[jboss-jira] [JBoss JIRA] (WFLY-9742) ClassLoader leak in JBoss Threads caused by MDBs

[David Lloyd (JIRA)]

    [ https://issues.jboss.org/browse/WFLY-9742?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13525852#comment-13525852 ] 

David Lloyd commented on WFLY-9742:
-----------------------------------

I think there isn't one single perfect answer, but the best answer is probably to clear TCCL in JBoss Threads.  The inherited TCCL behavior is usually not helpful, especially for pooled threads.

> ClassLoader leak in JBoss Threads caused by MDBs
> ------------------------------------------------
>
>                 Key: WFLY-9742
>                 URL: https://issues.jboss.org/browse/WFLY-9742
>             Project: WildFly
>          Issue Type: Bug
>    Affects Versions: 11.0.0.Final
>            Reporter: Markus Dlugi
>            Assignee: Jason Greene
>         Attachments: default-threads-tccl.png, jboss-threads-tccl-example.zip
>
>
> There is a classloader leak in JBoss Threads which is most noticable when deploying MDBs. When a new MDB is created and a new thread for the MDB is started in the JCA thread pool ("default-threads - x"), the thread will be created using the context classloader of the MDB's deployment unit. This is because [MessageDrivenComponent.activate()|https://github.com/wildfly/wildfly/blob/master/ejb3/src/main/java/org/jboss/as/ejb3/component/messagedriven/MessageDrivenComponent.java#L275] sets the context classloader of the ServerService thread in order to create the MDB, and this classloader will then also be used by the child thread.
> In the default configuration, the threads in the default thread pool will not be terminated and therefore the thread will keep the reference to the classloader even when the deployment unit is undeployed. This in turn can lead to "OutOfMemoryError: Metaspace" after a couple of redeployments.
> As a workaround, we changed [JBossThreadFactory.createThread()|https://github.com/jbossas/jboss-threads/blob/master/src/main/java/org/jboss/threads/JBossThreadFactory.java#L104] to set the context classloader to null after a new thread has been created. While this fixes the issue for us, I am not sure whether this is a good solution for all consumers of the thread factory, or if this should be fixed in the JCA subsystem instead. That's also the reason why I opened this issue against the WildFly project instead of JBoss Threads.



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