java設計模式與j2ee設計模式(編輯修改稿)

　

【文章內容簡介】 prone, reusebycopy andpaste environment.Typically, a controller coordinates with a dispatcher ponent. 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 ponent.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.. View Helper—視圖助手模式ContextThe system creates presentation content, which requires processing of dynamic business data.ProblemPresentation tier changes occur often and are difficult to develop and maintain when business data access logic and presentation formatting logic are interwoven. This makes the system less flexible, less reusable, and generally less resilient to change.Intermingling the business and systems logic with the view processing reduces modularity and also provides a poor separation of roles among Web production and software development teams.ForcesBusiness data assimilation requirements are nontrivial. Embedding business logic in the view promotes a copyandpaste type of reuse. This causes maintenance problems and bugs because a piece of logic is reused in the same or different view by simply duplicating it in the new location. It is desirable to promote a clean separation of labor by having different individuals fulfill the roles of software developer and Web production team member. One view is monly used to respond to a particular business request. SolutionA view contains formatting code, delegating its processing responsibilities to its helper classes, implemented as JavaBeans or custom tags. Helpers also store the view39。s intermediate data model and serve as business data adapters.There are multiple strategies for implementing the view ponent. The JSP View Strategy suggests using a JSP as the view ponent. This is the preferred strategy, and it is the one most monly used. The other principal strategy is the Servlet View Strategy, which utilizes a servlet as the view (see the section Strategies for more information).Encapsulating business logic in a helper instead of a view makes our application more modular and facilitates ponent reuse. Multiple clients, such as controllers and views, may leverage the same helper to retrieve and adapt similar model state for presentation in multiple ways. The only way to reuse logic embedded in a view is by copying and pasting it elsewhere. Furthermore, copyandpaste duplication makes a system harder to maintain, since the same bug potentially needs to be corrected in multiple places.A signal that one may need to apply this pattern to existing code is when scriptlet code dominates the JSP view. The overriding goal when applying this pattern, then, is the partitioning of business logic outside of the view. While some logic is best encapsulated within helper objects, other logic is better placed in a centralized ponent that sits in front of the views and the helpersthis might include logic that is mon across multiple requests, such as authentication checks or logging services, for example. Refer to the Intercepting Filter on page 4 and Front Controller on page 21 for more information on these issues.If a separate controller is not employed in the architecture, or is not used to handle all requests, then the view ponent bees the initial contact point for handling some requests. For certain requests, particularly those involving minimal processing, this scenario works fine. Typically, this situation occurs for pages that are based on static information, such as the first of a set of pages that will be served to a user to gather some information (see Dispatcher View on page 232). Additionally, this scenario occurs in some cases when a mechanism is employed to create posite pages (see Composite View on page 203).The View Helper pattern focuses on remending ways to partition your application responsibilities. For related discussions about issues dealing with directing client requests directly to a view, please refer to the section Dispatcher View on page 232.. Composite View—復合視圖模式ContextSophisticated Web pages present content from numerous data sources, using multiple subviews that prise a single display page. Additionally, a variety of individuals with different skill sets contribute to the development and maintenance of these Web pages.ProblemInstead of providing a mechanism to bine modular, atomic portions of a view into a posite whole, pages are built by embedding formatting code directly within each view.Modification to the layout of multiple views is difficult and error prone, due to the duplication of code.ForcesAtomic portions of view content change frequently. Multiple posite views use similar subviews, such as a customer inventory table. These atomic portions are decorated with different surrounding template text, or they appear in a different location within the page. Layout changes are more difficult to manage and code harder to maintain when subviews are directly embedded and duplicated in multiple views. Embedding frequently changing portions of template text directly into views also potentially affects the availability and administration of the system. The server may need to be restarted before clients see the modifications or updates to these template ponents. SolutionUse posite views that are posed of multiple atomic subviews. Each ponent of the template may be included dynamically into the whole and the layout of the page may be managed independently of the content.This solution provides for the creation of a posite view based on the inclusion and substitution of modular dynamic and static template fragments. It promotes t