Servlets Outline 1 Introduction 2 Servlet Overview and Architecture 2.1 Interface Servlet and the Servlet Life Cycle 2.2 HttpServlet Class 2.3 HttpServletRequest Interface 2.4 HttpServletResponse Interface 3 Handling HTTP get Requests 3.1 Setting Up the Apache Tomcat Server 3.2 Deploying a Web Application 4 Handling HTTP get Requests Containing Data 5 Handling HTTP post Requests 6 Redirecting Requests to Other Resources 7 Session Tracking 7.1 Cookies 7.2 Session Tracking with HttpSession 8 Multi-Tier Applications: Using JDBC from a Servlet 9 HttpUtils Class 10 Internet and World Wide Web Resources
1 Introduction Java networking capabilities –Socket-based and packet-based communications Package java.net –Remote Method Invocation (RMI) Package java.rmi –Servlets and Java Server Pages (JSP) Request-response model Packages javax.servlet javax.servlet.http javax.servlet.jsp javax.servlet.tagext Form the Web tier of J2EE
1 Introduction (Cont.) Servlets –Thin clients –Request/response mechanism –Session-tracking capabilities –redirection Tomcat –Jakarta project –Official reference implementation of the JSP and servlet standards
2 Servlet Overview and Architecture Servlet container (servlet engine) –Server that executes a servlet Web servers and application servers –Netscape iPlanet Application Server –Microsoft’s Internet Information Server (IIS) –Apache HTTP Server –BEA’s WebLogic Application Server –IBM’s WebSphere Application Server –World Wide Web Consortium’s Jigsaw Web Server
2.1 Interface Servlet and the Servlet Life Cycle Interface Servlet –All servlets must implement this interface –All methods of interface Servlet are invoked automatically Servlet life cycle –Servlet container invokes the servlet’s init method –Servlet’s service method handles requests –Servlet’s destroy method releases servlet resources when the servlet container terminates the servlet Servlet implementation –GenericServlet –HttpServlet
2.1 Interface Servlet and the Servlet Life Cycle (Cont.)
2.2 HttpServlet Class Overrides method service Two most common HTTP request types –get requests –post requests Method doGet responds to get requests Method doPost responds to post requests HttpServletRequest and HttpServletResponse objects
2.2 HttpServlet Class (Cont.)
2.3 HttpServletRequest Interface Web server –creates an HttpServletRequest object –passes it to the servlet’s service method HttpServletRequest object contains the request from the client
2.3 HttpServletRequest Interface (Cont.)
2.4 HttpServletResponse Interface Web server –creates an HttpServletResponse object –passes it to the servlet’s service method
2.4 HttpServletResponse Interface (Cont.)
3 Handling HTTP get Requests get request –Retrieve the content of a URL Example: WelcomeServlet –a servlet handles HTTP get requests
Fig. 5 WelcomeServlet that responds to a simple HTTP get request. Lines 5-6 Line 9 Lines Line 16 Line 17 Lines // Fig. 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 { // process "get" requests from clients 12 protected void doGet( HttpServletRequest request, 13 HttpServletResponse response ) 14 throws ServletException, IOException 15 { 16 response.setContentType( "text/html" ); 17 PrintWriter out = response.getWriter(); // send XHTML page to client // start XHTML document 22 out.println( " " ); out.println( "<!DOCTYPE html PUBLIC \"-//W3C//DTD " + 25 "XHTML 1.0 Strict//EN\" \" "/TR/xhtml1/DTD/xhtml1-strict.dtd\">" ); out.println( 29 " " ); // head section of document 32 out.println( " " ); 33 out.println( " A Simple Servlet Example " ); 34 out.println( " " ); 35 Import the javax.servlet and javax.servlet.http packages. Extends HttpServlet to handle HTTP get requests and HTTP post requests. Override method doGet to provide custom get request processing. Uses the response object’s setContentType method to specify the content type of the data to be sent as the response to the client. Uses the response object’s getWriter method to obtain a reference to the PrintWriter object that enables the servlet to send content to the client. Create the XHTML document by writing strings with the out object’s println method.
Fig. 5 WelcomeServlet that responds to a simple HTTP get request. Line // body section of document 37 out.println( " " ); 38 out.println( " Welcome to Servlets! " ); 39 out.println( " " ); // end XHTML document 42 out.println( " " ); 43 out.close(); // close stream to complete the page 44 } 45 } Closes the output stream, flushes the output buffer and sends the information to the client.
Fig. 6 HTML document in which the form’s action invokes WelcomeServlet through the alias welcome1 specified in web.xml. 1 2 <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" 3 " Handling an HTTP Get Request Click the button to invoke the servlet
Fig. 6 HTML document in which the form’s action invokes WelcomeServlet through the alias welcome1 specified in web.xml. Program output
3.1 Setting Up the Apache Tomcat Server Download Tomcat (version 3.2.3) –Jakarta.apache.org/builds/jakarta-tomcat/release/v3.2.3/bin/ Define environment variables –JAVA_HOME –TOMCAT_HOME Start the Tomcat server –tomcat start Launch the Tomcat server –
3.1 Setting Up the Apache Tomcat Server (Cont.). Fig. 7Tomcat documentation home page. (Courtesy of The Apache Software Foundation.)
3.2 Deploying a Web Application Web applications –JSPs, servlets and their supporting files Deploying a Web application –Directory structure Context root –Web application archive file (WAR file) –Deployment descriptor web.xml
3.2 Deploying a Web Application (Cont.)
Fig. 9 Deployment descriptor (web.xml) for the advjhtp1 Web application. Lines 5-37 Lines 8-11 Lines Lines Line 20 Lines Lines <!DOCTYPE web-app PUBLIC 2 "-//Sun Microsystems, Inc.//DTD Web Application 2.2//EN" 3 " Advanced Java How to Program JSP 10 and Servlet Chapter Examples This is the Web application in which we 15 demonstrate our JSP and Servlet examples welcome A simple servlet that handles an HTTP get request com.deitel.advjhtp1.servlets.WelcomeServlet Element web-app defines the configuration of each servlet in the Web application and the servlet mapping for each servlet. Element display-name specifies a name that can be displayed to the administrator of the server on which the Web application is installed. Element description specifies a description of the Web application that might be displayed to the administrator of the server. Element servlet describes a servlet.Element servlet-name is the name for the servlet. Element description specifies a description for this particular servlet. Element servlet-class specifies compiled servlet’s fully qualified class name.
Fig. 9 Deployment descriptor (web.xml) for the advjhtp1 Web application. Lines welcome1 34 /welcome Element servlet-mapping specifies servlet-name and url-pattern elements.
3.2 Deploying a Web Application (Cont.) Invoke WelcomeServlet example –/advjhtp1/welcome1 /advjhtp1 specifies the context root /welcome1 specifies the URL pattern URL pattern formats –Exact match /advjhtp1/welcome1 –Path mappings /advjhtp1/example/* –Extension mappings *.jsp –Default servlet /advjhtp1/example/
3.2 Deploying a Web Application (Cont.)
4 Handling HTTP get Requests Containing Data Servlet WelcomeServlet2 –Responds to a get request that contains data
Fig. 11 WelcomeServlet2 responds to a get request that contains data. Line 16 1 // Fig. 11: 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 { // process "get" request from client 12 protected void doGet( HttpServletRequest request, 13 HttpServletResponse response ) 14 throws ServletException, IOException 15 { 16 String firstName = request.getParameter( "firstname" ); response.setContentType( "text/html" ); 19 PrintWriter out = response.getWriter(); // send XHTML document to client // start XHTML document 24 out.println( " " ); out.println( "<!DOCTYPE html PUBLIC \"-//W3C//DTD " + 27 "XHTML 1.0 Strict//EN\" \" "/TR/xhtml1/DTD/xhtml1-strict.dtd\">" ); out.println( 31 " " ); 32 The request object’s getParameter method receives the parameter name and returns the corresponding String value.
Fig. 11 WelcomeServlet2 responds to a get request that contains data. Line // head section of document 34 out.println( " " ); 35 out.println( 36 " Processing get requests with data " ); 37 out.println( " " ); // body section of document 40 out.println( " " ); 41 out.println( " Hello " + firstName + ", " ); 42 out.println( "Welcome to Servlets! " ); 43 out.println( " " ); // end XHTML document 46 out.println( " " ); 47 out.close(); // close stream to complete the page 48 } 49 } Uses the result of line 16 as part of the response to the client.
Fig. 12 HTML document in which the form’s action invokes WelcomeServlet2 through the alias welcome2 specified in web.xml. Line <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" 3 " Processing get requests with data Type your first name and press the Submit button Get the first name from the user.
Fig. 12 HTML document in which the form’s action invokes WelcomeServlet2 through the alias welcome2 specified in web.xml. Program output
4 Handling HTTP get Requests Containing Data (Cont.)
5 Handling HTTP post Requests HTTP post request –Post data from an HTML form to a server-side form handler –Browsers cache Web pages Servlet WelcomeServlet3 –Responds to a post request that contains data
Fig. 14 WelcomeServlet3 responds to a post request that contains data. Lines // Fig. 14: 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 { // process "post" request from client 12 protected void doPost( HttpServletRequest request, 13 HttpServletResponse response ) 14 throws ServletException, IOException 15 { 16 String firstName = request.getParameter( "firstname" ); response.setContentType( "text/html" ); 19 PrintWriter out = response.getWriter(); // send XHTML page to client // start XHTML document 24 out.println( " " ); out.println( "<!DOCTYPE html PUBLIC \"-//W3C//DTD " + 27 "XHTML 1.0 Strict//EN\" \" "/TR/xhtml1/DTD/xhtml1-strict.dtd\">" ); out.println( 31 " " ); 32 Define a doPost method to responds to post requests.
WelcomeServlet3.java 33 // head section of document 34 out.println( " " ); 35 out.println( 36 " Processing post requests with data " ); 37 out.println( " " ); // body section of document 40 out.println( " " ); 41 out.println( " Hello " + firstName + ", " ); 42 out.println( "Welcome to Servlets! " ); 43 out.println( " " ); // end XHTML document 46 out.println( " " ); 47 out.close(); // close stream to complete the page 48 } 49 }
Fig. 15 HTML document in which the form’s action invokes WelcomeServlet3 through the alias welcome3 specified in web.xml. Lines <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" 3 " Handling an HTTP Post Request with Data Type your first name and press the Submit button Provide a form in which the user can input a name in the text input element firstname, then click the Submit button to invoke WelcomeServlet3.
Fig. 15 HTML document in which the form’s action invokes WelcomeServlet3 through the alias welcome3 specified in web.xml. Program output
5 Handling HTTP post Requests (Cont.)
6 Redirecting Requests to Other Resources Servlet RedirectServlet –Redirects the request to a different resource
Fig. 17 Redirecting requests to other resources. Line 16 Lines Line 21 Line 24 Lines // Fig. 17: 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 { // process "get" request from client 12 protected void doGet( HttpServletRequest request, 13 HttpServletResponse response ) 14 throws ServletException, IOException 15 { 16 String location = request.getParameter( "page" ); if ( location != null ) if ( location.equals( "deitel" ) ) 21 response.sendRedirect( " ); 22 else 23 if ( location.equals( "welcome1" ) ) 24 response.sendRedirect( "welcome1" ); // code that executes only if this servlet 27 // does not redirect the user to another page response.setContentType( "text/html" ); 30 PrintWriter out = response.getWriter(); 31 Obtains the page parameter from the request. Determine if the value is either “deitel” or “welcome1” Redirects the request to Redirects the request to the servlet WelcomeServlet. Output a Web page indicating that an invalid request was made if method sendRedirect is not called.
Fig. 17 Redirecting requests to other resources. 32 // start XHTML document 33 out.println( " " ); out.println( "<!DOCTYPE html PUBLIC \"-//W3C//DTD " + 36 "XHTML 1.0 Strict//EN\" \" "/TR/xhtml1/DTD/xhtml1-strict.dtd\">" ); out.println( 40 " " ); // head section of document 43 out.println( " " ); 44 out.println( " Invalid page " ); 45 out.println( " " ); // body section of document 48 out.println( " " ); 49 out.println( " Invalid page requested " ); 50 out.println( " <a href = " + 51 "\"servlets/RedirectServlet.html\">" ); 52 out.println( "Click here to choose again " ); 53 out.println( " " ); // end XHTML document 56 out.println( " " ); 57 out.close(); // close stream to complete the page 58 } 59 }
Fig. 18 RedirectServlet.html document to demonstrate redirecting requests to other resources. Lines Lines <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" 3 " Redirecting a Request to Another Site Click a link to be redirected to the appropriate page Welcome servlet Provide hyperlinks that allow the user to invoke the servlet RedirectServlet.
Fig. 18 RedirectServlet.html document to demonstrate redirecting requests to other resources. Program output
6 Redirecting Requests to other Resources (Cont.)
7 Session Tracking Personalization Privacy invasion HTTP – stateless protocol –Does not support persistent information Track clients individually –Cookies –Session tracking –hidden type input –URL rewriting
7.1 Cookies Stored on the user’s computer for retrieval later Text-based data sent by servlets Maximum age of a cookie Deleted automatically when they expire Servlet CookieServlet –Handles both get and post requests
Fig. 20 Storing user data on the client computer with cookies. Lines Line 28 Line 29 Line 30 Line 32 1 // Fig. 20: 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 { 11 private final Map books = new HashMap(); // initialize Map books 14 public void init() 15 { 16 books.put( "C", " " ); 17 books.put( "C++", " " ); 18 books.put( "Java", " " ); 19 books.put( "VB6", " " ); 20 } // receive language selection and send cookie containing 23 // recommended book to the client 24 protected void doPost( HttpServletRequest request, 25 HttpServletResponse response ) 26 throws ServletException, IOException 27 { 28 String language = request.getParameter( "language" ); 29 String isbn = books.get( language ).toString(); 30 Cookie cookie = new Cookie( language, isbn ); response.addCookie( cookie ); // must precede getWriter 33 response.setContentType( "text/html" ); 34 PrintWriter out = response.getWriter(); 35 Method init populates books with four key/value pair of books. Uses method getParameter to obtain the user’s language selection. Gets the ISBN number for the selected language from books. Creates a new Cookie object, using the language and isbn values as the cookie name and cookie value, respectively. Adds the cookie to the response with method addCookie of interface HttpServletResponse.
Fig. 20 Storing user data on the client computer with cookies. 36 // send XHTML page to client // start XHTML document 39 out.println( " " ); out.println( "<!DOCTYPE html PUBLIC \"-//W3C//DTD " + 42 "XHTML 1.0 Strict//EN\" \" "/TR/xhtml1/DTD/xhtml1-strict.dtd\">" ); out.println( 46 " " ); // head section of document 49 out.println( " " ); 50 out.println( " Welcome to Cookies " ); 51 out.println( " " ); // body section of document 54 out.println( " " ); 55 out.println( " Welcome to Cookies! You selected " + 56 language + " " ); out.println( " <a href = " + 59 "\"/advjhtp1/servlets/CookieSelectLanguage.html\">" + 60 "Click here to choose another language " ); out.println( " " + 63 "Click here to get book recommendations " ); 64 out.println( " " ); // end XHTML document 67 out.println( " " ); 68 out.close(); // close stream 69 } 70
Fig. 20 Storing user data on the client computer with cookies. Line // read cookies from client and create XHTML document 72 // containing recommended books 73 protected void doGet( HttpServletRequest request, 74 HttpServletResponse response ) 75 throws ServletException, IOException 76 { 77 Cookie cookies[] = request.getCookies(); // get cookies response.setContentType( "text/html" ); 80 PrintWriter out = response.getWriter(); // start XHTML document 83 out.println( " " ); out.println( "<!DOCTYPE html PUBLIC \"-//W3C//DTD " + 86 "XHTML 1.0 Strict//EN\" \" "/TR/xhtml1/DTD/xhtml1-strict.dtd\">" ); out.println( 90 " " ); // head section of document 93 out.println( " " ); 94 out.println( " Recommendations " ); 95 out.println( " " ); // body section of document 98 out.println( " " ); // if there are any cookies, recommend a book for each ISBN 101 if ( cookies != null && cookies.length != 0 ) { 102 out.println( " Recommendations " ); 103 out.println( " " ); 104 Retrieves the cookies from the client using HttpServletRequest method getCookies, which returns an array of Cookie objects.
Fig. 20 Storing user data on the client computer with cookies. 105 // get the name of each cookie 106 for ( int i = 0; i < cookies.length; i++ ) 107 out.println( cookies[ i ].getName() " How to Program. ISBN#: " cookies[ i ].getValue() + " " ); out.println( " " ); 112 } 113 else { // there were no cookies 114 out.println( " No Recommendations " ); 115 out.println( " You did not select a language. " ); 116 } out.println( " " ); // end XHTML document 121 out.println( " " ); 122 out.close(); // close stream 123 } 124 }
Fig. 21 CookieSelectLan guage.html document for selecting a programming language and posting the data to the CookieServlet. 1 2 <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" 3 " Using Cookies Select a programming language: <input type = "radio" name = "language" 18 value = "C" />C <input type = "radio" name = "language" 21 value = "C++" />C <input type = "radio" name = "language" 25 value = "Java" checked = "checked" />Java <input type = "radio" name = "language" 28 value = "VB6" />VB
Fig. 21 CookieSelectLan guage.html document for selecting a programming language and posting the data to the CookieServlet. Program output
7.1 Cookies (Cont.)
Fig. 23 (Part 1 of 2)Important methods of class Cookie.
7.1 Cookies (Cont.)
7.2 Session Tracking with HttpSession Interface HttpSession –Supports session tracking Servlet SessionServlet –Uses HttpSession objects –Handles both get and post requests
Fig. 24 Maintaining state information with HttpSession objects. Line 28 Line 32 Line 35 1 // Fig. 24: 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 { 11 private final Map books = new HashMap(); // initialize Map books 14 public void init() 15 { 16 books.put( "C", " " ); 17 books.put( "C++", " " ); 18 books.put( "Java", " " ); 19 books.put( "VB6", " " ); 20 } // receive language selection and create HttpSession object 23 // containing recommended book for the client 24 protected void doPost( HttpServletRequest request, 25 HttpServletResponse response ) 26 throws ServletException, IOException 27 { 28 String language = request.getParameter( "language" ); // Get the user's session object. 31 // Create a session (true) if one does not exist. 32 HttpSession session = request.getSession( true ); // add a value for user's choice to session 35 session.setAttribute( language, books.get( language ) ); Gets the user’s language selection. Uses method getSession of interface HttpServletRequest to obtain the HttpSession object for the client. Uses setAttribute to put the language and the corresponding recommended book’s ISBN number into the HttpSession object.
Fig. 24 Maintaining state information with HttpSession objects. Line 64 Line response.setContentType( "text/html" ); 38 PrintWriter out = response.getWriter(); // send XHTML page to client // start XHTML document 43 out.println( " " ); out.println( "<!DOCTYPE html PUBLIC \"-//W3C//DTD " + 46 "XHTML 1.0 Strict//EN\" \" "/TR/xhtml1/DTD/xhtml1-strict.dtd\">" ); out.println( 50 " " ); // head section of document 53 out.println( " " ); 54 out.println( " Welcome to Sessions " ); 55 out.println( " " ); // body section of document 58 out.println( " " ); 59 out.println( " Welcome to Sessions! You selected " + 60 language + ". " ); // display information about the session 63 out.println( " Your unique session ID is: " + 64 session.getId() + " " ); out.println( 67 "This " + ( session.isNew() ? "is" : "is not" ) + 68 " a new session " ); 69 Uses HttpSession method getID to obtain the session’s unique ID number. Determines whether the session is new or already exists with method isNew.
Fig. 24 Maintaining state information with HttpSession objects. Line 71 Line 74 Line 77 Line out.println( "The session was created at: " + 71 new Date( session.getCreationTime() ) + " " ); out.println( "You last accessed the session at: " + 74 new Date( session.getLastAccessedTime() ) + " " ); out.println( "The maximum inactive interval is: " + 77 session.getMaxInactiveInterval() + " seconds " ); out.println( " <a href = " + 80 "\"servlets/SessionSelectLanguage.html\">" + 81 "Click here to choose another language " ); out.println( " " + 84 "Click here to get book recommendations " ); 85 out.println( " " ); // end XHTML document 88 out.println( " " ); 89 out.close(); // close stream 90 } // read session attributes and create XHTML document 93 // containing recommended books 94 protected void doGet( HttpServletRequest request, 95 HttpServletResponse response ) 96 throws ServletException, IOException 97 { 98 // Get the user's session object. 99 // Do not create a session (false) if one does not exist. 100 HttpSession session = request.getSession( false ); // get names of session object's values 103 Enumeration valueNames; 104 Obtains the time at which the session was created with method getCreationTime. Obtains the time at which the session was last accessed with method getLastAccessedTime. Uses method getMaxInactiveInterval to obtain the maximum amount of time that an HttpSession object can be inactive before the servlet container discards it. Obtains the HttpSession object for the client with method getSession.
Fig. 24 Maintaining state information with HttpSession objects. Line if ( session != null ) 106 valueNames = session.getAttributeNames(); 107 else 108 valueNames = null; PrintWriter out = response.getWriter(); 111 response.setContentType( "text/html" ); // start XHTML document 114 out.println( " " ); out.println( "<!DOCTYPE html PUBLIC \"-//W3C//DTD " "XHTML 1.0 Strict//EN\" \" "/TR/xhtml1/DTD/xhtml1-strict.dtd\">" ); out.println( 121 " " ); // head section of document 124 out.println( " " ); 125 out.println( " Recommendations " ); 126 out.println( " " ); // body section of document 129 out.println( " " ); if ( valueNames != null && 132 valueNames.hasMoreElements() ) { 133 out.println( " Recommendations " ); 134 out.println( " " ); String name, value; 137 Uses HttpSession method getAttributeNames to retrieve an Enumeration of the attribute names.
Fig. 24 Maintaining state information with HttpSession objects. Line // get value for each name in valueNames 139 while ( valueNames.hasMoreElements() ) { 140 name = valueNames.nextElement().toString(); 141 value = session.getAttribute( name ).toString(); out.println( name + " How to Program. " "ISBN#: " + value + " " ); 145 } out.println( " " ); 148 } 149 else { 150 out.println( " No Recommendations " ); 151 out.println( " You did not select a language. " ); 152 } out.println( " " ); // end XHTML document 157 out.println( " " ); 158 out.close(); // close stream 159 } 160 } Invokes method getAttribute of HttpSession to retrieve the ISBN of a book from the HttpSession object.
Fig. 25 SessionSelectLa nguage.html document for selecting a programming language and posting the data to the SessionServlet. 1 2 <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" 3 " Using Sessions Select a programming language: <input type = "radio" name = "language" 18 value = "C" />C <input type = "radio" name = "language" 21 value = "C++" />C <input type = "radio" name = "language" 25 value = "Java" checked = "checked" />Java <input type = "radio" name = "language" 28 value = "VB6" />VB
Fig. 25 SessionSelectLa nguage.html document for selecting a programming language and posting the data to the SessionServlet. Program output
7.2 Session Tracking with HttpSession (Cont.)
8 Multi-Tier Applications: Using JDBC from a Servlet Three-tier distributed applications –User interface –Business logic –Database access Web servers often represent the middle tier Three-tier distributed application example –SurveyServlet –Survey.html –Cloudscape database
Fig. 27 Multi- tier Web-based survey using XHTML, servlets and JDBC. Lines Line 21 Lines Lines // Fig. 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.*; public class SurveyServlet extends HttpServlet { 12 private Connection connection; 13 private PreparedStatement updateVotes, totalVotes, results; // set up database connection and prepare SQL statements 16 public void init( ServletConfig config ) 17 throws ServletException 18 { 19 // attempt database connection and create PreparedStatements 20 try { 21 Class.forName( "COM.cloudscape.core.RmiJdbcDriver" ); 22 connection = DriverManager.getConnection( 23 "jdbc:rmi:jdbc:cloudscape:animalsurvey" ); // PreparedStatement to add one to vote total for a 26 // specific animal 27 updateVotes = 28 connection.prepareStatement( 29 "UPDATE surveyresults SET votes = votes + 1 " + 30 "WHERE id = ?" 31 ); 32 Servlets are initialized by overriding method init. Loads the database driver. Attempt to open a connection to the animalsurvey database. Create PreparedStatement updateVotes object.
Fig. 27 Multi- tier Web-based survey using XHTML, servlets and JDBC. Lines // PreparedStatement to sum the votes 34 totalVotes = 35 connection.prepareStatement( 36 "SELECT sum( votes ) FROM surveyresults" 37 ); // PreparedStatement to obtain surveyoption table's data 40 results = 41 connection.prepareStatement( 42 "SELECT surveyoption, votes, id " + 43 "FROM surveyresults ORDER BY id" 44 ); 45 } // for any exception throw an UnavailableException to 48 // indicate that the servlet is not currently available 49 catch ( Exception exception ) { 50 exception.printStackTrace(); 51 throw new UnavailableException(exception.getMessage()); 52 } } // end of init method // process survey response 57 protected void doPost( HttpServletRequest request, 58 HttpServletResponse response ) 59 throws ServletException, IOException 60 { 61 // set up response to client 62 response.setContentType( "text/html" ); 63 PrintWriter out = response.getWriter(); 64 DecimalFormat twoDigits = new DecimalFormat( "0.00" ); 65 Create PreparedStatement totalVotes and results objects.
Fig. 27 Multi- tier Web-based survey using XHTML, servlets and JDBC. Lines Lines Lines Lines // start XHTML document 67 out.println( " " ); out.println( "<!DOCTYPE html PUBLIC \"-//W3C//DTD " + 70 "XHTML 1.0 Strict//EN\" \" "/TR/xhtml1/DTD/xhtml1-strict.dtd\">" ); out.println( 74 " " ); // head section of document 77 out.println( " " ); // read current survey response 80 int value = 81 Integer.parseInt( request.getParameter( "animal" ) ); // attempt to process a vote and display current results 84 try { // update total for current surevy response 87 updateVotes.setInt( 1, value ); 88 updateVotes.executeUpdate(); // get total of all survey responses 91 ResultSet totalRS = totalVotes.executeQuery(); 92 totalRS.next(); 93 int total = totalRS.getInt( 1 ); // get results 96 ResultSet resultsRS = results.executeQuery(); 97 out.println( " Thank you! " ); 98 out.println( " " ); 99 Obtain the survey response Set the first parameter of PreparedStatement updateVotes to the survey response and update the database. Execute PreparedStatement totalVotes to retrieve the total number of votes received. Execute PreparedStatement results and process the ResultSet to create the survey summary for the client.
Fig. 27 Multi- tier Web-based survey using XHTML, servlets and JDBC. 100 out.println( " " ); 101 out.println( " Thank you for participating." ); 102 out.println( " Results: " ); // process results 105 int votes; while ( resultsRS.next() ) { 108 out.print( resultsRS.getString( 1 ) ); 109 out.print( ": " ); 110 votes = resultsRS.getInt( 2 ); 111 out.print( twoDigits.format( 112 ( double ) votes / total * 100 ) ); 113 out.print( "% responses: " ); 114 out.println( votes ); 115 } resultsRS.close(); out.print( "Total responses: " ); 120 out.print( total ); // end XHTML document 123 out.println( " " ); 124 out.close(); 125 } // if database exception occurs, return error page 128 catch ( SQLException sqlException ) { 129 sqlException.printStackTrace(); 130 out.println( " Error " ); 131 out.println( " " ); 132 out.println( " Database error occurred. " ); 133 out.println( "Try again later. " ); 134 out.close();
Fig. 27 Multi- tier Web-based survey using XHTML, servlets and JDBC. Lines } } // end of doPost method // close SQL statements and database when servlet terminates 140 public void destroy() 141 { 142 // attempt to close statements and database connection 143 try { 144 updateVotes.close(); 145 totalVotes.close(); 146 results.close(); 147 connection.close(); 148 } // handle database exceptions by returning error to client 151 catch( SQLException sqlException ) { 152 sqlException.printStackTrace(); 153 } 154 } // end of destroy method 155 } Method destroy closes each PreparedStatement and database connection.
Fig. 28 Survey.html document that allows users to submit survey responses to SurveyServlet. 1 2 <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" 3 " Survey What is your favorite pet? <input type = "radio" name = "animal" 19 value = "1" />Dog 20 <input type = "radio" name = "animal" 21 value = "2" />Cat 22 <input type = "radio" name = "animal" 23 value = "3" />Bird 24 <input type = "radio" name = "animal" 25 value = "4" />Snake 26 <input type = "radio" name = "animal" 27 value = "5" checked = "checked" />None
Fig. 28 Survey.html document that allows users to submit survey responses to SurveyServlet.
8 Multi-Tier Applications: Using JDBC from a Servlet (Cont.)
9 HttpUtils Class