[rules-users] Multithreaded update of facts in Drools 5.5 - how to ensure that a nested value isn't null

[Per Sterner]

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

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