Extension Interface

All contents are extracted from POSA2.

 

The Extension Interface design pattern allows multiple interfaces to be exported by a component, to prevent bloating of interfaces and breaking of client code when developers extend or modify the functionality of the component.

 

The structure of the Extension interface pattern includes four participants:

 

Components: Components aggregate and implement various types of service-specific functionality. This functionality can often be partitioned into several independent roles, each of which defines a set of semantically-related operations.

 

Extension interfaces: Extension interfaces export selected facets of a component's implementation. There is one extension interface for each role that a component implements. In addition, an extension interface implicitly specifies a contract that describes how clients should use the component's functionality. This contract defines the protocol for invoking the methods of the extension interface, such as the acceptable parameter types and the order in which methods must be called.

 

Root interface: The root interface is a special extension interface that provides three types of functionality, 1. Core functionality that all extension interfaces must support, for example functionality that allows clients to retrieve the interfaces they request. This functionality defines the basic mechanisms a component must implement to allow clients to retrieve and navigate among its interfaces. 2. Domain-independent functionality, such as  methods that manage component lifecycles. 3. Domain-specific functionality that should be provided by all components within a particular domain.

 

Clients: Clients access the functionality provided by components only via extension interfaces. After a client retrieves a reference to an initial extension interface, it can use this reference to retrieve any other extension interface supported by a component.