How to do null or empty check in drools - mvel dialect?

Question

When the configList is null, the AND logic should not proceed further, but I am getting this error - "array index out of bounds".

Below is the rule :

rule "testRule"
   when
       config : Config( configList != null && !configList.empty && configList[0].attribute != null )
   then
       // logic
end

Before answering your question, are you aware that checking for null is not the same as checking for "emptiness"? — Esteban Aliverti, Jul 16 '18 at 12:43
Hi, I do know the difference but I posted the wrong code, please check the edited code as it is still giving the same error. — Runtime Terror - BS, Jul 16 '18 at 13:54

score 1 · Accepted Answer · answered Jul 16 '18 at 14:46

Because of the way Drools executes the conditions of the rules, short-circuit logical operators are not guaranteed. In some cases they work, but in some other cases they don't.

As a workaround, you might split the single pattern you have into two:

rule "testRule"
when
  config : Config( $configList: configList != null, configList.empty == false)
  Attribute() from $configList.get(0)
then
  // logic
end

I'm assuming that $configList is a list of Attribute objects.

Hope it helps,

How to do null or empty check in drools - mvel dialect?

1 Answers1