Address of a Java Object

In conventional java programming, you will never need address or location of a java object from memory. When you discuss about this in forums, the first question raised is why do you need to know the address of a java object? Its a valid question. But always, we reserve the right to experiment. Nothing is wrong in exploring uncharted areas.

I thought of experimenting using a little known class from sun package. Unsafe is a class that belongs to sun.misc package. For some of you the package might be new. You need not worry about it. If you have sun’s JDK then you have this class already.

When a class name is “Unsafe” in java, it calls for your attention immediately. Then I decided to dig deep into it and find what is unsafe about that class. Voila, it really opens up the pandora’s box. Its difficult to find the source of Unsafe. Get the source and look at the methods you will know what I am referring to.

Java’s security manager provides sufficient cover and ensures you don’t fiddle with memory that easily. As a first step, I thought of getting the memory location of a java object. Until the exploration, I too was 100% confident that it was not possible to find the location / address of an object in java.

Sun’s Unsafe.java api documentation shows us an opportunity to get the address using the method objectFieldOffset. That method says, “Report the location of a given field in the storage allocation of its class“. It also says, “it is just a cookie which is passed to the unsafe heap memory accessors“. Whatsoever, I am able to get the storage memory location of an object from the storage allocation of its class.

You can argue that, what we have got is not the absolute physical memory address of an object. But we have got the logical memory address. The following program will be quite interesting for you!

As a first step, I have to get an object of Unsafe class. It is quite difficult as the constructor is private. There is a method named getUnsafe which returns the unsafe object. Java’s security manager asks you to make your java source code privileged. I used little bit of reflection and got an instance out. I know there are better ways to get the instance. But to bypass the security easily I chose the following.

Using Unsafe’s object just invoke objectFieldOffset and staticFieldOffset. The result is address / location of object in the storage allocation of its class.

Following example program runs well on JDK 1.6

import sun.misc.Unsafe;
import java.lang.reflect.Field;

public class ObjectLocation {

 private static int apple = 10;
 private int orange = 10;

 public static void main(String[] args) throws Exception {
  Unsafe unsafe = getUnsafeInstance();

  Field appleField = ObjectLocation.class.getDeclaredField("apple");
  System.out.println("Location of Apple: "
    + unsafe.staticFieldOffset(appleField));

  Field orangeField = ObjectLocation.class.getDeclaredField("orange");
  System.out.println("Location of Orange: "
    + unsafe.objectFieldOffset(orangeField));
 }

 private static Unsafe getUnsafeInstance() throws SecurityException,
   NoSuchFieldException, IllegalArgumentException,
   IllegalAccessException {
  Field theUnsafeInstance = Unsafe.class.getDeclaredField("theUnsafe");
  theUnsafeInstance.setAccessible(true);
  return (Unsafe) theUnsafeInstance.get(Unsafe.class);
 }
}