The Presentation Tier in Java

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CHAPTER 7 The Presentation Tier
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passwordMessage = ""; %> </head> <body> <h1>League Planet Login</h1> <form action="login" method="post"> <table> <tr> <th align="right >User id:</th> <td><input name="userId" type="text" value="<%= userId %>"></td> <td><span class="validator"><%= userIdMessage %></span></td> </tr> <tr> <th align="right">Password:</th> <td><input name="password" type="password" value=""></td> <td><span class="validator"><%= passwordMessage %></span></td> </tr> <tr> <td colspan="2"> </td> <td><input name="action" type="submit" value="Login">  <input name="reset"s type="reset" value="Reset" /></td> </tr> </table> </form> </body> </html>
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6 Create a second JSP named logoutjsp and edit it (see Example 716) logoutjsp also contains a page directive that enables HTTP session tracking The user session object is retrieved in the HTML <head> element using the <jsp:useBean> tag The userId is written into the page using the <jsp:getProperty> tag
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Example 716 Listing of logoutjsp
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<%@ page language="java" contentType="text/html; charset=ISO-8859-1" pageEncoding="ISO-8859-1" session="true"%> <!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 401 Transitional//EN"> <html> <head> <meta http-equiv="Content-Type" content="text/html; charset=ISO-8859-1"> <jsp:useBean class="comleagueplanetUser" id="user" scope="session" /> <title>League Planet Logout</title> <link rel="stylesheet" href="schedulecss" type="text/css"> </head> <body> <h1>League Planet Logout</h1> <form action="login" method="post">
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Iteration 7: JSP
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<table> <tr> <th align="right">User id:</th> <td><jsp:getProperty name="user" property="userId" /></td> </tr> <tr> <td colspan="2"></td> <td><input name="action" type="submit" value="Logout" /></td> </tr> </table> </form> </body> </html>
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7 In the Project Explorer, select the LoginServlet in either the src folder or under the Servlets category of the IceHockeyWeb item, right click, and select the Run As Run on Server menu item The project is published, the server starts, and the Web browser is opened on the URL
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http://localhost:8080/icehockey/login
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Note that occasionally the server may not have completely started before the browser requests the servlet, in which case you ll get a 404 error To fix this, restart the server and try again (see the sidebar in 3, What Do I Do If I Get a 404 ) The LoginServlet receives the GET request and forwards it to loginjsp The Web browser displays the League Planet Login page (see Figure 769) 8 Enter an invalid userId and password, and click the Login button to test the server-side validation logic Enter a valid userId, anne for example; and password, guest for example; and click the Login button The Web browser displays the League Planet Logout page (see Figure 770) Note that logoutjsp correctly retrieved the userId from the session object and displayed it in the Web page 9 Experiment with debugging by setting breakpoints in the servlet and JSP scriptlets, and repeat the above testing This time select the Debug As Debug on Server menu item instead of Run As Run on Server The familiar Java debugging perspective opens
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CHAPTER 7 The Presentation Tier
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Figure 769 League Planet Login
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Figure 770 League Planet Logout
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Iteration 8: Monitoring HTTP Sessions
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Summary of Iteration 7
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In this iteration you used the JSP source editor to generate dynamic Web content You also used the JSP debugger to step through your JSP code You are now ready to develop both the server and client sides of your presentation layer In the next iteration, you ll use the TCP/IP monitor to view HTTP message traffic This tool will help you debug Web applications and understand HTTP topics such as cookies and sessions
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Iteration 8: Monitoring HTTP Sessions
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HTTP Sessions
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In the preceding iteration, you explored the use of HTTP session tracking in servlets and JSPs HTTP is actually a sessionless protocol, so it is something of an abuse of terminology to talk about HTTP sessions In reality, Web application servers layer virtual HTTP sessions on top of the HTTP protocol There are several techniques for accomplishing this They all boil down to the server sending a session id in some form to the browser so that the browser sends it back to the server in the subsequent requests The server maintains state information associated with the session id and retrieves that information when it receives a request that contains the session id One way to implement session tracking is through URL rewriting and hidden form variables In this approach, every URL that the server sends in a response is rewritten to include the session id Also, every HTML form that gets sent to the server includes a hidden input field that contains the session id This ensures that any request back to the server will contain the session id Using this approach requires extra steps for the application developer For example, every URL must be explicitly rewritten when the response is generated A much simpler approach is to use client-side cookies, which were introduced by NetScape Communications In this approach, the server sends the session id using an HTTP Set-Cookie response header, and the browser returns it using a Cookie request header For a thorough treatment of cookies, see Cookies [StLaurent1998a] by Simon St Laurent J2EE includes support for session tracking If the user has enabled cookie support, then cookies are used Otherwise, URL rewriting is used The servlet runtime automatically detects whether cookies are enabled and selects the correct method However, if cookies are disabled, your application won t work correctly unless you explicitly rewrite your URLs using the encodeURL method of the HttpServletResponse class
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