1、模式一：-javaaent

1.1、编写 MyAgent.java

import java.lang.instrument.Instrumentation;

public class MyAgent {

    public static void premain(String agentArg, Instrumentation instrumentation) {
        System.out.println("premain");
    }
}

1.2、在META-INF文件夹创建 MANIFEST.MF 文件

Premain-Class: MyAgent 下面要有一行空行

Manifest-Version: 1.0
Premain-Class: MyAgent

1.3、 编译MyAgent.java

javac MyAgent.java

1.4、打包xx.jar

jar cfm xx.jar ./META-INF/MANIFEST.MF MyAgent.class

1.5、创建一个含有main函数的类

public class A {
    public static void main(String[] args) throws InterruptedException {
        int i = 0;
        while (true) {
            print(i++);
            Thread.sleep(1000);
        }
    }

    private static void print(Object obj) {
        System.out.println(obj);
    }
}

1.6、编译A.java

javac A.java

1.7、运行

-javaagent:D:/study/myAgent/xx.jar一定要写在A.class之前

java -javaagent:D:/study/myAgent/xx.jar A

1.8、效果