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Concurrency problem with persistent JBPM + Drools
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created by <a href="https://community.jboss.org/people/vercruysse_tim">Tim Vercruysse</a> in <i>jBPM</i> - <a href="https://community.jboss.org/message/797940#797940">View the full discussion</a>
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<div class="jive-rendered-content"><p>Hi all,</p><p style="min-height: 8pt; height: 8pt; padding: 0px;"> </p><p>We have a problem that when multiple threads try to fetch process variables an exception is thrown:</p><pre class="jive-pre"><code class="jive-code jive-java">java.lang.NullPointerException
               org.jbpm.process.instance.impl.ProcessInstanceImpl.getProcess(ProcessInstanceImpl.java:67)
               org.jbpm.persistence.processinstance.ProcessInstanceInfo.update(ProcessInstanceInfo.java:189)
</code></pre><p style="min-height: 8pt; height: 8pt; padding: 0px;"> </p><p>the code in our EJB is:</p><pre class="jive-pre"><code class="jive-code jive-java"><font color="navy"><b>public</b></font> Object getProcessVar(<font color="navy"><b>long</b></font> processInstanceId, String varName) <font color="navy">{</font>
     <font color="navy"><b>try</b></font><font color="navy">{</font>
          WorkflowProcessInstance p = (WorkflowProcessInstance) ksession.getProcessInstance(processInstanceId);
          var = p.getVariable(varName);
     <font color="navy">}</font> <font color="navy"><b>catch</b></font>(Throwable x) <font color="navy">{</font>x.printStackTrace()<font color="navy">}</font>
<font color="navy"><b>return</b></font> var;
<font color="navy">}</font>
</code></pre><p style="min-height: 8pt; height: 8pt; padding: 0px;"> </p><p>This exception is thrown when we do multiple calls (around 40) from our Spring context to this EJB method.</p><p style="min-height: 8pt; height: 8pt; padding: 0px;"> </p><p>We found that most of the time that when we do a context switch from Spring to EJB  drools automatically tries to update a ProcessInstanceInfo entity with procesInstanceInfo.update() or invokes jPAProcessInstanceManager.clearProcessInstances() to disconnect all the processInstances. <span style="font-size: 10pt;">It happens now that drools tries to update a processInstanceInfo that points to a processInstance that previously has been disconnected, hence the error.</span></p><p style="min-height: 8pt; height: 8pt; padding: 0px;"> </p><p>My guess is that the multiple threads mess up the order of the transactions so that some processInstanceInfo entities are in invalid states.</p><p style="min-height: 8pt; height: 8pt; padding: 0px;"> </p><p>Does anyone know how to solve this issue?</p><p style="min-height: 8pt; height: 8pt; padding: 0px;"> </p><p>We use JBPM 5.2, Spring 3 and Hibernate 4.</p><p>KnowledgeSession: statefullKnowledgeSession, with a commandBasedWSHumanTaskHandler.</p><p>Persistence transactions: JTA and JBossTransactionManagerLookup</p><p style="min-height: 8pt; height: 8pt; padding: 0px;"> </p><p>Thanks!</p></div>
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