[rules-users] Concurrency problem in PackageClassLoader

Hi, We are using a single (Drools 5.1.1) Stateless Sessions for the parallel processing of thousands of facts. org.drools.runtime.rule.ConsequenceException: rule: Priorite - Groupe: TMK2612G01, P10 at org.drools.runtime.rule.impl.DefaultConsequenceExceptionHandler.handleException(DefaultConsequenceExceptionHandler.java:39) at org.drools.common.DefaultAgenda.fireActivation(DefaultAgenda.java:927) at org.drools.common.DefaultAgenda.fireNextItem(DefaultAgenda.java:856) at org.drools.common.DefaultAgenda.fireAllRules(DefaultAgenda.java:1071) at org.drools.common.AbstractWorkingMemory.fireAllRules(AbstractWorkingMemory.java:785) at org.drools.common.AbstractWorkingMemory.fireAllRules(AbstractWorkingMemory.java:751) at org.drools.impl.StatefulKnowledgeSessionImpl.fireAllRules(StatefulKnowledgeSessionImpl.java:218) at acme.srv.ept.service.impl.traitement.MoteurTraitementDemandes.traiterDemande(MoteurTraitementDemandes.java:121) (...) Caused by: java.util.ConcurrentModificationException at java.util.HashMap $AbstractMapIterator.checkConcurrentMod(Unknown Source) at java.util.HashMap$AbstractMapIterator.makeNext(Unknown Source) at java.util.HashMap$KeyIterator.next(Unknown Source) at java.util.HashMap.analyzeMap(Unknown Source) at java.util.HashMap.rehash(Unknown Source) at java.util.HashMap.rehash(Unknown Source) at java.util.HashMap.putImpl(Unknown Source) at java.util.HashMap.put(Unknown Source) at org.drools.rule.JavaDialectRuntimeData $PackageClassLoader.loadClass(JavaDialectRuntimeData.java:503) at java.lang.ClassLoader.loadClass(ClassLoader.java:609) at acme.srv.ept.regle.Rule_Priorite___Groupe__TMK2612G01__P10_0DefaultConsequenceInvoker.evaluate(Rule_Priorite___Groupe__TMK2612G01__P10_0DefaultConsequenceInvoker.java:29) at org.drools.common.DefaultAgenda.fireActivation(DefaultAgenda.java:917) ... 49 more We get the same error when we used a pool of Stateless Sessions (serial access) based on the same KnowledgeBase for the processing. Thanks,