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

[Peter Levart]

Hello Jaroslav,

Regardless of where it is implemented (in JDK or in NetBeans) it would 
be impossible to create something that doesn't do polling in a single 
thread and yet wait for two ReferenceQueues at the same time - one is 
the queue of Runnable references which you run() and the other is the 
queue of WeakReference(s) to the before-mentioned queue(s) (the ACTIVE 
in ActiveQueue). Each of these two queues has it's own 'lock' and there 
is no way to wait() for two locks at the same time in a single thread. 
But you could do this in two threads. I created an alternative 
implementation using two threads:

http://cr.openjdk.java.net/~plevart/misc/nb.ActiveQueue2/ActiveQueue2.java

I deliberately commented-out the setDaemon(true) calls of both threads 
so it can be seen that JVM exits when both are finished in the little 
test at the end.

It's easy to adapt this to use polling instead of 2nd thread. Just 
specify a time-out in a call to CleanupHandler.remove(timeout) and scrap 
the code for 2nd thread. The remaining thread will eventually find out 
that ReferenceQueue has been collected and will exit.

Regards, Peter

On 07/28/2014 03:06 PM, Jaroslav Tulach wrote:
> 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