"Classes are dead, long live interfaces" was declared by Rickard Oberg at Oredev this week where he announced Qi4j. Qi4j brings the new idea of Composite Oriented Programing, in which is no behaviour at all is put in a class, instead the class becomes a 'composite' of mixins and interfaces declared on the class via annotations.
Qi4j itself is a Java framework built to make Composite Oriented Programming available to all Java developers. While Composite Oriented Programming will require a large mind-shift, it does not require separate tools, language, or XML. Qi4j applications will be able to run inside a Spring application, a web application container, OSGi, and elsewhere. Qi4j makes heavy use of annotations to push the knowledge of framework specifics away and bring developers closer to the business rules. Rickard Öberg and Niclas Hedhman, the founders of the project, describe it in a press release as:
"Qi4j is a framework for domain centric application development, including evolved concepts from Aspect Oriented Programming, Dependency Injection and Domain-Driven Design, leveraging the Java 5 platform and eco-system", says Niclas Hedhman a long-term Java framework developer and now active Apache member.
"I need a system that puts the domain model and business rules back in focus, allowing clear communication between domain experts and developers.", says Rickard Öberg, the former JBoss, Webwork and XDoclet founder.
As this is only the initial revealing of the framework and paradigm, documentation and examples are still sparse. However, the goal of Composite Oriented Programming is to switch from thinking about objects to thinking about composites. Composites are composed of re-useable fragments. Fragments are the building blocks of Qi4j. They carry the state of composites (in mixins), validate and constrain the usage of them (in constraints), handle cross-cutting concerns (in concerns) and provide notifications, cache and undo management, and other asynchronous needs (in side effects). The switch of programming paradigms is not an easy one. Oberg describes the reactions to Composite Oriented Programming as:
The pattern I have seen very consistently is two reactions: a headache from the brain frantically trying to unlearn the bad ideas from everything they previously did, and a smile from understanding the potential of what we are suggesting
Qi4j is still very new and unstable. At this point they are working on stabilizing the framework and trying to get the ideas of Composite Oriented Programming developed and presented to the community. They admit that they are not quite ready for people to dive in to the framework, and suggest people focus on understanding the concepts for the next couple months.