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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).

Lii
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Sergio
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    Reference counts aren't tracked in a JVM. The only way to know is to count them. – Gabe Mar 17 '11 at 02:25
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    There is a library that can be used to get notification when an object is garbage collected. Please refer to this link for more details http://sourceforge.net/projects/gcradar/ – R.daneel.olivaw Dec 13 '13 at 05:28
  • @AmrenduPandey, do not simply "bold some letters" -- that is not an appropriate way to improve a question. Edits should be substantial, not just be formatting changes. – Charles Jan 28 '14 at 05:07
  • Java has no such provision for counting/getting list of references to an Object. Allowing this could be a serious issue as anyone can finalize the object even if someones code overlying on it is still in need of it. – Siddappa Walake Aug 29 '18 at 11:07

3 Answers3

17

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:

Lii
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scorpiodawg
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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

dLobatog
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Java is NOT reference-counting

And we simply can not get something which does not exist.

See similar Q/A,
which explains why reference recursion does not cause memory-leak:
https://stackoverflow.com/a/72167978/8740349

See Javadoc for PhantomReference class:
download.oracle.com/javase/6/docs/api/java/lang/ref/PhantomReference.

Also, there is a library which can notify you when an object is garbage collected:
https://sourceforge.net/projects/gcradar

Top-Master
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