设计模式基于 Web 的应用程序

First, the Controller part should implement the Front Controller pattern (which is a specialized kind of Mediator pattern). It should consist of only a single servlet which provides a centralized entry point of all requests. It should create the Model based on information available by the request, such as the pathinfo or servletpath, the method and/or specific parameters. The Business Model is called Action in the below HttpServlet example.

protected void service(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
    try {
        Action action = ActionFactory.getAction(request);
        String view = action.execute(request, response);

        if (view.equals(request.getPathInfo().substring(1)) {
            request.getRequestDispatcher("/WEB-INF/" + view + ".jsp").forward(request, response);
        }
        else {
            response.sendRedirect(view); // We'd like to fire redirect in case of a view change as result of the action (PRG pattern).
        }
    }
    catch (Exception e) {
        throw new ServletException("Executing action failed.", e);
    }
}

执行操作应该返回一些标识符来定位视图.最简单的方法是将它用作 JSP 的文件名.将此 servlet 映射到 web.xml 中的特定 url-pattern，例如/pages/*、*.do 甚至只是 *.html.

public interface Action {
    public String execute(HttpServletRequest request, HttpServletResponse response) throws Exception;
}

public class LoginAction implements Action {

    public String execute(HttpServletRequest request, HttpServletResponse response) throws Exception {
        String username = request.getParameter("username");
        String password = request.getParameter("password");
        User user = userDAO.find(username, password);

        if (user != null) {
            request.getSession().setAttribute("user", user); // Login user.
            return "home"; // Redirect to home page.
        }
        else {
            request.setAttribute("error", "Unknown username/password. Please retry."); // Store error message in request scope.
            return "login"; // Go back to redisplay login form with error.
        }
    }

}

public static Action getAction(HttpServletRequest request) {
    return actions.get(request.getMethod() + request.getPathInfo());
}

actions.put("POST/register", new RegisterAction());
actions.put("POST/login", new LoginAction());
actions.put("GET/logout", new LogoutAction());
// ...

for (Entry entry : configuration) {
    actions.put(entry.getKey(), Class.forName(entry.getValue()).newInstance());
}

for (ClassFile classFile : classpath) {
    if (classFile.isInstanceOf(Action.class)) {
       actions.put(classFile.getAnnotation("mapping"), classFile.newInstance());
    }
}