JAVA

J2EE Design Patterns: The Presentation Layer Patterns: Model-View-Controller

In any application, it is the presentation that matters most to the end user. The usability of software depends heavily on the User Interface or Presentation Layer in the case of J2EE. Since it is the Presentation Layer that presents the UI to the user, the solutions for recurring problems have to be standard. That is where Presentation Layer Design Patterns come into the picture.

Author Info:

By: A.P.Rajshekhar

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November 14, 2006

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Now comes the Controller which is implemented as a servlet. It creates an instance of the MailingBean, sets the data, and calls the method for persisting the data. If the call is successful, it sets an attribute in the request scope specifying that the operation is successful; otherwise the attribute specifies that the operation is a failure. Then the request is redirected to the View.

import javax.servlet.*;

import javax.servlet.http.*;

import java.io.IOException;

public class ListController extends HttpServlet {

public static final String FIRST_PARAM = "first";

public static final String LAST_PARAM = "last";

public static final String EMAIL_PARAM = "email";

public static final String MAILINGBEAN_ATTR = "mailingbean";

public void init(ServletConfig config)

throws ServletException {

super.init(config);

}

public void destroy( ) {

}

// handle get requests

protected void doGet(HttpServletRequest request,

HttpServletResponse response)

throws ServletException, IOException {

// read the parameters from the request

String first = request.getParameter(FIRST_PARAM);

String last = request.getParameter(LAST_PARAM);

String email = request.getParameter(EMAIL_PARAM);

// get the mailing list bean for this list

MailingBean mb = MailingBeanFactory.newInstance( );

// set the parameters into the bean

mb.setFirst(first);

mb.setLast(last);

mb.setEmail(email);

// perform the business method

boolean result = mb.doSubscribe( );

// choose a page based on the result

String op = "success";

if (!result) op = "failure";

String nextPage="/redirect.jsp";

// store the operation state in the request context

request.setAttribute(MAILINGBEAN_ATTR, op);

RequestDispatcher dispatcher =

getServletContext( ).getRequestDispatcher(nextPage);

dispatcher.forward(request, response);

}

Next comes the JSP. It checks the attribute and redirects accordingly. Here the Controller functionality has been divided into a servlet as well as the following JSP:

<%@page contentType="text/html"%>

<jsp:useBean id="mailingbean" scope="request"

class="MailingBean" />

<html>

<head><title>Subscription Results</title></head>

<body>

 

<% if

((String)request.getAttribute
(MAILINGBEAN_ATTR).equalsIgnoreCase

("success"))

<%{%>

<jsp:forward page="/success.jsp"/>

<%}

else

{

<jsp:forward page="/failure.jsp"/>

}

</body>

</html>

Here is the success.jsp:

<%@page contentType="text/html"%>

<jsp:useBean id="mailingbean" scope="request"

class="MailingBean" />

<html>

<head><title>Subscription Results</title></head>

<body>

 

Dear <jsp:getProperty name="mailingbean" property="first"/>,

 

Congrats, the address

<jsp:getProperty name="mailingbean" property="email"/>

Has been added to the list. 

</body>

</html>

And here is the failure.jsp:

<%@page contentType="text/html"%>

<jsp:useBean id="mailingbean" scope="request"

             class="MailingBean" />

<html>

<head><title>Subscription Results</title></head>

<body>

<br><br>

Dear <jsp:getProperty name="mailingbean" property="first"/>,

<br><br>

We're sorry, the address

<jsp:getProperty name="mailingbean" property="email"/>

could not be added to the list.<br><br>

The problem was:

<jsp:getProperty name="mailingbean" property="errorString"/>.

</body>

</html>

That completes the application. Though MVC provides maintainable code, there are still situations in which MVC has limitations. These cases and their solutions will be the focus of next part. Till then...

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