I've initially posted this in the RichFaces users forum: http://www.jboss.com/index.html?module=bb&op=viewtopic&t=136368 I haven't got reply there so far and I'm unsure where is the exact problem so I've decided to post it here, too. The problem as it seems to be is the web session gets modified while it is been serialized. Specifically the serialization of an org.ajax4jsf.util.LRUMap instance holding some JSF(?) state is failing with ConcurrentModificationException, so I've first thought it could be a RichFaces library problem. This is my development configuration (1 node cluster): JBossAS 4.2.2.GA Seam 2.0.2.SP1 RichFaces 3.2.0.SR1 Sun's JDK 1.5.0_15 Windows XP Pro SP2 The same problem appers with 2 nodes configuration on FreeBSD 7.0 with Sun's JDK 1.5.0_11. Tell me what info I could supply more to clarify the issue. Here's the full stack trace of the exception: 15:27:13,187 ERROR [JBossCacheService] externalizeSession(): exception occurred externalizing session SessionBasedClusteredSession[id: F3m44XY0NwX7HVPjsweIDw** lastAccessedTime: 1211977633015 version: 39 lastOutdated: 0] | java.util.ConcurrentModificationException | at java.util.LinkedHashMap$LinkedHashIterator.nextEntry(LinkedHashMap.java:365) | at java.util.LinkedHashMap$EntryIterator.next(LinkedHashMap.java:384) | at java.util.LinkedHashMap$EntryIterator.next(LinkedHashMap.java:383) | at java.util.HashMap.writeObject(HashMap.java:1037) | at sun.reflect.GeneratedMethodAccessor109.invoke(Unknown Source) | at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25) | at java.lang.reflect.Method.invoke(Method.java:585) | at java.io.ObjectStreamClass.invokeWriteObject(ObjectStreamClass.java:917) | at java.io.ObjectOutputStream.writeSerialData(ObjectOutputStream.java:1339) | at java.io.ObjectOutputStream.writeOrdinaryObject(ObjectOutputStream.java:1290) | at java.io.ObjectOutputStream.writeObject0(ObjectOutputStream.java:1079) | at java.io.ObjectOutputStream.writeObject(ObjectOutputStream.java:302) | at java.util.HashMap.writeObject(HashMap.java:1039) | at sun.reflect.GeneratedMethodAccessor109.invoke(Unknown Source) | at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25) | at java.lang.reflect.Method.invoke(Method.java:585) | at java.io.ObjectStreamClass.invokeWriteObject(ObjectStreamClass.java:917) | at java.io.ObjectOutputStream.writeSerialData(ObjectOutputStream.java:1339) | at java.io.ObjectOutputStream.writeOrdinaryObject(ObjectOutputStream.java:1290) | at java.io.ObjectOutputStream.writeObject0(ObjectOutputStream.java:1079) | at java.io.ObjectOutputStream.defaultWriteFields(ObjectOutputStream.java:1375) | at java.io.ObjectOutputStream.writeSerialData(ObjectOutputStream.java:1347) | at java.io.ObjectOutputStream.writeOrdinaryObject(ObjectOutputStream.java:1290) | at java.io.ObjectOutputStream.writeObject0(ObjectOutputStream.java:1079) | at java.io.ObjectOutputStream.writeObject(ObjectOutputStream.java:302) | at java.util.concurrent.ConcurrentHashMap.writeObject(ConcurrentHashMap.java:1380) | at sun.reflect.GeneratedMethodAccessor290.invoke(Unknown Source) | at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25) | at java.lang.reflect.Method.invoke(Method.java:585) | at java.io.ObjectStreamClass.invokeWriteObject(ObjectStreamClass.java:917) | at java.io.ObjectOutputStream.writeSerialData(ObjectOutputStream.java:1339) | at java.io.ObjectOutputStream.writeOrdinaryObject(ObjectOutputStream.java:1290) | at java.io.ObjectOutputStream.writeObject0(ObjectOutputStream.java:1079) | at java.io.ObjectOutputStream.writeObject(ObjectOutputStream.java:302) | at org.jboss.web.tomcat.service.session.SessionBasedClusteredSession.writeExternal(SessionBasedClusteredSession.java:175) | at org.jboss.web.tomcat.service.session.JBossCacheService.externalizeSession(JBossCacheService.java:1027) | at org.jboss.web.tomcat.service.session.JBossCacheService.putSession(JBossCacheService.java:316) | at org.jboss.web.tomcat.service.session.JBossCacheClusteredSession.processSessionRepl(JBossCacheClusteredSession.java:121) | at org.jboss.web.tomcat.service.session.JBossCacheManager.processSessionRepl(JBossCacheManager.java:1097) | at org.jboss.web.tomcat.service.session.JBossCacheManager.storeSession(JBossCacheManager.java:652) | at org.jboss.web.tomcat.service.session.InstantSnapshotManager.snapshot(InstantSnapshotManager.java:49) | at org.jboss.web.tomcat.service.session.ClusteredSessionValve.invoke(ClusteredSessionValve.java:98) | at org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:432) | at org.jboss.web.tomcat.security.JaccContextValve.invoke(JaccContextValve.java:84) | at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:127) | at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:102) | at org.jboss.web.tomcat.service.jca.CachedConnectionValve.invoke(CachedConnectionValve.java:157) | at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:109) | at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:262) | at org.apache.coyote.http11.Http11Processor.process(Http11Processor.java:844) | at org.apache.coyote.http11.Http11Protocol$Http11ConnectionHandler.process(Http11Protocol.java:583) | at org.apache.tomcat.util.net.JIoEndpoint$Worker.run(JIoEndpoint.java:446) | at java.lang.Thread.run(Thread.java:595) View the original post : http://www.jboss.com/index.html?module=bb&op=viewtopic&p=4154849#... 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