Is it possible to get the object reference count?

Question

I'd like to know if there is a way to check how many references a Java object has. As far as I could check the only way to do that is using JVMTI through a JNI interface. Is there a pure Java (without using native libraries) solution to get this information?

We are developing an educational tool for data structure animation(to be used with students implementation of certain algorithms), so it would be very nice if we could check for "released" objects on the most non-intrusive way (I'm trying to avoid forcing the user of this tool to call a method such as objectReleased(objRef) in order to update the data structure animation for an element removal or something similar).

Answer 1

Java doesn't offer this option natively as far as I know.

Here you have some guidance on how to do it manually:

http://www.velocityreviews.com/forums/t363649-how-do-i-get-a-reference-count-of-a-object-in-java.html

Answer 2

From your description, it seems you care less about the actual count of references than to simply know when an object has been collected. If this is the case, you can use WeakReference or PhantomReference to determine when a referenced object is ready for finalization.

See:

• Javadoc for WeakReference: http://download.oracle.com/javase/6/docs/api/java/lang/ref/WeakReference.html
• Javadoc for PhantomReference: http://download.oracle.com/javase/6/docs/api/java/lang/ref/PhantomReference.html
• A great explanation of weak references: https://community.oracle.com/blogs/enicholas/2006/05/04/understanding-weak-references
• Garbage collector callbacks: http://java.dzone.com/articles/letting-garbage-collector-do-c
• Garbage collection notification?