I am running 2 rules that are very similar but with a slight change: rule One when $foo : Foo($name : Name, $msg : Message !="") $prev : ArrayList(size<1) from collect( Foo(Name==$name, Message == $msg, this before $foo) $list : Set(size>0) from accumulate( Foo(Name == $name, $msgs: Message != $msg, this after $foo), collectSet($msgs)) then System.out.println($name + "\twent from \"" + $msg + "\"\tto " + $list); end rule Two when $foo : Foo($name : Name, $msg : Message !="") Not( Foo(Name==$name, Message == $msg, this before $foo)) $list : Set(size>0) from accumulate( Foo(Name == $name, $msgs: Message != $msg, this after $foo), collectSet($msgs)) then System.out.println($name + "\twent A from \"" + $msg + "\"\tto " + $list); end rule One works just fine however its rule Two I seem to be having trouble with. In my main method I have an initial print statement, then I go through this whole process of parsing XML files into objects and then adding those objects into the session. Once all the objects are entered I do a print out, I then call fireAllRules on the session and then do another print out. When I use rule Two, my program starts firing the rules and displaying their results almost immediately, even before the 2nd print out (sometimes even before the first initial print out). Most of the time I get a Null Pointer Exception but it does occasionally finish running. In the cases it does finish it spits out the output from most of the rules then displays the 2nd printout, the print out from a rule and then the ending print out. When it does throw the null pointer exception I get: java.lang.NullPointerException at org.drools.core.util.LinkedList.remove(LinkedList.java:133) at org.drools.common.DefaultAgenda.addActivation(DefaultAgenda.java.446) at org.drools.common.DefaultAgenda.modifyActivation(DefaultAgenda.java:382) at org.drools.reteoo.RuleTerminalNode.modifyLeftTuple(RuleTerminalNode.java:301) at org.drools.reteoo.SingleLeftTupleSinkAdapter.propagateModifyChildLeftTuple(SingleLeftTupleSinkAdapter.java:259) at org.drools.reteoo.AccumulateNode.exaluateResultConstraints(AccumulateNode.java:676) at org.drools.reteoo.ReteooWorkingMemory$EvaluateResultConstraints.execute(ReteooWorkingMemory.java:590) at org.drools.common.PropagationContextImpl.evaluateActionQueue(PropagationContextimple:350) at org.drools.common.NamedEntryPoint.insert(NamedEntryPoint.java:355) at org.drools.common.NamedEntryPoint.insert(NamedEntryPoint.java:311) at org.drools.AbstractWorkingMemory.insert(AbstractWorkingMemory.java:903) at org.drools.AbstractWorkingMemory.insert(AbstractWorkingMemory.java:847) at org.drools.imp.StatefulKnowledgeSessionImpl.insert(StatefulKnowledgeSessionImpl.java:269) at test2.Engine2.main(Engine2.java:80) However sometimes when it throws the Null pointer exception, it prints out some of the results of the rules in the middle of the display of the exception, such as: java.lang.NullPointerExceptionName1 went A from "msg1" to [msg2] Name2 went A from "msg1" to [msg2] at org.drools.core.util.LinkledList.remove(LinkedList.java.133) and you know the rest. When I run both rules together, for some reason it seems to run rule One over the same name and message pair multiple times even though based on the logic it should only run it based on the first occurrence. Also I don't know if this matters but there are 361,694 objects that are entered into the session, 44,269 of which are Foo objects -- View this message in context: http://drools.46999.n3.nabble.com/fusion-weird-race-conditions-and-a-null... Sent from the Drools: User forum mailing list archive at Nabble.com. _______________________________________________ rules-users mailing list rules-users(a)lists.jboss.org https://lists.jboss.org/mailman/listinfo/rules-users