In current aspect-oriented systems, aspects usually carry, through their pointcuts, explicit references to the base code. Those references are fragile and hinder important software engineering properties such as modular reasoning and independent evolution of aspects and base code. In this work, we introduce a novel abstraction called Join Point Interface, which, by design, aids modular reasoning and independent evolution by decoupling aspects from base code and by providing a modular type-checking algorithm. Join point interfaces can be used both with implicit announcement through pointcuts, and with explicit announcement, using closure join points. Join point interfaces further offer polymorphic dispatch on join points, with an advice-dispatch semantics akin to multimethods. To support flexible join point matching, we incorporate into our language an earlier proposal for generic advice, and introduce a mechanism for controlled global quantification. We motivate each language feature in detail, showing that it is necessary to obtain a language design that is both type safe and flexible enough to support typical aspect-oriented programming idioms. We have implemented join point interfaces as an open-source extension to AspectJ. A case study on existing aspect-oriented programs supports our design, and in particular shows the necessity of both generic interfaces and some mechanism for global quantification.

While aspect-oriented programming supports the modular definition of crosscutting concerns, most approaches to aspect- oriented programming fail to improve, or even preserve, modular reasoning. The main problem is that aspects usually carry, through their pointcuts, explicit references to the base code. These dependencies make programs fragile. Changes in the base code can unwittingly break a pointcut definition, rendering the aspect ineffective or causing spurious matches. Conversely, a change in a pointcut definition may cause parts of the base code to be advised without notice. Therefore separate development of aspect-oriented programs is largely compromised, which in turns seriously hinders the adoption of aspect-oriented programming by practitioners. We propose to separate base code and aspects using Join Point Interfaces, which are contracts between aspects and base code. Base code can define pointcuts that expose selected join points through a Join Point Interface. Conversely, an aspect can offer to advise join points that pro- vide a given Join Point Interface. Crucially, however, aspect themselves cannot contain pointcuts, and hence cannot refer to base code elements. In addition, because a given join point can provide several Join Point Interfaces, and Join Point Interfaces can be organized in a subtype hierarchy, our approach supports join point polymorphism. We describe a novel advice dispatch mechanism that offers a flexible and type-safe approach to aspect reuse.