Rule extension is an extremely useful feature for authoring technical rules, and it deserves a high level of attention, which it hasn't really been given up to now.

But, for the scenario that's been presented here, I'm inclined to think that an approach without extends would be much better:

The RuleConstant object doesn't have to be bound to $CONSTANT - apparently fields such as SEDAN and COUPE are static fields and could be used simply as RuleConstant.SEDAN, etc.
Keeping objects in a single container fact ("Vehicles") and extracting them by using "from" in each rule is not good for performance. Simply inserting all individual Vehicle facts is better, and it avoids the inevitable loops.

rule "Custom|2"
enabled true
when
$vehicle:Vehicle(modelYear > 2000, type = RuleConstant.COUPE)
then
modify( $vehicle ){
setValue(100)
}
end

-W

On 5 April 2013 19:21, Arul Prashanth <arul.prashanth@gmail.com> wrote:

I have a Rule implementation as below which works as expected, here
Vehicles() and RuleConstant are facts.

/rule "Rule|1"
no-loop true
when
$CONSTANT : RuleConstant();
$vehicles : Vehicles();
$vehicle:Vehicle(modelYear > 2000, type = $CONSTANT.SEDAN) from
$vehicles.Vehicle;
then
$vehicle.setValue(100);
update($vehicles);
end /

But am planning to abstract the rules in the below fashion so that we can
reuse the LHS conditions which doesn't work and throws the below said Null
Pointer Exception

*BaseRule.drl*

/package com.xyz.base

rule "Base|1"
no-loop true
enabled false
when
$CONSTANT : RuleConstant();
$vehicles : Vehicles();
then

end /

*CustomRule.drl*

/package com.xyz.custom

rule "Custom|1" extends "Base|1"
no-loop true
enabled true
when
$vehicle:Vehicle(modelYear > 2000, type = $CONSTANT.SEDAN) from
$vehicles.Vehicle;
then
$vehicle.setValue(100);
update($vehicles);
end

rule "Custom|2" extends "Base|1"
no-loop true
enabled true
when
$vehicle:Vehicle(modelYear > 2000, type = $CONSTANT.COUPE) from
$vehicles.Vehicle;
then
$vehicle.setValue(100);
update($vehicles);
end/

*Exception:*

Caused by: [Error: drools.update($vehicles): null]
[Line: 1, Column: 1]
at
org.mvel2.optimizers.impl.refl.ReflectiveAccessorOptimizer.compileGetChain(ReflectiveAccessorOptimizer.java:416)
at
org.mvel2.optimizers.impl.refl.ReflectiveAccessorOptimizer.optimizeAccessor(ReflectiveAccessorOptimizer.java:143)
at org.mvel2.ast.ASTNode.optimize(ASTNode.java:159)
at org.mvel2.ast.ASTNode.getReducedValueAccelerated(ASTNode.java:115)
at org.mvel2.MVELRuntime.execute(MVELRuntime.java:85)
at
org.mvel2.compiler.CompiledExpression.getValue(CompiledExpression.java:123)
at
org.mvel2.compiler.CompiledExpression.getValue(CompiledExpression.java:116)
at org.mvel2.MVEL.executeExpression(MVEL.java:930)
at org.drools.base.mvel.MVELConsequence.evaluate(MVELConsequence.java:101)
at org.drools.common.DefaultAgenda.fireActivation(DefaultAgenda.java:1273)
... 33 more
Caused by: java.lang.reflect.InvocationTargetException
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
at java.lang.reflect.Method.invoke(Unknown Source)
at
org.mvel2.optimizers.impl.refl.ReflectiveAccessorOptimizer.getMethod(ReflectiveAccessorOptimizer.java:1105)
at
org.mvel2.optimizers.impl.refl.ReflectiveAccessorOptimizer.getMethod(ReflectiveAccessorOptimizer.java:987)
at
org.mvel2.optimizers.impl.refl.ReflectiveAccessorOptimizer.compileGetChain(ReflectiveAccessorOptimizer.java:377)
... 42 more
Caused by: java.lang.NullPointerException
at org.drools.common.DefaultAgenda.createActivation(DefaultAgenda.java:569)
at
org.drools.reteoo.RuleTerminalNode.modifyLeftTuple(RuleTerminalNode.java:297)
at
org.drools.reteoo.SingleLeftTupleSinkAdapter.propagateModifyChildLeftTuple(SingleLeftTupleSinkAdapter.java:246)
at org.drools.reteoo.JoinNode.modifyLeftTuple(JoinNode.java:363)
at
org.drools.reteoo.SingleLeftTupleSinkAdapter.propagateModifyChildLeftTuple(SingleLeftTupleSinkAdapter.java:272)
at
org.drools.reteoo.EvalConditionNode.modifyLeftTuple(EvalConditionNode.java:213)
at
org.drools.reteoo.SingleLeftTupleSinkAdapter.propagateModifyChildLeftTuple(SingleLeftTupleSinkAdapter.java:259)
at
org.drools.reteoo.FromNode.checkConstraintsAndPropagate(FromNode.java:337)
at org.drools.reteoo.FromNode.modifyLeftTuple(FromNode.java:281)
at
org.drools.reteoo.CompositeLeftTupleSinkAdapter.propagateModifyChildLeftTuple(CompositeLeftTupleSinkAdapter.java:316)
at org.drools.reteoo.JoinNode.modifyRightTuple(JoinNode.java:273)
at org.drools.reteoo.BetaNode.modifyObject(BetaNode.java:531)
at org.drools.reteoo.JoinNode.modifyObject(JoinNode.java:446)
at
org.drools.reteoo.CompositeObjectSinkAdapter.doPropagateModifyObject(CompositeObjectSinkAdapter.java:507)
at
org.drools.reteoo.CompositeObjectSinkAdapter.propagateModifyObject(CompositeObjectSinkAdapter.java:432)
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:470)
at org.drools.common.NamedEntryPoint.update(NamedEntryPoint.java:370)
at
org.drools.base.DefaultKnowledgeHelper.update(DefaultKnowledgeHelper.java:317)
at
org.drools.base.DefaultKnowledgeHelper.update(DefaultKnowledgeHelper.java:329)
at
org.drools.base.DefaultKnowledgeHelper.update(DefaultKnowledgeHelper.java:325)
... 49 more

-----
- Prashanth
--
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