Java Server Pages

A More Modular Design Using Action Objects

The approach in Example 14.3 is to implement each action as a separate method in the servlet, so every time you add a new action, you also have to update the servlet. In a large application, you may find yourself adding more and more code to the controller servlet, eventually ending up with a class that’s hard to maintain.

A more modular approach is to treat each action as a separate class that implements a common interface, for instance called Action.^[6] The Action interface may look like this:

import java.io.*;
import javax.servlet.*;
import javax.servlet.http.*;

public interface Action {
    public void perform(HttpServlet servlet, 
        HttpServletRequest request,
        HttpServletResponse response) 
        throws IOException, ServletException;
}

The single method, perform( ) , has arguments that give the action class access to all the same objects as a regular servlet: the request and response objects. In addition, the servlet argument lets the action class access the servlet context and servlet configuration objects if needed.

With this approach, each action ends up as a simple class. For instance, the logout action is handled by a nice little class like this:

import java.io.*; import javax.servlet.*; import javax.servlet.http.*; public class LogoutAction implements Action { public void perform(HttpServlet servlet, HttpServletRequest request, HttpServletResponse response) throws IOException, ServletException { HttpSession session = request.getSession( ); session.invalidate( ...