Garbage Collector Listeners

Question

I read an small article at some point about adding some callbacks to WeakReference objects that would be triggered upon garbage collection. Now, no mater how much I search, I cannot find it.

I need a way to execute some code whenever a weak referenced object is destroyed. I know that it can be done, I just don't remember how or whether I need a WeakReference or something else like a WeakHasMap?

Answer 1

Use ReferenceQueues to archieve that. You might want to look into PhantomReferences, too, depending on what you're actually trying to do (but WeakReferences work with the queues, too). Just create a reference queue and pass it as a second argument to the reference-constructor. When the GC remove the object, the reference is enqueued and you can get it using remove() (blocking) or poll() (non-blocking) on your queue.

There is an alternative: implementing finalize. It'S much less flexible though and runs in another thread, too (so you still have concurrency - with the addition of not even knowing which thread will execute it). The ReferenceQueue is superiour in all aspects.