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https://issues.apache.org/jira/browse/AMQ-6555?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15814978#comment-15814978
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Christopher L. Shannon commented on AMQ-6555:
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I think the main fix here is to prevent the start method from being called a
second time and a check can also be added to the Scheduler as a precaution as
well. However instead of a put if absent it might be better to cancel the old
task and then start/add the new one in case the period has changed. We can log
a warning as well. I can't really think of any current use cases where this
would/should happen but figure it's worth putting the check in there as a
precaution.
> Failed to browse topic warnings caused by orphaned expire message tasks in
> Sceduler
> -----------------------------------------------------------------------------------
>
> Key: AMQ-6555
> URL: https://issues.apache.org/jira/browse/AMQ-6555
> Project: ActiveMQ
> Issue Type: Bug
> Affects Versions: 5.14.1
> Reporter: jack
> Assignee: Christopher L. Shannon
>
> When using a FixedCountSubscriptionRecoveryPolicy on a topic I get millions
> of warnings when the topic is gc'd after inactivity. The impact is that warn
> logging has to be turned off which is not ideal. Looking in detail it seems
> this is caused by a couple of things. Firstly the Topic start method can be
> invoked twice, once when adding the destination to the RegionBroker and
> second when the regionBroker is started.
> That causes the same expireMessagesTask to get scheduled twice which reveals
> the crux of the problem. The Scheduler allows the same task to be scheduled
> again in new TimerTasks, even though it will use the same key and ophan any
> existing (and running) TimerTasks. This is what's happening here. When the
> topic gets stopped, the orphaned tasks continue to run causing these spurious
> warnings that clog up the logs.
> My suggestion would be for the scheduler to check if the task has already
> been registered using a putIfAbsent and fail if it already exists:
> https://github.com/apache/activemq/blob/d9350912984f12356e9d51b0f00b5a28f5cfa58d/activemq-client/src/main/java/org/apache/activemq/thread/Scheduler.java#L42
> These are the warnings we see:
> 2017-01-10 11:36:13,885 [host] Scheduler] - WARN Topic
> - Failed to browse Topic: myTopic
> java.lang.NullPointerException
> at
> org.apache.activemq.broker.region.policy.FixedCountSubscriptionRecoveryPolicy.browse(FixedCountSubscriptionRecoveryPolicy.java:103)
> at
> org.apache.activemq.broker.region.policy.RetainedMessageSubscriptionRecoveryPolicy.browse(RetainedMessageSubscriptionRecoveryPolicy.java:111)
> at org.apache.activemq.broker.region.Topic.doBrowse(Topic.java:677)
> at org.apache.activemq.broker.region.Topic.access$1(Topic.java:639)
> at org.apache.activemq.broker.region.Topic$2.run(Topic.java:791)
> at
> org.apache.activemq.thread.SchedulerTimerTask.run(SchedulerTimerTask.java:33)
> at java.util.TimerThread.mainLoop(Timer.java:555)
> at java.util.TimerThread.run(Timer.java:505
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