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

[Markus Dlugi (JIRA)]

Markus Dlugi created WFLY-9742:
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             Summary: 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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