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Using a Controller for JSP Pages

  • By .
  • Article is provided courtesy of Que.
  • Date: Feb 22, 2002.


  1. Planning Your JSP-MVC Application

Article Description

The Model-View-Controller, or MVC, method of programming is widely used in object-oriented programming and using J2EE. According to Drew Falkman, your JSP applications will be easy to update, easy to break up (for the purpose of scaling), and easier to maintain when you use this method.

The Model-View-Controller, or MVC, method of programming is well-known as a scalable and modular approach for object-oriented programming. This method breaks up programs into three sections: the Model, the View, and the Controller. Figure 1 shows a basic diagram of the MVC method.

Figure 1 The Model-View-Controller (MVC) method of programming is both modular and scalable.

The Model is where the true logic of a program is—including the data Model and any proprietary processing that must be done to this data.

The View is what the application user sees. This is the layout or GUI (graphical user interface), in which the user can enter data into the program and see results.

The Controller is the core of the program that controls all interactions within the application. For example, if a user enters data, it is first sent to the Controller, which sends the information to the appropriate area to be processed. The results are then sent back to the Controller, which finally returns the results to the appropriate display. The Controller is the nerve center of an application.

By breaking up an application in this way, a programmer can easily make changes to one section of the code without having to affect any of the others. If you need to change some logic, simply change the Model. If you need a new GUI, edit the View. If you want to add a more granular security construct, edit the Controller.

Often, JSP and other server-side scripting language programmers don't use this method, meaning that their code is all intertwined. If you want to change the GUI, you must maneuver around both the Controller and Model aspects of your JSP page. This can lead to unmanageable code (or spaghetti code, as we call it), and eventually requires a total redo of your code.

In the J2EE environment, it is generally recommended to use a servlet as the Controller. JavaBeans, Java classes, and Enterprise JavaBeans (EJB) components are then used for the Model. Then, JSP is reserved for the View. But as JSP programmers know, sometimes we don't have the Java know-how to create servlets or EJBs, or an application is simple enough that a JSP-only application is the simplest and best way to go.

So, what can you do to make your applications more modular and scalable? Well, what is a JSP page in the end but a servlet, anyway? So let's just use JSP as the Controller.

Planning Your JSP-MVC Application

The first step in using JSP-MVC, as with in any program, is to plan. Primarily, you will need to break up the design (View) logic (Model), and Controller issues. It might help to actually draw a flowchart of these pieces and how they will all work together.

The user interface is simple enough, and can be represented by simple HTML layout screens. It might be easiest to start with these. For example, in a shopping cart, you have the category display page, the search screen, the search results, and the form interfaces for the checkout process.

The Model section is a little more abstract, but can be done fairly easily. Your primary question in an all-JSP application is where to draw the lines. For example, querying the database to obtain the data for the product category display page in your shopping cart would technically be considered part of the Model. It might be easier (especially if you are using custom actions from your app server vendor) to simply include this in the View, however. Just be careful. If, for example, you want to change the database of product categories from Access to MS SQL Server, or even to data from a JavaBeans component, will this be difficult? If the answer is yes, then break it out. This is the whole point of the MVC method.

The Controller is probably the most abstract section of your code. This single JSP page controls everything in your application. The information passed from your users will always be sent to this page first. So you should make sure that this page is organized and well-commented. Remember, there should be no actual logic here, mostly just flow control. Checking for certain variables, checking security; then including files or redirecting to the appropriate display page.

Building the Controller

The actual Controller page is mostly made of if statements, includes, and forwards. Make sure to build this page in the correct order. First, ask yourself the following questions:

  1. What security is needed? Will certain users only be allowed to access this application?

  2. What will the different steps be? Are they linear? How does the flow look? Drawing out a decision tree, as shown in Figure 2, is a good idea.

  3. What data or pages need to be shown in every page? What will be in certain groups of pages (such as all the checkout)?

Figure 2 Drawing a decision-based flowchart can help plan the Controller page.

The name of your Controller page is irrelevant, but index.jsp usually works well because it is probably the default document.

In the beginning of this page, you perform any checks or include any files that will need to happen, no matter what part of the application the user is accessing. This includes performing security checks, setting cookies, setting the time the user started the application, and grabbing any application-wide variables. If all pages in the application have the same layout, you can also include the header file containing the HTML code for the top section of the page (up to the area in which the page-sensitive content will be placed).

So the top of your page might look like this:

<%-- check to see if user has logged in yet
   note: login is not part of this application, 
      so user is redirected to the login application
<% if (session.getAttribute("UserType") == null) { %>
 <jsp:forward url="/login.jsp" />
<% } %>

<%-- check to see if user can access this section,
   if not, then send user to Access Denied message
<% if (!session.getAttribute("UserType").equals("Customer") ||    
 !session.getAttribute("UserType").equals("Administrator")) { %>
 <jsp:forward url="/login.jsp">
  <jsp:param name="action" value="failed" />
<% } %>

<%-- include page with all application variables --%>
<%@include file="/Model/application.jsp" %>

<%-- include header template (design) --%>
<jsp:include page="/View/header.jsp" />

This file is fairly simple—as it should be. Notice that the comments are easy to read and widely used. This is always a good idea, but especially in this document. You also want to indent because you will likely have nested if statements as you move deeper into your application.

First, authentication is checked by looking at the value of a variable called UserType. If this variable doesn't exist, the user is forwarded to the /login.jsp page using a plain <jsp:forward> action. If the user gets past this, a second if block checks to see whether the user is a Customer or Administrator. If not, the user is forwarded to the /login.jsp, but this time, a variable is embedded in the <jsp:forward> using the <jsp:param> tag.

Using <jsp:param> is an important part of a Controller page because it allows you to forward request variables to be accessed in a receiving page—as if a form or URL variable were passed directly to it. This variable can now be available as a request variable in the receiving page using request.getParameter("action").

The action parameter is the next key component of your Controller page. This action can dictate the primary actions that will be performed in the application. For example, if there is no action, the first step of the application will execute—in our e-commerce application, this is probably the browse page, including the product categories. Then, you can pass action variables representing different actions. For example /index.jsp?action=updatecart will trigger the application to call on the shopping cart update functionality, /index.jsp?action=showcart will display the cart, and so on.

So, the next section of your Controller page will be made up of if/else blocks (or case blocks) that check this action variable:

<% // first check to make sure there is an action
  // if not, do default action 
  if (request.getParameter("action") == null) { %>

 <%-- display product categories --%>
 <jsp:include url="/View/productdisplay.jsp" />

<% } 
  // check to see if user is adding item to cart
  else if (request.getParameter("action").equals("additem")) { %>

 <%-- inlcude to add cart page --%>
 <jsp:include page="/Model/addtocart.jsp">
  <%-- include forwarding parameters
     note that this could be a loop that 
     autoincludes any form variables passed,
     thus protecting you from code changes --%>
  <jsp:param name="ProductID" 
     value="<%= request.getParameter(/"ProductID/") %>" />
  <jsp:param name="Quantity" 
     value="<%= request.getParameter(/"Quantity/") %>" />

<% } %>
<%-- add more else ifs for all of your processes and a final else in case of an error --%>

You can see how this will work by checking what the action is. You can even have a number of substeps within a single action (just remember to indent your ifs, and comment your code well). The key to remember is to keep all display and data handling out of this template.

The final step will be to create your actual display and processing pages. For display (or View) pages, remember that you will have your header and footer already included, so only the actual interface of the application step needs to be designed in the page.

In the processing (Model) pages, you will handle your processes; then reforward to the Controller with a new action value. For example, when you are adding an item to the shopping cart, do so and then forward the user to /index.jsp?action=displaycart, so that this will display the contents of the cart. You can even include additional variables using <jsp:param>, which can trigger additional options—such as displaying a message confirming that the appropriate item has just been added to the cart.

Overall, you will find that using an organized approach such as the MVC method to your JSP application will enable you to develop Web applications that are scalable and highly modular. Using this method, your JSP applications will be easy to update, easy to break up (for the purpose of scaling), and easier to maintain overall.