Contract4J for Design by Contract in Java: Design Pattern-Like Protocols and Aspect Interfaces

Recent trends in Aspect-oriented Design (AOD) have emphasized interface-based modularity constructs that support noninvasive advising of components by aspects in a robust and flexible way. We show how the AspectJ-based tool Contract4J supports Design by Contract in Java using two different forms of a design patternlike protocol, one based on Java 5 annotations and the other based on a JavaBeans-like method naming convention. Neither form resembles conventional Java-style interfaces, yet each promotes the same goals of abstraction and minimal coupling between the cross-cutting concern of contract enforcement and the components affected by the contracts. Unlike traditional implementations of design patterns, which tend to be ad hoc and require manual enforcement by the developer, the Contract4J protocol offers at least partial support for compile time enforcement of proper usage. This example suggests that the concept of an interface in AOD should more fully encompass usage protocols and conversely, aspects give us ways to enforce proper usage programmatically.