把下面的类编译好放到ObjectSizeFetcherAgent.jar中

import java.lang.instrument.Instrumentation;
public class ObjectSizeFetcher {
    private static Instrumentation instrumentation;
    public static void premain(String args, Instrumentation inst) {
        instrumentation = inst;
    }
    public static long getObjectSize(Object o) {
        return instrumentation.getObjectSize(o);
    }
}

在MANIFEST.MF中加入：

Premain-Class: ObjectSizeFetcher

然后写个test类

public class C {
    private int x;
    private int y;

    public static void main(String [] args) {
        System.out.println(ObjectSizeFetcher.getObjectSize(new C()));
    }
}

在命令行中这样执行：

java -javaagent:ObjectSizeFetcherAgent.jar C