Calculate size of Object in Java

Question

I want to record how much memory (in bytes, hopefully) an object takes up for a project (I'm comparing sizes of data structures) and it seems like there is no method to do this in Java. Supposedly, C/C++ has sizeOf() method, but this is nonexistant in Java. I tried recording the free memory in the JVM with Runtime.getRuntime().freeMemory() before and after creating the object and then recording the difference, but it would only give 0 or 131304, and nothing in between, regardless of the number of elements in the structure. Help please!

Answer 1

You can use the java.lang.instrumentation package.

It has a method that can be used to get the implementation specific approximation of object size, as well as overhead associated with the object.

The answer that Sergey linked has a great example, which I'll repost here, but you should have already looked at from his comment:

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);
    }
}

Use getObjectSize:

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

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

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