[rules-users] NPE from LHS of rule is not returned by Drools to calling program

[scottleff]

During development of our Drools rule base I had a case where a rule compiled
fine, but at runtime a NPE is thrown by MVEL when applying the predicate
(LHS) because the Coverage.vehicleSupplement attribute was NULL.  I had
expected the NPE to be returned to my calling java program so that my
try/catch could address the issue.  However, the exception was sent to
System.err and the rule was simply skipped.  First, is the the default
behavior in Drools for Exceptions thrown from the LHS?  If so, can it be
overridden to actually return the exception? 

rule "BR--UM Stacked Coverage"
	agenda-group "BR_Change"
	dialect "mvel"
	when
		coverage : Coverage( vehicleSupplement.stackable == true , coverageCode ==
"UDCSL")
		vehicle : Vehicle( unitAtRiskNumber : unitAtRiskNumber)
		Policy( state == "NM" )
	then
		WorkItemReason fact0 = new WorkItemReason();
		fact0.setType( "Policy" );
		fact0.setReason( "6408: UM/UDM stacking change" );
		fact0.setAddBookmark( "VE" );
		fact0.setAddBookmarkUnit( unitAtRiskNumber );
		insertLogical(fact0 );
end

*Here is the stacktrace from my console:*
WARNING: java.lang.RuntimeException: cannot invoke getter:
getVehicleSupplement [declr.class: com.fbfs.pc.ecd.model.Coverage;
act.class: com.fbfs.pc.ecd.model.Coverage] (see trace)
	at
org.mvel2.optimizers.impl.refl.nodes.GetterAccessor.getValue(GetterAccessor.java:74)
	at org.mvel2.ast.ASTNode.getReducedValueAccelerated(ASTNode.java:106)
	at org.mvel2.MVELRuntime.execute(MVELRuntime.java:87)
	at
org.mvel2.compiler.CompiledExpression.getValue(CompiledExpression.java:122)
	at org.mvel2.MVEL.executeExpression(MVEL.java:954)
	at
org.drools.base.extractors.MVELClassFieldReader.getValue(MVELClassFieldReader.java:100)
	at
org.drools.base.extractors.BaseObjectClassFieldReader.isNullValue(BaseObjectClassFieldReader.java:179)
	at
org.drools.base.evaluators.EqualityEvaluatorsDefinition$BooleanEqualEvaluator.evaluate(EqualityEvaluatorsDefinition.java:555)
	at org.drools.rule.LiteralRestriction.isAllowed(LiteralRestriction.java:87)
	at org.drools.rule.LiteralConstraint.isAllowed(LiteralConstraint.java:109)
	at org.drools.reteoo.AlphaNode.assertObject(AlphaNode.java:130)
	at
org.drools.reteoo.CompositeObjectSinkAdapter.doPropagateAssertObject(CompositeObjectSinkAdapter.java:458)
	at
org.drools.reteoo.CompositeObjectSinkAdapter.propagateAssertObject(CompositeObjectSinkAdapter.java:386)
	at org.drools.reteoo.ObjectTypeNode.assertObject(ObjectTypeNode.java:215)
	at org.drools.reteoo.EntryPointNode.assertObject(EntryPointNode.java:244)
	at org.drools.common.NamedEntryPoint.insert(NamedEntryPoint.java:330)
	at org.drools.common.NamedEntryPoint.insert(NamedEntryPoint.java:291)
	at
org.drools.common.AbstractWorkingMemory.insert(AbstractWorkingMemory.java:886)
	at
org.drools.base.DefaultKnowledgeHelper.insert(DefaultKnowledgeHelper.java:180)
	at
org.drools.base.DefaultKnowledgeHelper.insert(DefaultKnowledgeHelper.java:174)
	at
PolicyDefinitionRules.Rule_policyDefinition_UnitAtRiskCoverage.defaultConsequence(Rule_policyDefinition_UnitAtRiskCoverage.java:8)
	at
PolicyDefinitionRules.Rule_policyDefinition_UnitAtRiskCoverageDefaultConsequenceInvoker.evaluate(Unknown
Source)
	at org.drools.common.DefaultAgenda.fireActivation(DefaultAgenda.java:1091)
	at org.drools.common.DefaultAgenda.fireNextItem(DefaultAgenda.java:1029)
	at org.drools.common.DefaultAgenda.fireAllRules(DefaultAgenda.java:1251)
	at
org.drools.common.AbstractWorkingMemory.fireAllRules(AbstractWorkingMemory.java:708)
	at
org.drools.common.AbstractWorkingMemory.fireAllRules(AbstractWorkingMemory.java:672)
	at
org.drools.impl.StatefulKnowledgeSessionImpl.fireAllRules(StatefulKnowledgeSessionImpl.java:218)
	at
com.fbfs.pc.rulesengine.controller.RulesEngine.runRulesEngine(RulesEngine.java:318)


My expectation that Exceptions thrown from the consequence (RHS) are
returned to the calling program was confirmed when I added the following 2
lines to a rule that was not experiencing the above issue:
     String myString = null;
     System.out.println(myString.trim());

Is there any good documentation on Drools exception handling?
Current configuration is Drools 5.3.0, MVEL 2.1.0, Glassfish 2.1

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