Benefits of activeReferenceQueue was: ReferenceQueue.remove to allow GC of the queue itself

[Jaroslav Tulach]

Hello David,
thanks for being patient with me. I'll do my best to describe the original context.

Dne Po 28. července 2014 21:07:45, David Holmes napsal(a):
> I read the issue and still did not understand the nature of the problem.
> The netbeans bugs also did not shed any light on things for me. What is
> the functionality of the activeReferenceQueue 

The functionality of the active reference queue is described in NetBeans APIs[1]. I 
think the best way to describe it in context of existing JDK APIs, is to call it 
"lightweight finalizer without classical finalizer problems". To quote the Javadoc:

---
If you have a reference that needs cleanup, make it implement Runnable and register 
it with the queue:

 class MyReference extends WeakReference implements Runnable {
     private final OtherInfo dataToCleanUp;
     public MyReference(Thing ref, OtherInfo data) {
         super(ref, Utilities.activeReferenceQueue());
         dataToCleanUp = data;
     }
     public void run() {
         dataToCleanUp.releaseOrWhateverYouNeed();
     }
 }
 
When the ref object is garbage collected, your run method will be invoked by calling 
((Runnable) reference).run()
--

The benefit taken from "finalizer" is that one does not need to start own thread. The 
difference to "finalizer" is that the object is already gone, e.g. no chance to re-activate 
it again.

We introduced the activeReferenceQueue API when we realized that many modules 
over the code base start their own thread and try to do the classical poll() cleanup. 
Once upon a time we used to have more than twenty threads like this, and as 
overhead of a thread is not low, we improved the NetBeans memory consumption 
quite a lot by introducing the activeReferenceQueue.

> and what it is that there
> are problems with?

None in case of NetBeans. Once the activeReferenceQueue initializes itself and its 
thread, it runs up until the termination of the system and works great.

However NetBeans APIs can be used outside of NetBeans runtime container and, when 
used in a WAR file, people started to get problems during re-deploys. 

> Once we got a bug report[2] that it behaves poorly 
> when used inside of a WAR file. Whenever the WAR was redeployed, the number
> of cleanup threads increased by one, which also caused major memory leaks.

Those problems could be fixed by using active polling as I wrote in today's morning 
email:

> class Impl extends ReferenceQueue {}
> Reference<Impl> ref = new WeakReference<Impl>(new Impl());
> 
> while (true) {
> 
>   Impl impl = ref.get();
>   if (impl == null) {
>   
>     // no other Reference objects using the Impl queue.
>     // exit this cleaner thread
>     return;
>   
>   }
>   Reference<?> ref = impl.remove(15000);
>   if (ref == null) {
>   
>     impl = null; // don't hold strong ref to Impl queue
>     System.gc(); // XXX: is anyone else holding reference to Impl queue?
>     continue;
>   
>   }
>   // do something with ref
> 
> }
> 
> this could work, althrough the problem is the XXX part.
> 
> I need to release my own pointer to the Impl queue, tell the system to try
> to garbage collect it. If it has not been removed, grap new strong pointer
> to the Impl queue and wait again. I am not aware of any other way to ask
> for GC than System.gc, and having System.gc being called every 15s will
> likely decrease VM performance a bit.
> 
> The proper solution (no reflection, no repeated polling) would in fact be
> simple: Being able to call:
> 
> impl.remove();
> 
> without anyone having strong reference to impl - e.g. without impl being on
> stack during the remove call.

I don't know what else to add. So I wait for further question.
-jt

[1] http://bits.netbeans.org/dev/javadoc/org-openide-util/org/openide/util/Utilities.html#activeReferenceQueue()
[2] https://netbeans.org/bugzilla/show_bug.cgi?id=206621