[hibernate-dev] Example of WildFly JPA container safely closing the EntityManager after the application calls transaction.rollback, which also handles background tx reaper thread time outs

[Scott Marlow]

Hi,

[1] is an example of how the WildFly JPA container is ensuring that
the background (transaction manager) reaper (transaction timeout)
thread does not cause EntityManager.close() to be called, while the
application is actively executing calls to EntityManager.*().

The idea, as suggested by Tom Jenkinson, is to track when the
application (container) calls transaction.commit/rollback.  We also
register a Synchronization.  When we detect that
transaction.commit/rollback and the Synchronization.afterCompletion
have both been called, it is safe to perform the EntityManager clean
up action, as we know that only one thread will then be invoking the
EntityManager.close().

>From a state point of view, please look at how [2] is checking for the
EventType.DISASSOCIATING event, which means that
transaction.rollback/commit was called by the application.  If the
Synchronization.afterCompletion has already run as well, then we clean
up the EntityManager, otherwise, we defer the cleanup action until the
call to Synchronization.afterCompletion occurs.

Also see [3] which contains the recommendation of using the above solution.

Scott

[1] https://github.com/wildfly/wildfly/blob/master/jpa/subsystem/src/main/java/org/jboss/as/jpa/transaction/TransactionUtil.java#L146

[2] https://github.com/wildfly/wildfly/blob/master/jpa/subsystem/src/main/java/org/jboss/as/jpa/transaction/TransactionUtil.java#L197

[3] https://developer.jboss.org/message/919807