javaagent 是类似一个JVM的插件，利用JVM提供的Instrumentation API实现获取或者修改加载到JVM中的类字节码。
编写一个javagent的jar的方式如下：
1.实现一个ClassFileTransformer
1
2
3
4
5
6
7
8
9
10
|
public class SimpleTransformer implements ClassFileTransformer {
@Override
public byte[] transform(ClassLoader loader, String className, Class<?> classBeingRedefined, ProtectionDomain protectionDomain, byte[] classfileBuffer) throws IllegalClassFormatException {
System.out.println(className);
System.out.println(protectionDomain.toString());
return new byte[0];
}
}
|
2.实现一个Premain-Class
1
2
3
4
5
6
7
8
9
10
|
public class Main {
public static void premain(String agentOps, Instrumentation inst) {
inst.addTransformer(new SimpleTransformer());
}
public static void main(String[] args) {
System.out.println("This is a javaagent!");
}
}
|
3.MANIFEST.MF配置
1
2
3
4
5
|
Manifest-Version: 1.0
Premain-Class: com.github.liaojiacan.Main
Can-Redefine-Classes: true
Can-Retransform-Classes: true
Can-Set-Native-Method-Prefix: true
|
4.运行命令
1
|
java -javaagent:agent.jar -jar app.jar
|
代码和assembly的打包配置可以参考，github
近期评论