J2SE 1.6 特性：java.lang.instrument

1.

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);
    }
}

2.

jar cvf ObjectSizeFetcher.jar  ObjectSizeFetcher.class

3.

Add the following to your MANIFEST.MF:

Premain-Class: ObjectSizeFetcher

4.

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

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

5.调用

java -javaagent:ObjectSizeFetcherAgent.jar C