ex6_14.txt

j2ee core design patterns

Example 6.14 Servlet Front Strategy 
public class FrontController extends HttpServlet {
  // Initializes the servlet.
  public void init(ServletConfig config) throws ServletException {
    super.init(config);
  }

  // Destroys the servlet.
  public void destroy() { }

  /** Processes requests for both HTTP  
    * <code>GET</code> and <code>POST</code> methods.
    * @param request servlet request
    * @param response servlet response
    */
  protected void processRequest(HttpServletRequest request,
      HttpServletResponse response) throws ServletException,
      java.io.IOException {

    String page;
    /**ApplicationResources provides a simple API 
     * for retrieving constants and other 
     * preconfigured values**/
    ApplicationResources resource = ApplicationResources.getInstance();
    try {
      // create Context Object
      RequestContext requestContext = 
          new RequestContext(request, response);

      // invoke request-processing components to handle
      // incoming request
      ApplicationController applicationController = new
          ApplicationControllerImpl();
      ResponseContext responseContext =
          applicationController.handleRequest(requestContext);
 
       // invoke response-processing components to
       // handle response logic
       applicationController.handleResponse(
           requestContext, responseContext);
    } catch (Exception e) {
      LogManager.logMessage("FrontController:exception : " +
          e.getMessage());
      request.setAttribute(resource.getMessageAttr(), 
          "Exception occurred : " + e.getMessage());
      page = resource.getErrorPage(e);
      // dispatch control to site-unavailable view
      dispatch(request, response, page);
    }
  }

  /** Handles the HTTP <code>GET</code> method.
    * @param request servlet request
    * @param response servlet response
    */
  protected void doGet(HttpServletRequest request, 
      HttpServletResponse response) throws ServletException,
      java.io.IOException {

    processRequest(request, response);
  }

  /** Handles the HTTP <code>POST</code> method.
    * @param request servlet request
    * @param response servlet response
    */
  protected void doPost(HttpServletRequest request, 
      HttpServletResponse response) 
      throws ServletException, java.io.IOException {

    processRequest(request, response);
  }

  protected void dispatch(HttpServletRequest request,
      HttpServletResponse response, String page) 
      throws javax.servlet.ServletException, java.io.IOException {

    RequestDispatcher dispatcher = this.getServletContext().
        getRequestDispatcher(page);
    dispatcher.forward(request, response);
  }

  /** Returns a short description of the servlet	 */
  public String getServletInfo() {
    return "Front Controller Pattern" + 
    " Servlet Front Strategy Example";
  }
}