[aspectj-users] Generics in return types causing warnings

[Rens van Leeuwen <rens.van.leeuwen@xxxxxxxxxxxx>]

Hi everybody,

I've been trying to get a hierarchy of classes return defensive copies of instances of the Java Collections Framework. The idea is, that a caller should not be able to add/remove elements of a returned List<?> for instance, because the entity that returned that list should be the one managing it.

I thought of using an aspect, and when trying to do so, I found out that it works, but it produces quite a bunch of warnings in both my Eclipse workspace as well as during weaving during a Maven build. I'm wondering if there's anything I can do to get rid of those warnings.

I wrote the following class to define the aspect (stripped to only contain an example method):

@Aspect
public final class DefensiveCopyCollectionsReturningAspect {

    /**
     * Every 'getter' method in the portal model hierarchy that returns a 
     * {@link List} should return a defensive copy so that calling clients 
     * don't change the internal structure of the portal model.
     *
     * @param   joinPoint  The joinpoint.
     * @return  An defensive copy of the returned list.
     */
    @SuppressWarnings("unchecked", "rawtypes")
    @Around("execution(public !static java.util.List<*> (AbstractPM+).get*())")
    public List makeDefensiveCopyOfReturnedList(final ProceedingJoinPoint joinPoint) {
        try {
            return new ArrayList((List) joinPoint.proceed());
        } catch (final Throwable exception) {
            throw new PortalRuntimeException(exception);
        }
    }
}

I've defined methods that do similar things for Set and Map as well, which works like a charm. However, since the return type of the method is java.util.List, without any generics involved, I see the following warnings:

-- unchecked conversion when advice applied at shadow method-execution(java.util.List com.leanapps.portal.model.pension.participation.ParticipationPM.getDocuments()), expected java.util.List<com.leanapps.portal.model.report.DocumentPM> but advice uses java.util.List [Xlint:uncheckedAdviceConversion]

Now here's the million (OK, maybe a little less ;)) dollar question: Is there any way to get the generics to work? I've tried several things, but they all don't seem to work. I've tried

    <T> List<T> makeDefensiveCopyOfReturnedList(final ProceedingJoinPoint joinPoint) { .. }

which results in compilation errors:

-- incompatible return type applying to method-execution(java.util.List com.leanapps.portal.model.pension.participation.ParticipationPM.getDocuments())

If I try to add generics to the annotation:

    @Around("execution(public !static java.util.List<T+> (AbstractPM+).get*())")

I see that no advice has been applied at all:

-- advice defined in com.leanapps.portal.model.DefensiveCopyCollectionsReturningAspect has not been applied

Hope that somebody on this list has an answer to this.

Kind regards,

Rens