javaagent简单理解

1. javajaent是什么

javaagent可以理解为是一个插件，需要有一个jvm进程才能运行。例如arthas这个工具就是用到了javaagent。

2. 如何使用

1. 一种调用方式：java -javaagent:path[=参数=参数值]

2. 方法签名

    public static void premain(String args, Instrumentation inst) {
    }

    public static void agentmain(String args, Instrumentation inst) {
    }

看到签名就应该能够猜到，一个是在main函数之前运行的；一个是attach到jvm进程之后运行的。

3. premain的案例

新建一个maven工程

1. pom文件

<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>4.0.0modelVersion>

    <groupId>com.lagougroupId>
    <artifactId>ljavaagentartifactId>
    <version>1.0-SNAPSHOTversion>
    
    <build>
        <plugins>
            <plugin>
                <groupId>org.apache.maven.pluginsgroupId>
                <artifactId>maven-assembly-pluginartifactId>

                <configuration>
                    <descriptorRefs>
                        <descriptorRef>
                            jar-with-dependencies
                        descriptorRef>
                    descriptorRefs>

                    <archive>
                        <manifestFile>
                            src/main/resources/META-INF/MANIFEST.MF
                        manifestFile>
                    archive>
                configuration>
                <executions>
                    <execution>
                        <goals>
                            <goal>attachedgoal>
                        goals>
                        <phase>packagephase>
                    execution>
                executions>
            plugin>
        plugins>
    build>
project>

2. MANIFEST.MF文件

Manifest-Version: 1.0
Premain-Class: com.lagou.SimpleAgent
Agent-Class: com.lagou.SimpleAgent
Can-Redefine-Classes: true
Can-Retransform-Classes: true

3. 类

package com.lagou;

import java.lang.instrument.Instrumentation;

public class SimpleAgent {

    public static void premain(String args, Instrumentation inst) {
        System.out.println("args:" + args);
        System.out.println("instrumentation: " + inst);
        System.out.println(SimpleAgent.class.getSimpleName() + "public static void premain(String args, Instrumentation inst)");
    }

    public static void agentmain(String args, Instrumentation inst) {
        System.out.println("args:" + args);
        System.out.println("instrumentation: " + inst);
        System.out.println(SimpleAgent.class.getSimpleName() + "public static void agentmain(String args, Instrumentation inst)");
    }
}

4. 编译

使用

 ~/software/arthas/ java -javaagent:/Users/lixi/ljavaagent/target/ljavaagent-1.0-SNAPSHOT-jar-with-dependencies.jar=args=tom -jar math-game.jar
args:args=tom
instrumentation: sun.instrument.InstrumentationImpl@33909752
SimpleAgentpublic static void premain(String args, Instrumentation inst)
135397=71*1907
75004=2*2*17*1103
illegalArgumentCount:  1, number is: -148459, need >= 2                                                                                                                         ~/software/arthas/