A Definition, Overview and Taxonomy of Generic Aspect Languages

Aspect-orientation promises better modularity than pure object-oriented decomposition. A typical benefit of increased modularity is ease of maintenance, evolution and reuse. However, it has been noted by various researchers that many of the first generation aspect languages do not provide the degree of reusability initially hoped for. In this paper, we argue that the problem of insufficient reusability is due to a lack of support for aspect genericity. We analyze various problems of traditional aspect languages and define aspect genericity to be the ability to concisely express aspect effects that vary depending on the context of a join point known at weave-time, without falling back to runtime reflection. We identify the ability to parameterize aspect effects with meta-variables ranging over base language elements as the technical prerequisite for achieving genericity and describe the design space for generic aspect languages in terms of five basic questions that a language designer should consider. Within this space we review the different proposals for generic aspect languages made so far and compare them by showing which part of the possible design space is explored within these languages. Among the reviewed design options we identify predicate-based binding of logic metavariables, uniform genericity and fine-grained genericity as optimal answers to three of the discussed questions. Last but not least, we identify open research problems that are specific to aspect genericity or aggravated in the presence of genericity. We expect that genericity will be a key ingredient of a second generation of aspect languages. It will significantly reduce the dependency of aspects on base programs and will automatically select behavior depending on the weave-time context of a join point in the same way as dynamic binding does depending on the type of a message receiver.