RFR: 7161229 - PriorityBlockingQueue keeps hard reference to last removed element

David Holmes david.holmes at oracle.com
Wed Jun 27 05:26:11 UTC 2012

On 27/06/2012 2:35 AM, Alan Bateman wrote:
> On 25/06/2012 05:26, David Holmes wrote:
>> webrev:
>> http://cr.openjdk.java.net/~dholmes/7161229/webrev/
>> When removing the last element from the PBQ the "sift down" logic
>> would store it back in as array[0]. The simple fix is for the
>> sift-down to be a no-op if the queue size is zero.
>> The fix has been contributed by Doug Lea. We are taking this
>> opportunity to synchronize PBQ with Doug's latest version at:
>> http://gee.cs.oswego.edu/cgi-bin/viewcvs.cgi/jsr166/src/main/java/util/concurrent/PriorityBlockingQueue.java?view=log
>> So in addition to the above functional fix there are a number of
>> miscellaneous code and comment cleanups. The only non-trivial, but
>> still very simple, change is to the drainTo method to make it more
>> robust if the add() on the destination collection throws an exception.
>> I am of course a Reviewer for this.
>> I have provided the update to the LastElement test (as this is
>> certainly related to the last element) but there are some reservations
>> about examining the PBS internals this way. Unfortunately there is no
>> good way to test for these kinds of retention issues. You either need
>> a whitebox test like this, or need to rely on non-guaranteed GC and
>> finalization behaviour.
> I looked through the changes to siftDown* and the other clean-ups and
> they look fine to me.

Thanks Alan.

> As this test doesn't set a security manager then I assume it doesn't
> really need L92-94.

The IllegalAccessException has to be caught as it is a checked exception 
from getDeclaredField(). I removed the AccessControlException though.

> It is a bit icky to use reflection and pull out a
> private field but I don't think it's is easily tested otherwise.

The only other way is resorting to gc/finalization hacks - and that is 
even more icky in my opinion. If the field vanishes one day the test 
will fail and we'll know to update it.

I've updated the webrev for reference:


but will go ahead with the push.


> -Alan,

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