Vladyslav Kosulin reported in JBWEB-56:

I see the fillowing exception few times a day, and it does not look like the cause is in application code:

java.util.ConcurrentModificationException
at java.util.HashMap$HashIterator.nextEntry(Unknown Source)
at java.util.HashMap$EntryIterator.next(Unknown Source)
at java.util.HashMap$EntryIterator.next(Unknown Source)
at java.util.HashMap.writeObject(Unknown Source)
at sun.reflect.GeneratedMethodAccessor93.invoke(Unknown Source)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
at java.lang.reflect.Method.invoke(Unknown Source)
at java.io.ObjectStreamClass.invokeWriteObject(Unknown Source)
at java.io.ObjectOutputStream.writeSerialData(Unknown Source)
at java.io.ObjectOutputStream.writeOrdinaryObject(Unknown Source)
at java.io.ObjectOutputStream.writeObject0(Unknown Source)
at java.io.ObjectOutputStream.writeObject(Unknown Source)
at org.jboss.web.tomcat.tc5.session.SessionBasedClusteredSession.writeExternal(SessionBasedClusteredSession.java:288)
at org.jboss.web.tomcat.tc5.session.JBossCacheService.externalizeSession(JBossCacheService.java:771)
at org.jboss.web.tomcat.tc5.session.JBossCacheService.putSession(JBossCacheService.java:229)
at org.jboss.web.tomcat.tc5.session.SessionBasedClusteredSession.processSessionRepl(SessionBasedClusteredSession.java:165
at org.jboss.web.tomcat.tc5.session.JBossCacheManager.processSessionRepl(JBossCacheManager.java:606)
at org.jboss.web.tomcat.tc5.session.JBossCacheManager.storeSession(JBossCacheManager.java:375)
at org.jboss.web.tomcat.tc5.session.InstantSnapshotManager.snapshot(InstantSnapshotManager.java:38)
at org.jboss.web.tomcat.tc5.session.ClusteredSessionValve.invoke(ClusteredSessionValve.java:91)
at org.jboss.web.tomcat.security.JaccContextValve.invoke(JaccContextValve.java:59)
at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:126)
at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:105)
at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:107)
at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:148)
at org.apache.jk.server.JkCoyoteHandler.invoke(JkCoyoteHandler.java:307)
at org.apache.jk.common.HandlerRequest.invoke(HandlerRequest.java:385)
at org.apache.jk.common.ChannelSocket.invoke(ChannelSocket.java:748)
at org.apache.jk.common.ChannelSocket.processConnection(ChannelSocket.java:678)
at org.apache.jk.common.SocketConnection.runIt(ChannelSocket.java:871)
at org.apache.tomcat.util.threads.ThreadPool$ControlRunnable.run(ThreadPool.java:684)
at java.lang.Thread.run(Unknown Source)