引入javaAgent

我们知道，简单的实现一个java agent是实现如下接口

public static void premain(String agentArgs, Instrumentation inst);  //【1】
public static void premain(String agentArgs);  //【2】

其中，【1】和【2】同时存在时，【1】会优先被执行，而【2】则会被忽略。
这两个都适合在应用启动的时候加载，我们也可以提供一种动态挂载的方式，代码如下

    public static void agentmain(String agentArgs, Instrumentation inst){
        System.out.println("load agent...");
        inst.addTransformer(new AgentClazzTransformer(),true);
    }

基于此，我们把agent的基本项目结构搭建起来

<?xml version="1.0" encoding="UTF-8"?>
<project xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns="http://maven.apache.org/POM/4.0.0"
         xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
  <parent>
    <artifactId>jvm_sandbox_demo</artifactId>
    <groupId>com.deer</groupId>
    <version>1.0-SNAPSHOT</version>
    <relativePath>../pom.xml</relativePath>
  </parent>
  <modelVersion>4.0.0</modelVersion>
  <groupId>com.deer.agent</groupId>
  <artifactId>jvm_sandbox_agent</artifactId>
  <name>jvm_sandbox_agent</name>
  <properties>
    <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
    <maven.compiler.source>1.8</maven.compiler.source>
    <maven.compiler.target>1.8</maven.compiler.target>
  </properties>
  <build>
    <plugins>
      <plugin>
        <groupId>org.apache.maven.plugins</groupId>
        <artifactId>maven-assembly-plugin</artifactId>
        <version>2.5.5</version>
        <configuration>
          <descriptorRefs>
            <descriptorRef>jar-with-dependencies</descriptorRef>
          </descriptorRefs>
          <archive>
            <manifestEntries>
              <Premain-Class>com.deer.agent.AgentMain</Premain-Class>
              <Agent-Class>com.deer.agent.AgentMain</Agent-Class>
              <Can-Redefine-Classes>true</Can-Redefine-Classes>
              <Can-Retransform-Classes>true</Can-Retransform-Classes>
            </manifestEntries>
          </archive>
        </configuration>
        <executions>
          <execution>
            <goals>
              <goal>attached</goal>
            </goals>
            <phase>package</phase>
          </execution>
        </executions>
      </plugin>
      <plugin>
        <groupId>org.apache.maven.plugins</groupId>
        <artifactId>maven-surefire-plugin</artifactId>
        <configuration>
          <skipTests>true</skipTests>
        </configuration>
      </plugin>
    </plugins>
  </build>
</project>

package com.deer.agent;
import com.deer.agent.sandbox.AgentClazzTransformer;
import java.lang.instrument.Instrumentation;
public class AgentMain {
    public static void premain(String agentArgs, Instrumentation inst) {
        System.out.println("premain");
    }
    public static void agentmain(String agentArgs, Instrumentation inst){
        System.out.println("load agent...");
        inst.addTransformer(new AgentClazzTransformer(),true);
    }
}

package com.deer.agent.sandbox;
import java.lang.instrument.ClassFileTransformer;
import java.lang.instrument.IllegalClassFormatException;
import java.security.ProtectionDomain;
public class AgentClazzTransformer implements ClassFileTransformer {
    @Override
    public byte[] transform(ClassLoader loader, String className, Class<?> classBeingRedefined, ProtectionDomain protectionDomain, byte[] classfileBuffer) throws IllegalClassFormatException {
        if(className!=null && className.startsWith("com/deer/base/service")){
            System.out.println("I can see you ...."+className);
        }
        return classfileBuffer;
    }
}

基于此，我们把要测试的代码稍微修改一下

package com.deer.base.controller;
import com.deer.base.service.BaseService;
import com.deer.base.service.HelloService;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.web.bind.annotation.PathVariable;
import org.springframework.web.bind.annotation.RequestMapping;
import org.springframework.web.bind.annotation.RestController;
@RestController
public class HelloController {
    @Autowired
    private HelloService helloService;
    @RequestMapping("/sayHi/{userName}")
    public String sayHello(@PathVariable(name ="userName") String userName) {
        //新增加的代码
        BaseService.record(userName);
        return helloService.sayHello(userName);
    }
}

package com.deer.base.service;
public class BaseService {
    public static void record(String value){
        System.out.println("one record:"+value);
    }
}