Hello, My question is: How can I ensure that a nested property isn't null (in a multithreaded environment)? This is the rule: rule "Drools Test3: Concurrency" dialect "java" when $testObj : TestObject( $ref : subObject, $ref != null, $ref.message != null ) then System.out.println("Received-04-A: " + $testObj); end I am getting the following exception if I try to modify the fact in different threads. java.lang.NullPointerException: null at ConditionEvaluator8d1e3555e8c34810957bbb1670f51176.evaluate(null:-1) at org.drools.rule.constraint.MvelConstraint.evaluate(MvelConstraint.java:200) at org.drools.rule.constraint.MvelConstraint.isAllowed(MvelConstraint.java:157) at org.drools.reteoo.AlphaNode.modifyObject(AlphaNode.java:154) at org.drools.reteoo.SingleObjectSinkAdapter.propagateModifyObject(SingleObjectSinkAdapter.java:68) at org.drools.reteoo.AlphaNode.modifyObject(AlphaNode.java:157) at org.drools.reteoo.SingleObjectSinkAdapter.propagateModifyObject(SingleObjectSinkAdapter.java:68) at org.drools.reteoo.ObjectTypeNode.modifyObject(ObjectTypeNode.java:314) at org.drools.reteoo.EntryPointNode.modifyObject(EntryPointNode.java:265) at org.drools.common.NamedEntryPoint.update(NamedEntryPoint.java:483) at org.drools.common.AbstractWorkingMemory.update(AbstractWorkingMemory.java:976) at org.drools.common.AbstractWorkingMemory.update(AbstractWorkingMemory.java:949) at org.drools.impl.StatefulKnowledgeSessionImpl.update(StatefulKnowledgeSessionImpl.java:284) [...] There are two threads running which modify the same reference in a testobject ("subObject"). They trigger a queued update of the fact. One solution would be: - insert the nested object in drools - and modify the rule: rule "Drools Test3: Concurrency 2" dialect "java" when $subObj : TestObject( message != null ) $testObj : TestObject( subObject == $subObj ) then System.out.println("Received-04-A: " + $testObj); end Can I achieve a thread safety differently? Thanks and regards, Per Sterner