Question

I´ve got a problem with the following rule:

rule "Término sin Traducción"
    salience -100
    dialect "mvel"
    when
        traductor : TraductorDeEventosTratados()
            eventoGenerico : EventoGenerico() from traductor.eventoGenerico
    then
        System.out.println("Evento generico: " + eventoGenerico);
            traductor.setEventoGenerico( null );
            update( traductor );
            retract( eventoGenerico );
end

It causes a NullPointerException when retracting the “eventoGenerico”, as though it doesn’t exist in the working memory (it exists actually, and another rule sets the eventoGenerico to traductor previously):

Exception executing consequence for rule "Término sin Traducción" in RULA_PROV.SYSTEM_RULES: [Error: drools.retract( eventoGenerico ): null]
[Near : {... System.out.println("Evento gen ....}]
             ^
[Line: 1, Column: 1]
        at org.drools.runtime.rule.impl.DefaultConsequenceExceptionHandler.handleException(DefaultConsequenceExceptionHandler.java:39)
        at org.drools.common.DefaultAgenda.fireActivation(DefaultAgenda.java:1297)

However, if I make this little change it works fine (this verifies that the eventoGenerico really exists in the working memory):

rule "Término sin Traducción"
    salience -100
    dialect "mvel"
    when
        traductor : TraductorDeEventosTratados()
            eventoGenerico : EventoGenerico()
            eventoGenerico2 : EventoGenerico( this == eventoGenerico ) from traductor.eventoGenerico
    then
        System.out.println("Evento generico: " + eventoGenerico);
            traductor.setEventoGenerico( null );
            update( traductor );
            retract( eventoGenerico );
end

It looks like a bug, any ideas?

Thanks in advance

Answer 1

This appears to be a bug in Drools 5.5 caused by the usage of mvel dialect. I managed to reproduce your error using this simple test:

Java:

Model model = new Model("Model A");
DataSample data = new DataSample(model);
ksession.insert(model);
ksession.insert(data);

ksession.fireAllRules();

Drools:

rule "Rule 1"
dialect "mvel"
when
    $d: DataSample()
    $m: Model() from $d.model
then
    $d.setModel(null);
    update($d);
    retract($m);
end

If you remove the dialect “mvel” modifier, the rule works as expected. Recently in Drools’ mailing list there were some bug reports regarding mvel. Maybe this is one of them.

Best Regards,