I'm currently using Drools Flow with JPA session persistence enabled as described in section 5.1 of the User Guide.

As a process executes, session information is saved to the H2 database correctly.

The problem comes if a process only executes part way through, and I then try to finish the process by recreating a session using the code:

// recreate the session from database using the sessionId
ksession = JPAKnowledgeService.loadStatefulKnowledgeSession( sessionId, kbase, null, env );

This gives me a NullPointerException.

Having looked into the Drools Flow source code, I think the problem is in the class drools-persistence-jpa\src\main\java\org\drools\persistence\session\SingleSessionCommandService.java.

The 4th constructor of this class is called as part of the loading of a knowledge session from the H2 database; this in turn leads to the afterCompletion method of the inner class SynchronizationImpl being called. The final two lines of this method are:

((JPAProcessInstanceManager) ((ReteooWorkingMemory) session).getProcessInstanceManager()).clearProcessInstances();
((JPAWorkItemManager) ((ReteooWorkingMemory) session).getWorkItemManager()).clearWorkItems();

However, when called via the route described above, the session member variable of SingleSessionCommandService has not been set, thus causing the NullPointerException.

(Interestingly, running the unit tests for drools-persistence-jpa does result in the NullPointerException being thrown multiple times, although the tests are still listed as having passed.)

I believe enclosing the above two lines as follows will fix the problem:

if (session != null) {
    ((JPAProcessInstanceManager) ((ReteooWorkingMemory) session).getProcessInstanceManager()).clearProcessInstances();
    ((JPAWorkItemManager) ((ReteooWorkingMemory) session).getWorkItemManager()).clearWorkItems();
}

Also, in the 4th constructor of SingleSessionCommandService, after the session has been set, the following line needs to be added to ensure the session id is the same as the session info id:

((ReteooStatefulSession) this.session).setId( this.sessionInfo.getId() );

(This line is present at the end of the 3rd constructor.)

It would be great if this fix could make it into the Drools 5.1 release.

Thanks.

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