agent.
this is fun 1.
private void com.example.demo.AgentTest.fun1() throws java.lang.Exception: took 501ms
this is fun 2.
private void com.example.demo.AgentTest.fun2() throws java.lang.Exception: took 500ms
public static void com.example.demo.AgentTest.main(java.lang.String[]) throws java.lang.Exception: took 1001ms