ByteBuddy不仅仅是为了生成Java-Agent，它提供的API甚至可以改变重写一个Java类，本文我们使用其API实现和第二节一样的功能，给目标类中的函数统计其调用耗时。
二、实现
1、修改pom.xml
本节和上节的不同点，主要有两个。一个是引入ByteBuddy的依赖，另一个是需要将ByteBuddy的包通过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>
2、实现一个Agent
与之前相同的是，这里仍然是在premain处进行处理。通过AgentBuilder方法，生成一个Agent。这里有两点需要特别说明：其一是在AgentBuilder.type处，这里可以指定需要拦截的类；其二是在builder.method处，这里可以指定需要拦截的方法。当然其API支持各种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")) // 指定需要拦截的类
.transform(transformer)
.with(listener)
.installOn(inst);
}
}
3、实现一个用来委托的Interceptor
在上一步实现Transformer的过程中，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，我们在之前打了时间戳，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");
}
}
}
三、运行
这里需要注意的是，我们定义的包路径要和Agent中定义的相同，否则Agent无法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
可以看到，我们的Agent成功Hook并增强了其调用方法。