ex6_75.txt

j2ee core design patterns

Example 6.75 Dispatcher View Controller Servlet
public class Controller extends HttpServlet {

	/** 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 nextview;
		try {
			LogManager.recordStrategy(request, "Dispatcher View",
				" Servlet Front Strategy;" + 
				" Template-Based View Strategy;" +
				" Custom tag helper Strategy");
			LogManager.logMessage(request, getSignature(), 
				"Process incoming request. ");

			// Use a helper object to gather parameter 
			// specific information.
			RequestHelper helper = new 
				RequestHelper(request, response);
			LogManager.logMessage(request, getSignature(),
				" Authenticate user");

			Authenticator auth = new BasicAuthenticator();
			auth.authenticate(helper);

			// This is an oversimplification for the sake of 
			// simplicity. Typically, there will be a 
			// mapping from logical name to resource name at 
			// this point
			LogManager.logMessage(request, getSignature(), 
				"Getting nextview");
			nextview = request.getParameter("nextview");

			LogManager.logMessage(request, getSignature(), 
				"Dispatching to view: " + nextview);
		}
		catch (Exception e) {
			LogManager.logMessage("Handle exception appropriately", 
				e.getMessage() );


			/** ApplicationResources provides a simple API 
			 * for retrieving constants and other 
			 * preconfigured values**/
			nextview = ApplicationResources.getInstance().
				getErrorPage(e);
		}
		dispatch(request, response, nextview);
	}

	/** 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);
	}

	/** Returns a short description of the servlet. */
	public String getServletInfo() {
		return getSignature();
	}

	public void init(ServletConfig config) throws ServletException {
		super.init(config);
	}

	public void destroy() { }

	/**
	 * dispatcher method
	 */
	protected void dispatch(HttpServletRequest request, 
			HttpServletResponse response, 				String page) 
			throws javax.servlet.ServletException, java.io.IOException {
		RequestDispatcher dispatcher = 
			getServletContext().getRequestDispatcher(page);
		dispatcher.forward(request, response);
	}

	private String getSignature() {
		return "DispatcherView-Controller"; 
	}
}