[rules-users] ClassCastException and other problems

[Edson Tirelli]

   Sibylle,

   Unfortunately, this is the way it was implemented to be, under a kind-of
"fail-fast" philosophy we were following. I do think there is value in
changing things in 5.0 so that, for instance:

ProcessModel($activities : activities) from $models

   Simply does not match, instead of raises a CCE, in case the model is a
TypeModel.
   But for 4.0.x, that is how things work.

    The only suggestion I can give you is, in your second problem, to not
return null on your collections. Simply return empty collections:

class Project {
   ...
   public List getModels() {
        if( this.models == null ) {
            return Collections.emptyList();
        }
        return this.models;
   }
   ...
}

   This is usually a good thing, unless you have a specific semantics for
null collections in your domain model.

    []s
    Edson

2008/6/6 Sibylle Duerr <sibylle.duerr at mymail.ch>:

> Hi there,
>
> I have three issues, where I'm not sure if these are bugs in drools, or if
> this is just the way it has to be.
>
> the used classes in this case are:
>
> Project
> Model
> ProcessModel extends Model
>
> only the Project is in the working memory, because I didn't want to load
> all elements due to performance issues (there can be thousends of elements
> to load at startup).
>
> if I have the following rule:
>
> Project($models : models)
> ProcessModel($activities : activities) from $models
>
> there happen to be two problems. if the project does not only contain
> ProcessModels but also TypeModels or others (all extend Model), this leads
> to a ClassCastException.
> I can solve this by writing the rule as follows:
>
> Project($models : models)
> $m : Model(eval($m instanceof ProcessModel)) from $models
> ProcessModel($activities : activities) from $m
>
> but this is not really nice and I hope there is a better solution for this,
> is there?
>
> the second problem this rule gives me, is when the set 'models' is empty.
> that will also throw an exception. to solve this I write:
>
> $p : Project(eval($p.hasModels()), $models : models)
> $m : Model(eval($m instanceof ProcessModel)) from $models
> ProcessModel($activities : activities) from $m
>
> and I need to use this in each case where I have possible empty sets.
>
>
> the last problem I encountered when I tried to load all elements to the
> working memory and then redefine some rules. used classes:
>
> Aggregation extends DomainElement
>
> rule:
>
> Aggregation(owner.owner != type.owner)
>
> causes some exception. the root exception is a ClassCastException:
> ProxyShadowProxy cannot be cast to Aggregation.
> when I rewrite the rule as:
>
> $element : DomainElement(eval($element instanceof Aggregation))
> Aggregation(owner.owner != type.owner) from $element
>
> everything works fine. but that again is not a nice solution.
>
> any help?
>
> best regards
> sibylle
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>



-- 
Edson Tirelli
JBoss Drools Core Development
Office: +55 11 3529-6000
Mobile: +55 11 9287-5646
JBoss, a division of Red Hat @ www.jboss.com
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