Question

In Spring MVC, one can define interceptors that can perform work before and after a particular controller is invoked. This can be used, for example, to do logging, authentication etc.

The programmer who wishes to write a custom interceptor is supposed to implement the HandlerInterceptor interface. To aid this task, the HandlerInterceptorAdaptor abstract base class has been provided, which provides default implementations of all the methods specified in the interface. So, if just wants to do some pre processing, one can just extend HandlerInterceptorAdaptor and @Override public boolean preHandle(...), and not worry about implementing the postHandle function.

My doubt concerns the name. From what I understand of the Adapter pattern, it adapts syntactic impedance mismatches between interfaces.

1. Is that so? If yes, should the class providing the boilerplate implementations be called HandlerInterceptorDefaultImpl, or something along those lines?

2. Is there a different nomenclature/pattern for what is happening here?

3. Is the fact that we need a boilerplate class a code smell, and could be removed by refactoring the HandlerInterceptor interface into two: HandlerPreInterceptor and HandlerPostInterceptor? Or is that overkill?

Answer 1

From GOF book about the Adapter pattern:

Adapters vary in the amount of work they do to adapt Adaptee to the Target Interface. There is a spectrum of possible work, from simple interface conversion-for example,changing the names of operations-to supporting an entirely different set of operations. The amount of work Adapter does depends on how similar the Target interface is to Adaptee’s.

The boilerplate class that you are referring to is called skeletal implementation class. This is mentioned in Effective Java by Joshua Bloch. From the book:

You can combine the virtues of interfaces and abstract classes by providing an abstract skeletal implementation class to go with each nontrivial interface that you export. The interface still defines the type, but the skeletal implementation takes all of the work out of implementing it.

By convention, skeletal implementations are called AbstractInterface, where Interface is the name of the interface they implement. For example, the Collections Framework provides a skeletal implementation to go along with each main collection interface: AbstractCollection, AbstractSet, AbstractList, and
AbstractMap. Arguably it would have made sense to call them SkeletalCollection, SkeletalSet, SkeletalList, and SkeletalMap, but the Abstract convention is now firmly established.