Question

When running

java -javaagent:ObjectSizeFetcherAgent.jar PersistentTime

I get

24

when ObjectSizeFetcherAgent does this

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);
    }
}

and when PersistentTime looks as follows

public class PersistentTime {

    List<String>    list    = new ArrayList<String>();

    public static void main(String[] args) {

        PersistentTime p = new PersistentTime();

        p.list.add("a");  // The number is the same with or without this
        p.list.add("b");  // The number is the same with or without this
        p.list.add("c");  // The number is the same with or without this

        System.out.println(ObjectSizeFetcher.getObjectSize(p));
    }
}

Why is adding elements to the list have no affect?

Answer 1

Your PersistentTime object consists solely of one reference (to an array list).

24 bytes is typical for objects containing a single reference.

Note: referenced objects are not included in the computation. getObjectSize is not recursively collecting the combined object sizes. This is not generally possible: there could be infinite reference loops and such; I don’t think there is a “deep size” computation easily available.