表示层模式:Front Controller—前端控制器模式

Context
The presentation-tier request handling mechanism must control and coordinate
processing of each user across multiple requests. Such control mechanisms may be
managed in either a centralized or decentralized manner.
Problem
The system requires a centralized access point for presentation-tier request handling
to support the integration of system services, content retrieval, view management, and
navigation. When the user accesses the view directly without going through a centralized
mechanism, two problems may occur:
Each view is required to provide its own system services, often resulting in duplicate
code.
View navigation is left to the views. This may result in commingled view content and
view navigation.
Additionally, distributed control is more difficult to maintain, since changes will
often need to be made in numerous places.
Forces
Common system services processing completes per request. For example, the security
service completes authentication and authorization checks.
Logic that is best handled in one central location is instead replicated within
numerous views.
Decision points exist with respect to the retrieval and manipulation of data.
Multiple views are used to respond to similar business requests.
A centralized point of contact for handling a request may be useful, for example, to
control and log a user's progress through the site.
System services and view management logic are relatively sophisticated.
Solution
Use a controller as the initial point of contact for handling a request. The controller
manages the handling of the request, including invoking security services such as
authentication and authorization, delegating business processing, managing the choice of an
appropriate view, handling errors, and managing the selection of content creation strategies.
The controller provides a centralized entry point that controls and manages Web
request handling. By centralizing decision points and controls, the controller also helps
reduce the amount of Java code, called scriptlets, embedded in the JavaServer Pages (JSP)
page.
Centralizing control in the controller and reducing business logic in the view
promotes code reuse across requests. It is a preferable approach to the
alternative-embedding code in multiple views-because that approach may lead to a more
error-prone, reuse-by-copy- and-paste environment.
Typically, a controller coordinates with a dispatcher component. Dispatchers are
responsible for view management and navigation. Thus, a dispatcher chooses the next view
for the user and vectors control to the resource. Dispatchers may be encapsulated within the
controller directly or can be extracted into a separate component.
While the Front Controller pattern suggests centralizing the handling of all requests,
it does not limit the number of handlers in the system, as does a Singleton. An application
may use multiple controllers in a system, each mapping to a set of distinct services.