jmx-dev [PATCH] JDK-6809322: Missing notifications from javax.management.timer.Timer

[Jaroslav Bachorik]

On Fri 12 Oct 2012 04:44:31 AM CEST, David Holmes wrote:
> Hi Jaroslav,
>
>
> On 11/10/2012 6:07 PM, Jaroslav Bachorik wrote:
>> Dmitry has put the webrev on the public CR -
>> http://cr.openjdk.java.net/~dsamersoff/sponsorship/jbachorik/JDK-6809322-v2/
>>
>>
>> Thanks!
>>
>> -JB-
>>
>> On 10/10/2012 04:17 PM, Jaroslav Bachorik wrote:
>>> I am looking for a review and a sponsor.
>>>
>>> The issue is about some javax.management.timer.Timer notifications not
>>> being received by the listeners if the notifications are generated
>>> rapidly.
>>>
>>> The problem is caused by ConcurrentModificationException being thrown -
>>> the exception itself is ignored but the dispatcher logic is skipped.
>>> Therefore the currently processed notification gets lost.
>
> Can you point out where exactly in the code the exception is thrown
> and caught. I'd like to understand the problem better.

The CME is thrown in Timer.notifyAlarmClock() method in this case - but 
may happen in other places as well.

Actually, in some places the access to the timerTable map is 
synchronized while in others it isn't. While switching the Hashtable 
for ConcurrentHashMap resolves this particular issue it might be 
beneficial to correct the partial synchronization instead.

>
>>>
>>> The CME is thrown due to the Timer.timerTable being iterated over while
>>> other threads try to remove some of its elements. Fix consists of
>>> replacing the Hashtable used for Timer.timerTable by ConcurrentHashMap
>>> which handles such situations with grace.
>
> Be aware that it may also give surprising results as removal is no
> longer synchronized at all with processing. So it could now appear
> that a notification is processed after a listener has been removed.

Indeed, the CME is the symptom of the out-of-order processing - the 
removal method is synchronized on (Timer.this) while the 
notifyAlarmClock() method, processing the notifications, runs 
unsynchronized.

Thanks for pointing this out. I will have something to think about.

-JB-

>
> David
> -----
>
>>> The patch webrev is available @
>>> https://jbs.oracle.com/bugs/browse/JDK-6809322
>>>
>>> Thanks,
>>>
>>> -JB-
>>>
>>