最近几年来，地理信息系统无论是在理论上还是应用上都处在一个飞速发展的阶段。 GIS被应用于多个领域的建模和决策支持，如城市管理、区划、环境整治等等，地理信息成为信息时代重要的组成部分之一； “数字地球”概念的提出，更进一步推动了作为其技术支撑的GIS的发展。 与此同时，一些学者致力于相关的理论研究，如空间感知、空间数据误差、空间关系的形式化等等。 这恰好说明了地理信息系统作为应用技术和学科的两个方面，并且这两个方面构成了相互促进的发展过程。
The front-end controller pattern (Front Controller Pattern) is used to provide a centralized request processing mechanism, and all requests will be processed by a single handler. The handler can do authentication / authorization / log, or track the request and then pass the request to the appropriate handler. The following are the entities of this design pattern.
前端控制器（Front Controller） -A single handler that handles all types of requests from an application, either web-based or desktop-based.
调度器（Dispatcher） -the front-end controller may use a scheduler object to schedule requests to the corresponding specific handler.
视图（View） -views are objects created for requests.
6.32.1. Realize ¶
We will create FrontController 、 Dispatcher As a front-end controller and a scheduler, respectively. HomeView And StudentView Represents various views created for requests received by the front-end controller.
FrontControllerPatternDemo Our demo class uses the FrontController To demonstrate the front-end controller design pattern.
6.32.2. Step 1 ¶
Create a view.HomeView.java ¶
publicclassHomeView{publicvoidshow(){System.out.println("Displaying Home Page");}}
StudentView.java ¶
publicclassStudentView{publicvoidshow(){System.out.println("Displaying Student Page");}}
6.32.3. Step 2 ¶
Create a scheduler Dispatcher.Dispatcher.java ¶
publicclassDispatcher{privateStudentViewstudentView;privateHomeViewhomeView;publicDispatcher(){studentView=newStudentView();homeView=newHomeView();}publicvoiddispatch(Stringrequest){if(request.equalsIgnoreCase("STUDENT")){studentView.show();}else{homeView.show();}}}
6.32.4. Step 3 ¶
Create the front controller FrontController.FrontController.java ¶
publicclassFrontController{privateDispatcherdispatcher;publicFrontController(){dispatcher=newDispatcher();}privatebooleanisAuthenticUser(){System.out.println("User is authenticated successfully.");returntrue;}privatevoidtrackRequest(Stringrequest){System.out.println("Page requested:"+request);}publicvoiddispatchRequest(Stringrequest){//记录每一个请求trackRequest(request);//对用户进行身份验证if(isAuthenticUser()){dispatcher.dispatch(request);}}}
6.32.5. Step 4 ¶
Use FrontController To demonstrate the front-end controller design pattern.FrontControllerPatternDemo.java ¶
publicclassFrontControllerPatternDemo{publicstaticvoidmain(String[]args){FrontControllerfrontController=newFrontController();frontController.dispatchRequest("HOME");frontController.dispatchRequest("STUDENT");}}
6.32.6. Step 5 ¶
Execute the program and output the result:
Page requested: HOME User is authenticated successfully. Displaying Home Page Page requested: STUDENT User is authenticated successfully. Displaying Student Page