How to get object size in java.

It is easy to use intrumentation.

Source

import java.lang.instrument.Instrumentation;
import java.util.Calendar;
import java.util.HashMap;

public class Test {
 private static Instrumentation inst;

 public static void premain(String options, Instrumentation inst) {
  Test.inst = inst;
 }

 public static long sizeOf(Object o) {
  assert inst != null;
  return inst.getObjectSize(o);
 }

 public static void main(String[] args) {
  System.out.println("Size of Object: " + sizeOf(new Object()));
  System.out.println("Size of String: " + sizeOf(new String()));
  System.out.println("Size of String111: " + sizeOf(new String("111.111.111.111")));
  System.out.println("Size of StringBuffer: " + sizeOf(new String()));
  System.out.println("Size of HashMap: " + sizeOf(new HashMap()));
  System.out.println("Size of direct subclass: " + sizeOf(new Test()));
  System.out.println("Size of Calendar: " + sizeOf(Calendar.getInstance()));
 }
}

manifest.mf

Manifest-Version: 1.0
Premain-Class: com.google.laputa.sms.sample.Test
Main-Class: com.google.laputa.sms.sample.Test

jaring
$ jar cvfm test.jar test-classes/MANIFEST.MF -C test-classes .

execution
$ java -ea -javaagent:test.jar -jar test.jar

Result
Size of Object: 8
Size of String: 24
Size of String111: 24
Size of StringBuffer: 24
Size of HashMap: 40
Size of direct subclass: 8
Size of Calendar: 112