Determining java Obj size with java.lang.instrument package. Compile and put this class in a JAR:

ObjectSizeFetcher.java

// http://stackoverflow.com/questions/52353/in-java-what-is-the-best-way-to-determine-the-size-of-an-object
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);
    }
}
/*
Add the following to your MANIFEST.MF:

Premain-Class: ObjectSizeFetcher
Use getObjectSize:
*/

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

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

/*
Invoke with:

java -javaagent:ObjectSizeFetcherAgent.jar C

NOTE: also check http://www.eclipse.org/mat/
http://www.javaworld.com/article/2077408/core-java/sizeof-for-java.html
*/