1. Chapter 30 - Servlets: Bonus for Java Developers
Outline
Introduction
Servlet Overview and Architecture
Interface Servlet and the Servlet Life Cycle
HttpServlet Class
HttpServletRequest Interface
HttpServletResponse Interface
Handling HTTP get Requests
Setting Up the Apache Tomcat Server
Deploying a Web Application
Handling HTTP get Requests Containing Data
Handling HTTP post Requests
Redirecting Requests to Other Resources
Session Tracking
Cookies
Session Tracking with HttpSession
Multi-tier Applications: Using JDBC from a Servlet
HttpUtils Class
Internet and World Wide Web Resources

2. 30.1 Introduction Java Networking Fundamental capabilities
Network connections are sockets
Contained in package java.net
Higher-level packages
Java.rmi (Remote Method Invocation classes)
Allows JVMs to communicate via remote method calls
Based on capabilities of package java.net
Org.omg (Contains CORBA classes)
Allows any two CORBA enabled applications to communicate.

3. Client-Server Relationship
30.1 Introduction
Client-Server Relationship
Client sends request, server responds
Common implemetation – comunication between Web Server and Web Browser

4. 30.1 Introduction Servlets Extend server functionality
Often used with Web Servers
To generate dynamic XHTML
To provide secure access
To interact with a database
Packages javax.servlet, javax.servlet.http
Provide classes and interfaces for servlets

5. 30.2 Servlet Overview and Architecture
Hypertext Transfer Protocol
Basis to World Wide Web Communication
Uses URIs (Uniform Resource Identifiers)
URIs locate resources on internet

6. 30.2 Servlet Overview and Architecture
Servlet Containers (Servlet Engines)
Provide servlet runtime environment
Manage servlet lifecycle
Incorporated in many popular web servers
Microsoft Internet Information Server (IIS)
Apache HTTP Server
Many more
Redirects HTTP requests to appropriate servlet

7. 30.2.1 Interface Servlet and the Servlet Life Cycle
In package javax.servlet
Must be implemented by all servlets
Servlet methods automatically invoked by servlet container

8. 30.2.1 Interface Servlet and the Servlet Life Cycle

9. 30.2.1 Interface Servlet and the Servlet Life Cycle
Servlet Container Manages Life Cycle
Invokes method init when servlet is initially loaded
Usually in response to its first request
Invokes method service to handle request
Servlet processes request
Obtains request from ServletRequest object
Servlet responds to client
Writes to ServletResponse object
Called once per request
Invokes method destroy to terminate servlet
Releases servlet resources

10. 30.2.1 Interface Servlet and the Servlet Life Cycle
Servlet Abstract Implementations
Provide default Servlet implementations
GenericServlet
Package javax.servlet
Used in non-web servlets
HttpServlet
Package javax.servlet.http
Used for Web-based servlets
Defines enhanced Web processing capabilities

11. 30.2.2 HttpServlet Class Class HttpServlet Overrides method service
Differentiates between HTTP get and post requests
Defines methods doGet and doPost to process requests
Receives HttpServletRequest and HttpServletResponse objects

12. HttpServlet Class

13. 30.2.3 HttpServletRequest Interface
Created by servlet container and passed to service method
HttpServlet relays the object to doGet or doPost
Contains client request and request processing methods

14. 30.2.3 HttpServletRequest Interface

15. 30.2.4 HttpServletResponse Interface
Created by servlet container and passed to service method
HttpServlet relays the object to doGet or doPost
Provides methods to formulate responses

16. 30.2.4 HTTP ServletResponse Interface

17. 30.3 Handling HTTP get Requests
Primarily used to retrieve content of a URI
Usually HTML or XHTML
Can be images or binary data

18. Upcoming example demonstrates get request handling

19. WelcomeServlet.java Define WelcomeServlet doGet
1 // Fig. 9.5: WelcomeServlet.java
2 // A simple servlet to process get requests.
3 package com.deitel.advjhtp1.servlets; 4
5 import javax.servlet.*;
6 import javax.servlet.http.*;
7 import java.io.*; 8
9 public class WelcomeServlet extends HttpServlet { 10
// process "get" requests from clients
protected void doGet( HttpServletRequest request,
HttpServletResponse response )
throws ServletException, IOException {
response.setContentType( "text/html" );
PrintWriter out = response.getWriter(); 18
// send XHTML page to client 20
// start XHTML document
out.println( "<?xml version = \"1.0\"?>" ); 23
out.println( "<!DOCTYPE html PUBLIC \"-//W3C//DTD " +
"XHTML 1.0 Strict//EN\" \" +
"/TR/xhtml1/DTD/xhtml1-strict.dtd\">" ); 27
out.println(
"<html xmlns = \" ); 30
// head section of document
out.println( "<head>" );
out.println( "<title>A Simple Servlet Example</title>" );
out.println( "</head>" ); 35
WelcomeServlet.java Define WelcomeServlet doGet
Extends HttpServlet to create a Web-based Servlet.
Process get request by creating an XHTML document.

20. WelcomeServlet.java 36 // body section of document
out.println( "<body>" );
out.println( "<h1>Welcome to Servlets!</h1>" );
out.println( "</body>" ); 40
// end XHTML document
out.println( "</html>" );
out.close(); // close stream to complete the page }
45 }
WelcomeServlet.java

21. Main XHTML page which sends a get request to the servlet.
1 <?xml version = "1.0"?>
2 <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN"
" 4
5 <!-- Fig. 9.6: WelcomeServlet.html --> 6
7 <html xmlns = "
8 <head>
<title>Handling an HTTP Get Request</title>
10 </head> 11
12 <body>
<form action = "/advjhtp1/welcome1" method = "get"> 14
<p><label>Click the button to invoke the servlet
<input type = "submit" value = "Get HTML Document" />
</label></p> 18
</form>
20 </body>
21 </html>
Main XHTML page which sends a get request to the servlet.
WelcomeServlet.html

22. WelcomeServlet.java

23. 30.3.1 Setting up the Apache Tomcat Server
Fully functional JSP and Servlet container
Includes a Web Server
Integrates with popular Web Servers
Apache Web Server
Microsoft Internet Information Server (IIS)
Default Port is 8080

24. Testing and Debugging Tip 30.1
Fig Tomcat documentation home page. (Courtesy of The Apache Software Foundation.)

25. 30.3.2 Deploying a Web Application
Consists of JSPs and servlets
Rigid directory structure
Often within a Web Application Archive (WAR)
Configured via deployment descriptor
Web.xml
Maps servlet name and location
Defines URL pattern

26. 30.3.2 Deploying a Web Application

27. 1 <!-- Advanced Java How to Program JSP/servlet context -->
2 <Context path = "/advjhtp1"
docBase = "webapps/advjhtp1"
reloadable = "true">
5 </Context>
Fig Context element for servlet and JSP examples in Chapters 30 and 31.

28. Web.xml Map servlet name and class Map servlet name and URL
1 <!DOCTYPE web-app PUBLIC
"-//Sun Microsystems, Inc.//DTD Web Application 2.2//EN"
" 4
5 <web-app> 6
<!-- General description of your Web application -->
<display-name>
Advanced Java How to Program JSP
and Servlet Chapter Examples
</display-name> 12
<description>
This is the Web application in which we
demonstrate our JSP and Servlet examples.
</description> 17
<!-- Servlet definitions -->
<servlet>
<servlet-name>welcome1</servlet-name> 21
<description>
A simple servlet that handles an HTTP get request.
</description> 25
<servlet-class>
com.deitel.advjhtp1.servlets.WelcomeServlet
</servlet-class>
</servlet> 30
<!-- Servlet mappings -->
<servlet-mapping>
<servlet-name>welcome1</servlet-name>
Web.xml Map servlet name and class Map servlet name and URL
Maps a servlet name to its fully qualified class name.

29. Fig. 30.10 Deployment descriptor for the advjhtp1 Web application.
<url-pattern>/welcome1</url-pattern>
</servlet-mapping> 36
37 </web-app>
Map servlet to URL pattern. The pattern is relative to the server address and application context root.
Fig Deployment descriptor for the advjhtp1 Web application.

30. 30.3.2 Deploying a Web Application

31. 30.4 Handling HTTP get Requests Containing Data
Requests and Data
Http request follows the format Servlet_url?query_string
Query string format
Parameter_name=value
Name/value pairs separated by &

32. 1 // Fig. 9.12: WelcomeServlet2.java
2 // Processing HTTP get requests containing data.
3 package com.deitel.advjhtp1.servlets; 4
5 import javax.servlet.*;
6 import javax.servlet.http.*;
7 import java.io.*; 8
9 public class WelcomeServlet2 extends HttpServlet { 10
// process "get" request from client
protected void doGet( HttpServletRequest request,
HttpServletResponse response )
throws ServletException, IOException {
String firstName = request.getParameter( "firstname" ); 17
response.setContentType( "text/html" );
PrintWriter out = response.getWriter(); 20
// send XHTML document to client 22
// start XHTML document
out.println( "<?xml version = \"1.0\"?>" ); 25
out.println( "<!DOCTYPE html PUBLIC \"-//W3C//DTD " +
"XHTML 1.0 Strict//EN\" \" +
"/TR/xhtml1/DTD/xhtml1-strict.dtd\">" ); 29
out.println(
"<html xmlns = \" ); 32
// head section of document
out.println( "<head>" );
WelcomeServlet2.java Define Servlet WelcomeServlet2 Write XHTML response
Obtain the parameter from the request and assign the value to a string.

33. Include the value in the response.
out.println(
"<title>Processing get requests with data</title>" );
out.println( "</head>" ); 38
// body section of document
out.println( "<body>" );
out.println( "<h1>Hello " + firstName + ",<br />" );
out.println( "Welcome to Servlets!</h1>" );
out.println( "</body>" ); 44
// end XHTML document
out.println( "</html>" );
out.close(); // close stream to complete the page }
49 }
WelcomeServlet2.java
Include the value in the response.

34. WelcomeServlet2.html Send get request
1 <?xml version = "1.0"?>
2 <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN"
" 4
5 <!-- Fig. 9.13: WelcomeServlet2.html --> 6
7 <html xmlns = "
8 <head>
<title>Processing get requests with data</title>
10 </head> 11
12 <body>
<form action = "/advjhtp1/welcome2" method = "get"> 14
<p><label>
Type your first name and press the Submit button
<br /><input type = "text" name = "firstname" />
<input type = "submit" value = "Submit" />
</p></label> 20
</form>
22 </body>
23 </html>
WelcomeServlet2.html Send get request

35. Program Output

36. 30.4 Handling HTTP get Requests Containing Data

37. 30.5 Handling HTTP post Requests
Posts data from HTML form
Server-side form handler parses data
Not cached
Ensures client has most updated version
Default HttpServlet doPost method disables post

38. Obtain the firstname parameter from the post request.
1 // Fig. 9.15: WelcomeServlet3.java
2 // Processing post requests containing data.
3 package com.deitel.advjhtp1.servlets; 4
5 import javax.servlet.*;
6 import javax.servlet.http.*;
7 import java.io.*; 8
9 public class WelcomeServlet3 extends HttpServlet { 10
// process "post" request from client
protected void doPost( HttpServletRequest request,
HttpServletResponse response )
throws ServletException, IOException {
String firstName = request.getParameter( "firstname" ); 17
response.setContentType( "text/html" );
PrintWriter out = response.getWriter(); 20
// send XHTML page to client 22
// start XHTML document
out.println( "<?xml version = \"1.0\"?>" ); 25
out.println( "<!DOCTYPE html PUBLIC \"-//W3C//DTD " +
"XHTML 1.0 Strict//EN\" \" +
"/TR/xhtml1/DTD/xhtml1-strict.dtd\">" ); 29
out.println(
"<html xmlns = \" ); 32
// head section of document
out.println( "<head>" );
WelcomeServlet3.java Define Servlet WelcomeServlet3 Handle doPost Write XHTML response
Obtain the firstname parameter from the post request.

39. WelcomeServlet3.java 35 out.println(
"<title>Processing post requests with data</title>" );
out.println( "</head>" ); 38
// body section of document
out.println( "<body>" );
out.println( "<h1>Hello " + firstName + ",<br />" );
out.println( "Welcome to Servlets!</h1>" );
out.println( "</body>" ); 44
// end XHTML document
out.println( "</html>" );
out.close(); // close stream to complete the page }
49 }
WelcomeServlet3.java

40. Submit a post request with a firstname parameter
1 <?xml version = "1.0"?>
2 <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN"
" 4
5 <!-- Fig. 9.16: WelcomeServlet3.html --> 6
7 <html xmlns = "
8 <head>
<title>Handling an HTTP Post Request with Data</title>
10 </head> 11
12 <body>
<form action = "/advjhtp1/welcome3" method = "post"> 14
<p><label>
Type your first name and press the Submit button
<br /><input type = "text" name = "firstname" />
<input type = "submit" value = "Submit" />
</label></p> 20
</form>
22 </body>
23 </html>
WelcomeServlet3.html
Submit a post request with a firstname parameter

41. Program Output

42. 30.5 Handling HTTP post Requests

43. 30.6 Redirecting Requests to Other Resources
Redirection
Allows a Servlet to redirect a request
Use sendRedirect method of HttpServletResponse

44. Obtain the page parameter for redirection location.
1 // Fig. 9.18: RedirectServlet.java
2 // Redirecting a user to a different Web page.
3 package com.deitel.advjhtp1.servlets; 4
5 import javax.servlet.*;
6 import javax.servlet.http.*;
7 import java.io.*; 8
9 public class RedirectServlet extends HttpServlet { 10
// process "get" request from client
protected void doGet( HttpServletRequest request,
HttpServletResponse response )
throws ServletException, IOException {
String location = request.getParameter( "page" ); 17
if ( location != null ) 19
if ( location.equals( "deitel" ) )
response.sendRedirect( " );
else
if ( location.equals( "welcome1" ) )
response.sendRedirect( "welcome1" ); 25
// code that executes only if this servlet
// does not redirect the user to another page 28
response.setContentType( "text/html" );
PrintWriter out = response.getWriter(); 31
// start XHTML document
out.println( "<?xml version = \"1.0\"?>" );
RedirectServlet.java Define Servlet RedirectServlet Process get request
Obtain the page parameter for redirection location.
Redirect user to another page or servlet.

45. RedirectServlet.java Create error page34
out.println( "<!DOCTYPE html PUBLIC \"-//W3C//DTD " +
"XHTML 1.0 Strict//EN\" \" +
"/TR/xhtml1/DTD/xhtml1-strict.dtd\">" ); 38
out.println(
"<html xmlns = \" ); 41
// head section of document
out.println( "<head>" );
out.println( "<title>Invalid page</title>" );
out.println( "</head>" ); 46
// body section of document
out.println( "<body>" );
out.println( "<h1>Invalid page requested</h1>" );
out.println( "<p><a href = " +
"\"servlets/RedirectServlet.html\">" );
out.println( "Click here to choose again</a></p>" );
out.println( "</body>" ); 54
// end XHTML document
out.println( "</html>" );
out.close(); // close stream to complete the page }
59 }
RedirectServlet.java Create error page

46. Send get request to RedirectServlet with the page parameter.
1 <?xml version = "1.0"?>
2 <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN"
" 4
5 <!-- Fig. 9.19: RedirectServlet.html --> 6
7 <html xmlns = "
8 <head>
<title>Redirecting a Request to Another Site</title>
10 </head> 11
12 <body>
<p>Click a link to be redirected to the appropriate page</p>
<p>
<a href = "/advjhtp1/redirect?page=deitel">
/>
<a href = "/advjhtp1/redirect?page=welcome1">
Welcome servlet</a>
</p>
20 </body>
21 </html>
RedirectServlet.html
Send get request to RedirectServlet with the page parameter.

47. Program Output

48. 30.6 Redirecting Requests to Other Resources

49. 30.7 Session Tracking Session Tracking Information techniques
Track consumer internet movement
Integrate user supplied information
Personalization benefits
Product targeting
Individualized attention
Bypass irrelevant content
Tracking pitfalls
Privacy issues
Security of sensitive information

50. Tracking Technologies
30.7 Session Tracking
Tracking Technologies
Cookies
Section
Session tracking
Section
URL rewriting
Information embedded in URL parameters
Hidden form elements
Information contained in hidden form elements
Each form retains previous form information

51. 30.7.1 Cookies Cookies Small text data files
Often contain unique user identifiers
Locates server-side client information
Transmitted in HTTP header information
Persists for browsing session or maximum age
Expires when maximum age reached
Lasts for browsing session by default

52. 1 // Fig. 9.21: CookieServlet.java
2 // Using cookies to store data on the client computer.
3 package com.deitel.advjhtp1.servlets; 4
5 import javax.servlet.*;
6 import javax.servlet.http.*;
7 import java.io.*;
8 import java.util.*; 9
10 public class CookieServlet extends HttpServlet {
private final Map books = new HashMap(); 12
// initialize Map books
public void init() {
books.put( "C", " " );
books.put( "C++", " " );
books.put( "Java", " " );
books.put( "VB6", " " ); } 21
// receive language selection and send cookie containing
// recommended book to the client
protected void doPost( HttpServletRequest request,
HttpServletResponse response )
throws ServletException, IOException {
String language = request.getParameter( "language" );
String isbn = books.get( language ).toString();
Cookie cookie = new Cookie( language, isbn ); 31
response.addCookie( cookie ); // must precede getWriter
response.setContentType( "text/html" );
PrintWriter out = response.getWriter(); 35
CookieServlet.java Define Servlet CookieServlet Init book ISBN doPost writes cookie
Create a new cookie containing the client language and ordered book ISBN number.

53. CookieServlet.java Write XHTML response
// send XHTML page to client 37
// start XHTML document
out.println( "<?xml version = \"1.0\"?>" ); 40
out.println( "<!DOCTYPE html PUBLIC \"-//W3C//DTD " +
"XHTML 1.0 Strict//EN\" \" +
"/TR/xhtml1/DTD/xhtml1-strict.dtd\">" ); 44
out.println(
"<html xmlns = \" ); 47
// head section of document
out.println( "<head>" );
out.println( "<title>Welcome to Cookies</title>" );
out.println( "</head>" ); 52
// body section of document
out.println( "<body>" );
out.println( "<p>Welcome to Cookies! You selected " +
language + "</p>" ); 57
out.println( "<p><a href = " +
"\"/advjhtp1/servlets/CookieSelectLanguage.html\">" +
"Click here to choose another language</a></p>" ); 61
out.println( "<p><a href = \"/advjhtp1/cookies\">" +
"Click here to get book recommendations</a></p>" );
out.println( "</body>" ); 65
// end XHTML document
out.println( "</html>" );
out.close(); // close stream } 70
CookieServlet.java Write XHTML response

54. CookieServlet.java doGet process get request Obtain user cookies
// read cookies from client and create XHTML document
// containing recommended books
protected void doGet( HttpServletRequest request,
HttpServletResponse response )
throws ServletException, IOException {
Cookie cookies[] = request.getCookies(); // get cookies 78
response.setContentType( "text/html" );
PrintWriter out = response.getWriter(); 81
// start XHTML document
out.println( "<?xml version = \"1.0\"?>" ); 84
out.println( "<!DOCTYPE html PUBLIC \"-//W3C//DTD " +
"XHTML 1.0 Strict//EN\" \" +
"/TR/xhtml1/DTD/xhtml1-strict.dtd\">" ); 88
out.println(
"<html xmlns = \" ); 91
// head section of document
out.println( "<head>" );
out.println( "<title>Recommendations</title>" );
out.println( "</head>" ); 96
// body section of document
out.println( "<body>" ); 99
// if there are any cookies, recommend a book for each ISBN
if ( cookies != null && cookies.length != 0 ) {
out.println( "<h1>Recommendations</h1>" );
out.println( "<p>" );
104
// get the name of each cookie
Get any cookies from client.
CookieServlet.java doGet process get request Obtain user cookies

55. Cycle through cookies and display book recommendation.
for ( int i = 0; i < cookies.length; i++ )
out.println( cookies[ i ].getName() +
" How to Program. ISBN#: " +
cookies[ i ].getValue() + "<br />" );
110
out.println( "</p>" ); }
else { // there were no cookies
out.println( "<h1>No Recommendations</h1>" );
out.println( "<p>You did not select a language.</p>" ); }
117
out.println( "</body>" );
119
// end XHTML document
out.println( "</html>" );
out.close(); // close stream }
124 }
Cycle through cookies and display book recommendation.
CookieServlet.java

56. CookieSelectLanguage.html Static XHTML page
1 <?xml version = "1.0"?>
2 <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN"
" 4
5 <!-- Fig. 9.22: CookieSelectLanguage.html --> 6
7 <html xmlns = "
8 <head>
<title>Using Cookies</title>
10 </head> 11
12 <body>
<form action = "/advjhtp1/cookies" method = "post"> 14
<p>Select a programming language:</p>
<p>
<input type = "radio" name = "language"
value = "C" />C <br /> 19
<input type = "radio" name = "language"
value = "C++" />C++ <br /> 22
<!-- this radio button checked by default -->
<input type = "radio" name = "language"
value = "Java" checked = "checked" />Java<br /> 26
<input type = "radio" name = "language"
value = "VB6" />VB 6
</p> 30
<p><input type = "submit" value = "Submit" /></p> 32
</form>
34 </body>
35 </html>
CookieSelectLanguage.html Static XHTML page
Post user selected language.

57. Program Output

58. Program Output

59. Program Output

60. Cookies

61. Cookies

62. Cookies

63. 30.7.2 Session Tracking with HttpSession
HttpSession interface
Assigns a unique ID to user
Stores name/value pairs, attributes
Values are any Java object
Expire
Browser Session ends
Servlet invalidated
Servlet container restart

64. SessionServlet.java Define Servlet SessionServlet
1 // Fig. 9.25: SessionServlet.java
2 // Using HttpSession to maintain client state information.
3 package com.deitel.advjhtp1.servlets; 4
5 import javax.servlet.*;
6 import javax.servlet.http.*;
7 import java.io.*;
8 import java.util.*; 9
10 public class SessionServlet extends HttpServlet {
private final Map books = new HashMap(); 12
// initialize Map books
public void init() {
books.put( "C", " " );
books.put( "C++", " " );
books.put( "Java", " " );
books.put( "VB6", " " ); } 21
// receive language selection and create HttpSession object
// containing recommended book for the client
protected void doPost( HttpServletRequest request,
HttpServletResponse response )
throws ServletException, IOException {
String language = request.getParameter( "language" ); 29
// Get the user's session object.
// Create a session (true) if one does not exist.
HttpSession session = request.getSession( true ); 33
// add a value for user's choice to session
SessionServlet.java Define Servlet SessionServlet
Obtain client session, if the client does not have a session, create one.

65. SessionServlet.java Process post request
session.setAttribute( language, books.get( language ) ); 36
response.setContentType( "text/html" );
PrintWriter out = response.getWriter(); 39
// send XHTML page to client 41
// start XHTML document
out.println( "<?xml version = \"1.0\"?>" ); 44
out.println( "<!DOCTYPE html PUBLIC \"-//W3C//DTD " +
"XHTML 1.0 Strict//EN\" \" +
"/TR/xhtml1/DTD/xhtml1-strict.dtd\">" ); 48
out.println(
"<html xmlns = \" ); 51
// head section of document
out.println( "<head>" );
out.println( "<title>Welcome to Sessions</title>" );
out.println( "</head>" ); 56
// body section of document
out.println( "<body>" );
out.println( "<p>Welcome to Sessions! You selected " +
language + ".</p>" ); 61
// display information about the session
out.println( "<p>Your unique session ID is: " +
session.getId() + "<br />" ); 65
out.println(
"This " + ( session.isNew() ? "is" : "is not" ) +
" a new session<br />" ); 69
SessionServlet.java Process post request

66. SessionServlet.java process get request
out.println( "The session was created at: " +
new Date( session.getCreationTime() ) + "<br />" ); 72
out.println( "You last accessed the session at: " +
new Date( session.getLastAccessedTime() ) + "<br />" ); 75
out.println( "The maximum inactive interval is: " +
session.getMaxInactiveInterval() + " seconds</p>" ); 78
out.println( "<p><a href = " +
"\"servlets/SessionSelectLanguage.html\">" +
"Click here to choose another language</a></p>" ); 82
out.println( "<p><a href = \"sessions\">" +
"Click here to get book recommendations</a></p>" );
out.println( "</body>" ); 86
// end XHTML document
out.println( "</html>" );
out.close(); // close stream } 91
// read session attributes and create XHTML document
// containing recommended books
protected void doGet( HttpServletRequest request,
HttpServletResponse response )
throws ServletException, IOException {
// Get the user's session object.
// Do not create a session (false) if one does not exist.
HttpSession session = request.getSession( false );
101
// get names of session object's values
Enumeration valueNames;
104
SessionServlet.java process get request
Obtain the client session, but do not create one if it does not exist.

67. Obtain the attributes of the client. Then display any recommendations.
if ( session != null )
valueNames = session.getAttributeNames();
else
valueNames = null;
109
PrintWriter out = response.getWriter();
response.setContentType( "text/html" );
112
// start XHTML document
out.println( "<?xml version = \"1.0\"?>" );
115
out.println( "<!DOCTYPE html PUBLIC \"-//W3C//DTD " +
"XHTML 1.0 Strict//EN\" \" +
"/TR/xhtml1/DTD/xhtml1-strict.dtd\">" );
119
out.println(
"<html xmlns = \" );
122
// head section of document
out.println( "<head>" );
out.println( "<title>Recommendations</title>" );
out.println( "</head>" );
127
// body section of document
out.println( "<body>" );
130
if ( valueNames != null &&
valueNames.hasMoreElements() ) {
out.println( "<h1>Recommendations</h1>" );
out.println( "<p>" );
135
String name, value;
137
// get value for each name in valueNames
while ( valueNames.hasMoreElements() ) {
Obtain the attributes of the client. Then display any recommendations.
SessionServlet.java

68. SessionServlet.java 140 name = valueNames.nextElement().toString();
value = session.getAttribute( name ).toString();
142
out.println( name + " How to Program. " +
"ISBN#: " + value + "<br />" ); }
146
out.println( "</p>" ); }
else {
out.println( "<h1>No Recommendations</h1>" );
out.println( "<p>You did not select a language.</p>" ); }
153
out.println( "</body>" );
155
// end XHTML document
out.println( "</html>" );
out.close(); // close stream }
160 }
SessionServlet.java

69. 1 <?xml version = "1.0"?>
2 <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN"
" 4
5 <!-- Fig. 9.26: SessionSelectLanguage.html --> 6
7 <html xmlns = "
8 <head>
<title>Using Sessions</title>
10 </head> 11
12 <body>
<form action = "/advjhtp1/sessions" method = "post"> 14
<p>Select a programming language:</p>
<p>
<input type = "radio" name = "language"
value = "C" />C <br /> 19
<input type = "radio" name = "language"
value = "C++" />C++ <br /> 22
<!-- this radio button checked by default -->
<input type = "radio" name = "language"
value = "Java" checked = "checked" />Java<br /> 26
<input type = "radio" name = "language"
value = "VB6" />VB 6
</p> 30
<p><input type = "submit" value = "Submit" /></p> 32
</form>
34 </body>
35 </html>
SessionSelectLanguage.html
XHTML page that allows the client to select a programming language they would like a recommendation for.

70. Program Output

71. Program Output

72. Program Output

73. 30.7.2 Session Tracking with HttpSession

74. 30.8 Multi-tier Applications: Using JDBC from a Servlet
Multi-tier Architecture
Client
Commonly Web browser
XHTML
HTML
Applet
Middle Tier
Logic component examples:
Servlets
JSPs
Provides interface between client and database

75. 30.8 Multi-tier Applications: Using JDBC from a Servlet
Multi-tier Architecture
Backend tier
Usually a Database
Accessed via JDBC from servlets
Stores information
Client information
Product information

76. 30.8 Multi-tier Applications: Using JDBC from a Servlet
Java Database Connectivity (JDBC)
Provides uniform access to database systems
Middle tier will work with any JDBC database
Developer not required to write for a specific database
Works with SQL-based queries
Database interaction handled by JDBC driver
Driver usually provided by database vendor

77. SurveyServlet.java Define servlet SurveyServlet set up database
1 // Fig. 9.27: SurveyServlet.java
2 // A Web-based survey that uses JDBC from a servlet.
3 package com.deitel.advjhtp1.servlets; 4
5 import java.io.*;
6 import java.text.*;
7 import java.sql.*;
8 import javax.servlet.*;
9 import javax.servlet.http.*; 10
11 public class SurveyServlet extends HttpServlet {
private Connection connection;
private PreparedStatement updateVotes, totalVotes, results; 14
// set up database connection and prepare SQL statements
public void init( ServletConfig config )
throws ServletException {
// attempt database connection and create PreparedStatements
try {
Class.forName( "COM.cloudscape.core.RmiJdbcDriver" );
connection = DriverManager.getConnection(
"jdbc:rmi:jdbc:cloudscape:animalsurvey" ); 24
// PreparedStatement to add one to vote total for a
// specific animal
updateVotes =
connection.prepareStatement(
"UPDATE surveyresults SET votes = votes + 1 " +
"WHERE id = ?" ); 32
// PreparedStatement to sum the votes
totalVotes =
connection.prepareStatement(
SurveyServlet.java Define servlet SurveyServlet set up database
Specify database driver. Connect to database through that driver.
Define SQL query to obtain database information

78. SurveyServlet.java handle post request
"SELECT sum( votes ) FROM surveyresults" ); 38
// PreparedStatement to obtain surveyoption table's data
results =
connection.prepareStatement(
"SELECT surveyoption, votes, id " +
"FROM surveyresults ORDER BY id" ); } 46
// for any exception throw an UnavailableException to
// indicate that the servlet is not currently available
catch ( Exception exception ) {
exception.printStackTrace();
throw new UnavailableException(exception.getMessage()); } 53
} // end of init method 55
// process survey response
protected void doPost( HttpServletRequest request,
HttpServletResponse response )
throws ServletException, IOException {
// set up response to client
response.setContentType( "text/html" );
PrintWriter out = response.getWriter();
DecimalFormat twoDigits = new DecimalFormat( "0.00" ); 65
// start XHTML document
out.println( "<?xml version = \"1.0\"?>" ); 68
out.println( "<!DOCTYPE html PUBLIC \"-//W3C//DTD " +
"XHTML 1.0 Strict//EN\" \" +
SurveyServlet.java handle post request

79. Update database with post parameter.
"/TR/xhtml1/DTD/xhtml1-strict.dtd\">" ); 72
out.println(
"<html xmlns = \" ); 75
// head section of document
out.println( "<head>" ); 78
// read current survey response
int value =
Integer.parseInt( request.getParameter( "animal" ) ); 82
// attempt to process a vote and display current results
try { 85
// update total for current survey response
updateVotes.setInt( 1, value );
updateVotes.executeUpdate(); 89
// get total of all survey responses
ResultSet totalRS = totalVotes.executeQuery();
totalRS.next();
int total = totalRS.getInt( 1 ); 94
// get results
ResultSet resultsRS = results.executeQuery();
out.println( "<title>Thank you!</title>" );
out.println( "</head>" ); 99
out.println( "<body>" );
out.println( "<p>Thank you for participating." );
out.println( "<br />Results:</p><pre>" );
103
// process results
int votes;
SurveyServlet.java
Update database with post parameter.
Obtain the survey totals and information via SQL requests.

80. Display results in XHTML response. SurveyServlet.java
106
while ( resultsRS.next() ) {
out.print( resultsRS.getString( 1 ) );
out.print( ": " );
votes = resultsRS.getInt( 2 );
out.print( twoDigits.format(
( double ) votes / total * 100 ) );
out.print( "% responses: " );
out.println( votes ); }
116
resultsRS.close();
118
out.print( "Total responses: " );
out.print( total );
121
// end XHTML document
out.println( "</pre></body></html>" );
out.close(); }
126
// if database exception occurs, return error page
catch ( SQLException sqlException ) {
sqlException.printStackTrace();
out.println( "<title>Error</title>" );
out.println( "</head>" );
out.println( "<body><p>Database error occurred. " );
out.println( "Try again later.</p></body></html>" );
out.close(); }
136
} // end of doPost method
138
// close SQL statements and database when servlet terminates
public void destroy()
Display results in XHTML response.
SurveyServlet.java

81. SurveyServlet.java Close database{
// attempt to close statements and database connection
try {
updateVo\tes.close();
totalVotes.close();
results.close();
connection.close(); }
149
// handle database exceptions by returning error to client
catch( SQLException sqlException ) {
sqlException.printStackTrace(); }
} // end of destroy method
155 }
SurveyServlet.java Close database

82. Static survey XHTML page.
1 <?xml version = "1.0"?>
2 <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN"
" 4
5 <!-- Survey.html --> 6
7 <html xmlns = "
8 <head>
<title>Survey</title>
10 </head> 11
12 <body>
13 <form method = "post" action = "/advjhtp1/animalsurvey"> 14
<p>What is your favorite pet?</p> 16
<p>
<input type = "radio" name = "animal"
value = "1" />Dog<br />
<input type = "radio" name = "animal"
value = "2" />Cat<br />
<input type = "radio" name = "animal"
value = "3" />Bird<br />
<input type = "radio" name = "animal"
value = "4" />Snake<br />
<input type = "radio" name = "animal"
value = "5" checked = "checked" />None
</p> 29
<p><input type = "submit" value = "Submit" /></p> 31
32 </form>
33 </body>
34 </html>
Survey.html
Static survey XHTML page.

83. Program Output

84. 30.8.1 Configuring animalsurvey Database and SurveyServlet

85. 30.9 HttpUtils Class HttpUtils Class
Three static utility methods to simplify servlet development

86. 30.9 HttpUtils Class