This section covers some of the best practices that can be applied when designing modular systems, and in particular, modular applications that are built on either OSGi or using the standard Eclipse extension mechanisms.
It is very common for OSGi applications to have a separate API and implementation. This allows the API to be versioned independently from any implementations that may follow.
To implement this effectively, most APIs are specified in terms of pure Java interfaces. However, it is possible that classes are present as well; exceptions must be represented as classes, as are common POJO data structures.
Eclipse documents the interfaces that are not suitable to implement and the POJOs that are not suitable to subclass with the @noimplement and @noextend JavaDoc tags. These indicate to clients that the interface is not intended to be used outside of the API and that the classes are not designed to be subclassed. The annotations are not...