I also was getting this error. I brought out package building logic to execute once, and KnowledgeSession related logic to repeat the required number of times. Seems there required some delay in preparing rules related classes into one package during iteration and loading the same. Above step was working fine for me. Pseudo code: KnowledgeBuilder kbuilder = KnowledgeBuilderFactory.newKnowledgeBuilder(); // this will parse and compile in one step // get the compiled packages (which are serializable) Collection<KnowledgePackage> pkgs = null; // add the packages to a knowledgebase (deploy the knowledge packages). KnowledgeBase kbase = null; StatefulKnowledgeSession ksession = null; // constructor to be called once public RuleEngine (byte[] rules) { kbuilder.add(ResourceFactory.newByteArrayResource(rules),ResourceType.DRL); // Check the builder for errors if (kbuilder.hasErrors()) { System.out.println(kbuilder.getErrors().toString()); throw new RuntimeException("Unable to compile drl."); } pkgs = kbuilder.getKnowledgePackages(); // add the packages to a knowledgebase (deploy the knowledge packages). kbase = KnowledgeBaseFactory.newKnowledgeBase(); kbase.addKnowledgePackages(pkgs); } // this method will be called required number of times public void fire(Object obj) { ksession = kbase.newStatefulKnowledgeSession(); ksession.addEventListener(new DebugAgendaEventListener()); ksession.addEventListener(new DebugWorkingMemoryEventListener()); // setup the audit logging //KnowledgeRuntimeLogger logger = KnowledgeRuntimeLoggerFactory.newFileLogger(ksession, "log/drools.log"); ksession.insert(obj); ksession.fireAllRules(); //logger.close(); ksession.dispose(); } Check this out. Thanks & Regards Santhosh Pallerla From: rules-users-bounces(a)lists.jboss.org [mailto:rules-users-bounces@lists.jboss.org] On Behalf Of nicolas Sent: Thursday, September 29, 2011 8:09 AM To: rules-users(a)lists.jboss.org Subject: [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,