2005 Pearson Education, Inc. All rights reserved. 1 26 Servlets.

1

2005 Pearson Education, Inc. All rights reserved.

2626

Servlets

2


A fair request should be followed by the deed in silence.

— Dante Alighieri

The longest part of the journey is said to be the passing of the gate.

— Marcus Terentius Varro

If nominated, I will not accept; if elected, I will not serve.

— General William T. Sherman

Friends share all things.— Pythagoras

3


OBJECTIVES

In this chapter you will learn: How servlets can be used to extend a Web

server's functionality. The servlet life cycle. To execute servlets with the Apache

Tomcat server. To be able to respond to HTTP requests

from an HttpServlet. To be able to redirect requests to static

and dynamic Web resources. To use JDBC from a servlet.

4


26.1 Introduction

26.2 Servlet Overview and Architecture

26.2.1 Interface Servlet and the Servlet Life Cycle

26.2.2 HttpServlet Class

26.2.3 HttpServletRequest Interface

26.2.4 HttpServletResponse Interface

26.3 Setting Up the Apache Tomcat Server

26.4 Handling HTTP get Requests

26.4.1 Deploying a Web Application

26.5 Handling HTTP get Requests Containing Data

26.6 Handling HTTP post Requests

26.7 Redirecting Requests to Other Resources

26.8 Multitier Applications: Using JDBC from a Servlet

26.9 Welcome Files

26.10 Wrap-Up

26.11 Internet and Web Resources

5


26.1 Introduction

• Java networking capabilities– Socket-based and packet-based communications

• Package java.net

– Remote Method Invocation (RMI)• Package java.rmi

– Common Object Request Broker Architecture (COBRA)• Package org.omg

– Remote Method Invocation over the Internet Inter-Orb Protocol (RMI-IIOP)

6


26.1 Introduction (Cont.)

• Client-server relationship– Servlets and JavaServer Pages (JSP)

• Request-response model

• Packages javax.servlet

javax.servlet.http

javax.servlet.jsp

javax.servlet.tagext• Common implementation of request-response model

– Web browsers and Web servers

• Form the Web components of J2EE

7



• Thin clients– Provide presentation

– Do not process data

– Require fewer computing resources

8



• Apache Jakarta Project and the Tomcat Server

• Tomcat– Jakarta project

– Official reference implementation of the JSP and servlet standards

9


26.2 Servlet Overview and Architecture

• Servlet– Small portion of the content is static text or markup– Do not produce content– Perform a task on behalf of the client

• JavaServer Pages– Extension of servlet technology– Most of the content is static text or markup– Small portion of the content is generated dynamically

• Servlet container (servlet engine)– Server that executes a servlet

10


26.2 Servlet Overview and Architecture (Cont.)

• Web servers and application servers– Sun Java System 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

11


Fig. 26.1 | Servlet architecture.

12


26.2.1 Interface Servlet and the Servlet Life Cycle

• Interface Servlet– All servlets must implement this interface– All methods of interface Servlet are invoked by servlet

container

• 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

13


Software Engineering Observation 26.1

Servlets implement the Servlet interface of package javax.servlet.

14


Fig. 26.2 | Servlet interface methods.

Method Description

void init( ServletConfig config ) The servlet container calls this method once during a servlet’s execution cycle to initialize the

servlet. The ServletConfig argument is supplied by the servlet container that executes the servlet.

ServletConfig getServletConfig()

This method returns a reference to an object that implements interface ServletConfig. This object provides access to the servlet’s configuration information, such as its initialization parameters and ServletContext, which provides the servlet with access to its environment (i.e., the servlet container in which the servlet executes).

String getServletInfo()

This method is defined by a servlet programmer to return a string containing servlet information, such as the servlet’s author and version.

void service( ServletRequest request, ServletResponse response )

The servlet container calls this method to respond to a client request to the servlet.

void destroy()

This “cleanup” method is called when a servlet is terminated by its servlet container. Resources used by the servlet, such as open files or open database connections, should be deallocated here.

15


26.2.1 Interface Servlet and the Servlet Life Cycle (Cont.)

• Interface Servlet implementation– GenericServlet

• Abstract class

• Package javax.servlet• Protocol-independent servlet

– HttpServlet• Abstract class

• Package javax.servlet.http• Use the HTTP protocol to exchange information

• Key method service

– ServletRequest and ServletResponse

16


26.2.2 HttpServlet Class

• Overrides method service• Two most common HTTP request types

– get requests• Get/retrieve information from server

– post requests• Post/send data to server

• Method doGet responds to get requests• Method doPost responds to post requests•HttpServletRequest and HttpServletResponse objects

17


Fig. 26.3 | HttpServlet class’s other methods.

Method Description

doDelete Called in response to an HTTP delete request. Such a request is normally used to delete a file from a server. This may not be available on some servers because of its inherent security risks (e.g., the client could delete a file that is critical to the execution of the server or an application).

doHead Called in response to an HTTP head request. Such a request is normally used when the client wants only the response’s headers, such as its content type and content length. By overriding this method, the servlet does not compute the response body, thus improving performance.

doOptions Called in response to an HTTP options request. This returns information to the client indicating the HTTP options supported by the server, such as the HTTP version (1.0 or 1.1) and the request methods the server supports.

doPut Called in response to an HTTP put request. Such a request is normally used to store a file on the server. This may not be available on some servers because of its inherent security risks (e.g., the client could place an executable application on the server, which, if executed, could damage the server—perhaps by deleting critical files or occupying resources).

doTrace Called in response to an HTTP trace request. Such a request is normally used for debugging. The implementation of this method automatically returns an HTML document to the client containing the request header information (data sent by the browser as part of the request).

18



Do not override method service in an HttpServlet subclass. Doing so prevents the servlet from distinguishing between request types.

19


26.2.3 HttpServletRequest Interface

• Servlet container– creates an HttpServletRequest object

– passes it to the servlet’s service method

•HttpServletRequest object – contains the request from the client

– provides methods that enable the servlet to process the request

20


Fig. 26.4 | HttpServletRequest methods.

Method Description

String getParameter( String name )

Obtains the value of a parameter sent to the servlet as part of a get or post request. The name argument represents the parameter name.

Enumeration getParameterNames()

Returns the names of all the parameters sent to the servlet as part of a post request.

String[] getParameterValues( String name )

For a parameter with multiple values, this method returns an array of strings containing the values for a specified servlet parameter.

Cookie[] getCookies()

Returns an array of Cookie objects stored on the client by the server. Cookie objects can be used to uniquely identify clients to the servlet.

HttpSession getSession( boolean create )

Returns an HttpSession object associated with the client’s current browsing session. This method can create an HttpSession object (true argument) if one does not already exist for the client. HttpSession objects and Cookies are used in similar ways for uniquely identifying clients.

String getLocalName()

Gets the host name on which the request was received.

String getLocalAddr()

Gets the Internet Protocol (IP) address on which the request was received.

int getLocalPort()

Gets the Internet Protocol (IP) port number on which the request was received.

21


26.2.4 HttpServletResponse Interface

• Servlet container – creates an HttpServletResponse object

– passes it to the servlet’s service method

•HttpServletResponse object – provides methods that enable the servlet to formulate the

response to the client

22


Fig. 26.5 | HttpServletResponse methods.

Method Description

void addCookie( Cookie cookie ) Used to add a Cookie to the header of the response to the client. The Cookie’s

maximum age and whether Cookies are enabled on the client determine whether Cookies are stored on the client.

ServletOutputStream getOutputStream() Obtains a byte-based output stream for sending binary data to the client.

PrintWriter getWriter() Obtains a character-based output stream for sending text data (usually HTML formatted

text) to the client.

void setContentType( String type ) Specifies the content type of the response to the browser. The content type helps the

browser determine how to display the data (or possibly what other application to execute to process the data). The content type is also known as MIME (Multipurpose Internet Mail Extension) type of the data. For examples, content type "text/html" indicates that the response is an HTML document, so the browser displays the HTML page; content type "image/gif" indicates that the response is an image, so the browser displays the image. For a complete list of content types, visit www.isi.edu/in-notes/iana/assignments/media-types/media-types.

String getContentType() Gets the content type of the response.

23


26.3 Setting Up the Apache Tomcat Server

• Download Tomcat (version 5.0.25)– apache.towardex.com/jakarta/tomcat-5/v5.0.25/bin

• Define environment variables– JAVA_HOME

• C:\Program Files\Java\jdk1.5.0

– CATALINA_HOME• C:\jakarta-tomcat-5.-.25

24


Error-Prevention Tip 26.1

On some platforms you may need to restart your computer for the new environment variables to take effect

25


26.3 Setting Up the Apache Tomcat Server (Cont.)

• Start the Tomcat server– startup

• Launch the Tomcat server– http://localhost:8080/

• Shutdown the Tomcat server– shutdown

26


Fig. 26.6 | Tomcat documentation home page. Copyright 2000-2004 The Apache Software Foundation (http://www.apache.org/). All rights reserved.

27



If the host name localhost does not work on your computer, substitute the IP address 127.0.0.1 instead.

28


26.4 Handling HTTP get Requests

•get request– Retrieve the content of a URL

• Example: WelcomeServlet – a servlet handles HTTP get requests

29


Outline

WelcomeServlet.java

(1 of 2)

Lines 3-6

Line 10

Lines 13-44

Line 17

Line 18

Line 23

1 // Fig. 26.7: WelcomeServlet.java

2 // A simple servlet to process get requests.

3 import javax.servlet.ServletException;

4 import javax.servlet.http.HttpServlet;

5 import javax.servlet.http.HttpServletRequest;

6 import javax.servlet.http.HttpServletResponse;

7 import java.io.IOException;

8 import java.io.PrintWriter;

9

10 public class WelcomeServlet extends HttpServlet

11 {

12 // process "get" requests from clients

13 protected void doGet( HttpServletRequest request,

14 HttpServletResponse response )

15 throws ServletException, IOException

16 {

17 response.setContentType( "text/html" );

18 PrintWriter out = response.getWriter();

19

20 // send XHTML page to client

21

22 // start XHTML document

23 out.println( "<?xml version = \"1.0\"?>" );

24

25 out.printf( "%s%s%s", "<!DOCTYPE html PUBLIC",

26 " \"-//W3C//DTD XHTML 1.0 Strict//EN\"",

27 " \"http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd\">\n" );

28

29 out.println( "<html xmlns = \"http://www.w3.org/1999/xhtml\">" );

30

Import the classes and interfaces in 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

30


Outline

WelcomeServlet.java

(2 of 2)

Line 43

31 // head section of document

32 out.println( "<head>" );

33 out.println( "<title>A Simple Servlet Example</title>" );

34 out.println( "</head>" );

35

36 // body section of document

37 out.println( "<body>" );

38 out.println( "<h1>Welcome to Servlets!</h1>" );

39 out.println( "</body>" );

40

41 // end XHTML document

42 out.println( "</html>" );

43 out.close(); // close stream to complete the page

44 } // end method doGet

45 } // end class WelcomeServlet

Closes the output stream, flushes the output buffer and sends the information to the client

31


Outline

WelcomeServlet.html

(1 of 2)

Line 13

Line 13

Line 15

1 <?xml version = "1.0"?>

2 <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN"

3 "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">

4

5 

6

7 <html xmlns = "http://www.w3.org/1999/xhtml">

8 <head>

9 <title>Handling an HTTP Get Request</title>

10 </head>

11

12 <body>

13 <form action = "/jhtp6/welcome1" method = "get">

14 <p><label>Click the button to invoke the servlet

15 <input type = "submit" value = "Get HTML Document" />

16 </label></p>

17 </form>

18 </body>

19 </html>

The form’s action attribute (/jhtp6/welcome1) specifies the URL path that invokes the servlet

The form’s method attribute indicates that the browser sends a get request to the server, which results a call to the servlet’s doGet method

Create a button, when clicked, the form’s action is performed

32


Outline

WelcomeServlet.html

(2 of 2)

Program output

33



The Tomcat documentation specifies how to integrate Tomcat with popular Web server applications such as the Apache HTTP Server and Microsoft’s IIS.

34


Common Programming Error 26.1

Using “servlet” or “servlets” as a context root may prevent a servlet from working correctly on some servers.

35


26.4.1 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

36


Fig. 26.9 | Web application standard directories.

Directory Description

context root This is the root directory for the Web application. All the JSPs, HTML documents, servlets and supporting files such as images and class files reside in this directory or its subdirectories. The name of this directory is specified by the Web application creator. To provide structure in a Web application, subdirectories can be placed in the context root. For example, if your application uses many images, you might place an images subdirectory in this directory. The examples of this chapter use jhtp6 as the context root.

WEB-INF This directory contains the Web application deployment descriptor (web.xml).

WEB-INF/classes This directory contains the servlet class files and other supporting class files used in a Web application. If the classes are part of a package, the complete package directory structure would begin here.

WEB-INF/lib This directory contains Java archive (JAR) files. The JAR files can contain servlet class files and other supporting class files used in a Web application.

37


Outline

Web.xml

(1 of 2)

Lines 1-37

Lines 8-11

Lines 13-16

Lines 19-29

Line 20

Lines 22-24

Lines 26-28

1 <web-app xmlns="http://java.sun.com/xml/ns/j2ee"

2 xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"

3 xsi:schemaLocation="http://java.sun.com/xml/ns/j2ee

4 http://java.sun.com/xml/ns/j2ee/web-app_2_4.xsd"

5 version="2.4">

6

7 

8 <display-name>

9 Java How to Program JSP

10 and Servlet Chapter Examples

11 </display-name>

12

13 <description>

14 This is the Web application in which we

15 demonstrate our JSP and Servlet examples.

16 </description>

17

18 

19 <servlet>

20 <servlet-name>welcome1</servlet-name>

21

22 <description>

23 A simple servlet that handles an HTTP get request.

24 </description>

25

26 <servlet-class>

27 WelcomeServlet

28 </servlet-class>

29 </servlet>

30

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.

38


Outline

Web.xml

(2 of 2)

Lines 32-35

Line 34

31 

32 <servlet-mapping>

33 <servlet-name>welcome1</servlet-name>

34 <url-pattern>/welcome1</url-pattern>

35 </servlet-mapping>

36

37 </web-app>

Element servlet-mapping specifies servlet-name and url-pattern elements.Element url-pattern helps the server determine

which requests are sent to the servlet

39


26.4.1 Deploying a Web Application (Cont.)

• Invoke WelcomeServlet example– /jhtp6/welcome1

• /jhtp6 specifies the context root• /welcome1 specifies the URL pattern

• URL pattern formats– Exact match

• /jhtp5/welcome1

– Path mappings• /jhtp5/example/*

– Extension mappings• *.jsp

– Default servlet• /

40


Fig. 26.11 | Web application directory and file structure for WelcomeServlet.

WelcomeServlet Web application directory and file structure

jhtp6 servlets WelcomeServlet.html WEB-INF web.xml classes WelcomeServlet.class

41


Common Programming Error 26.2

Not placing a servlet or other class files in the appropriate directory structure prevents the server from locating those classes properly. This results in an error response to the client Web browser.

42



You can test a servlet that handles HTTP get requests by typing the URL that invokes the servlet directly into your browser’s Address or Location field because get is the default HTTP method when browsing.

43


26.5 Handling HTTP get Requests Containing Data

• Servlet WelcomeServlet2– Responds to a get request that contains data

44


Outline

WelcomeServlet2.java

(1 of 2)

Line 17

1 // Fig. 26.12: WelcomeServlet2.java

2 // Processing HTTP get requests containing data.







9

10 public class WelcomeServlet2 extends HttpServlet

11 {

12 // process "get" request from client




16 {

17 String firstName = request.getParameter( "firstname" );

18



21

22 // send XHTML document to client

23



26




The request object’s getParameter method receives the parameter name and returns the corresponding String value

45


Outline


(2 of 2)

Line 41

30


32



35 out.println(

36 "<title>Processing get requests with data</title>" );


38



41 out.println( "<h1>Hello " + firstName + ",<br />" );

42 out.println( "Welcome to Servlets!</h1>" );


44





49 } // end class WelcomeServlet2

Uses the result of line 17 as part of the response to the client

46


Outline

WelcomeServlet2.html

(1 of 2)

Line 16




4

5 

6


8 <head>

9 <title>Processing get requests with data</title>

10 </head>

11

12 <body>

13 <form action = "/jhtp6/welcome2" method = "get">

14 <p><label>

15 Type your first name and press the Submit button

16 <br /><input type = "text" name = "firstname" />

17 <input type = "submit" value = "Submit" />

18 </p></label>

19 </form>

20 </body>

21 </html>

Get the first name from the user.

47


Outline


(2 of 2)

Program output

form data specified in URL’s query string as part of a get request

48


Fig. 26.14 | Deployment descriptor information for servlet WelcomeServlet2.

Descriptor element Value

servlet element

servlet-name welcome2

description Handling HTTP get requests with data.

servlet-class WelcomeServlet2

servlet-mapping element


url-pattern /welcome2

49



If an error occurs during the servlet invocation, the log files in the logs directory of the Tomcat installation can help you determine the error and debug the problem.

50



A get request is limited to standard characters, which means that you cannot submit any special characters via a get request. The length of the URL in a get request is limited. For example, the maximum URL length in Internet Explorer is 2,083 characters. Some Web servers might restrict this even more.

51


Good Programming Practice 26.1

A get request should not be used for sending sensitive data (e.g., a password) because the form data is placed in a query string that is appended to the request URL as plain text and can be intercepted.

52


26.6 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

53


Outline


(1 of 2)

Lines 13-48

Line 17

1 // Fig. 26.15: WelcomeServlet3.java

2 // Processing post requests containing data.







9

10 public class WelcomeServlet3 extends HttpServlet

11 {

12 // process "post" request from client

13 protected void doPost( HttpServletRequest request,



16 {

17 String firstName = request.getParameter( "firstname" );

18



21

22 // send XHTML page to client

23



26




30

Declare a doPost method to responds to post requests

The request object’s getParameter method receives the parameter name and returns the corresponding String value

54


Outline


(2 of 2)


32



35 out.println(

36 "<title>Processing post requests with data</title>" );


38



41 out.println( "<h1>Hello " + firstName + ",<br />" );

42 out.println( "Welcome to Servlets!</h1>" );


44




48 } // end method doPost

49 } // end class WelcomeServlet3

55


Outline


(1 of 2)

Lines 13-19




4

5 

6


8 <head>

9 <title>Handling an HTTP Post Request with Data</title>

10 </head>

11

12 <body>

13 <form action = "/jhtp6/welcome3" method = "post">

14 <p><label>

15 Type your first name and press the Submit button

16 <br /><input type = "text" name = "firstname" />

17 <input type = "submit" value = "Submit" />

18 </label></p>

19 </form>

20 </body>

21 </html>

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

56


Outline


(2 of 2)

Program output

57


Fig. 26.17 | Deployment descriptor information for servlet WelcomeServlet3.


servlet element


description Handling HTTP post requests with data.

servlet-class WelcomeServlet3



url-pattern /welcome3

58


26.7 Redirecting Requests to Other Resources

• Servlet RedirectServlet– Redirects the request to a different resource

59


Outline

RedirectServlet.java

(1 of 2)

Line 17

Lines 21 and 23

Line 22

Line 24

1 // Fig. 26.18: RedirectServlet.java

2 // Redirecting a user to a different Web page.







9

10 public class RedirectServlet extends HttpServlet

11 {

12 // process "get" request from client




16 {

17 String location = request.getParameter( "page" );

18

19 if ( location != null )

20 {

21 if ( location.equals( "deitel" ) )

22 response.sendRedirect( "http://www.deitel.com" );

23 else if ( location.equals( "welcome1" ) )

24 response.sendRedirect( "welcome1" );

25 } // end if

26

27 // code that executes only if this servlet

28 // does not redirect the user to another page



Obtains the page parameter from the request.

Determine if the value is either “deitel” or “welcome1”Redirects the request to

www.deitel.com.Redirects the request to the servlet WelcomeServlet.

60


Outline

RedirectServlet.java

(2 of 2)

31



34




38

39 out.println(

40 "<html xmlns = \"http://www.w3.org/1999/xhtml\">" );

41



44 out.println( "<title>Invalid page</title>" );


46



49 out.println( "<h1>Invalid page requested</h1>" );

50 out.println( "<p><a href = " +

51 "\"servlets/RedirectServlet.html\">" );

52 out.println( "Click here to choose again</a></p>" );


54





59 } // end class RedirectServlet

61



Using relative paths to reference resources in the same context root makes your Web application more flexible. For example, you can change the context root without making changes to the static and dynamic resources in the application.

62


Outline

RedirectServlet.html

(1 of 2)

Lines 15-16 and 17-18




4

5 

6


8 <head>

9 <title>Redirecting a Request to Another Site</title>

10 </head>

11

12 <body>

13 <p>Click a link to be redirected to the appropriate page</p>

14 <p>

15 <a href = "/jhtp6/redirect?page=deitel">

16 www.deitel.com</a><br />

17 <a href = "/jhtp6/redirect?page=welcome1">

18 Welcome servlet</a>

19 </p>

20 </body>

21 </html>

Provide links that allow the user to invoke the servlet RedirectServlet

63


Outline

RedirectServlet.html

(2 of 2)

Program output

64


Fig. 26.20 | Deployment descriptor information for servlet RedirectServlet.


servlet element

servlet-name redirect

description Redirecting to static Web pages and other servlets.

servlet-class RedirectServlet


servlet-name redirect

url-pattern /redirect

65


26.8 Multitier Applications: Using JDBC from a Servlet

• Three-tier distributed applications– User interface

– Business logic

– Database

• Web servers often represent the middle tier

• Three-tier distributed application example– SurveyServlet

– Survey.html

– MySQL database

66


Outline

SurveyServlet.java

(1 of 6)

Lines 7-11

Line 21

Line 22

1 // Fig. 26.21: SurveyServlet.java

2 // A Web-based survey that uses JDBC from a servlet.

3 package com.deitel.jhtp6.servlets;

4



7 import java.sql.Connection;

8 import java.sql.DriverManager;

9 import java.sql.Statement;

10 import java.sql.ResultSet;

11 import java.sql.SQLException;

12 import javax.servlet.ServletConfig;


14 import javax.servlet.UnavailableException;




18

19 public class SurveyServlet extends HttpServlet

20 {

21 private Connection connection;

22 private Statement statement;

23

Import interfaces and classes for database manipulation

Declare a Connection to manage the database connectionDeclare a Statement for updating the vote count, totaling all the votes and obtaining the complete survey result

67


Outline

SurveyServlet.java

(2 of 6)

Line 30

Lines 31-34

Line 37

24 // set up database connection and create SQL statement

25 public void init( ServletConfig config ) throws ServletException

26 {

27 // attempt database connection and create Statements

28 try

29 {

30 Class.forName( config.getInitParameter( "databaseDriver" ) );

31 connection = DriverManager.getConnection(

32 config.getInitParameter( "databaseName" ) );

33 config.getInitParameter( "username" ),

34 config.getInitParameter( "password" ) );

35

36 // create Statement to query database

37 statement = connection.createStatement();

38 } // end try

39 // for any exception throw an UnavailableException to

40 // indicate that the servlet is not currently available

41 catch ( Exception exception )

42 {

43 exception.printStackTrace();

44 throw new UnavailableException(exception.getMessage());

45 } // end catch

46 } // end method init

47

Loads the database driver, which is specified in the initialization parameter “databaseDriver”

Attempt to open a connection to the animalsurvey database, the database name, username and password are specified in the initialization parameters

Create Statement to query database.

68


Outline

SurveyServlet.java

(3 of 6)

Lines 71-72

48 // process survey response

49 protected void doPost( HttpServletRequest request,



52 {

53 // set up response to client



56



59




63

64 out.println(

65 "<html xmlns = \"http://www.w3.org/1999/xhtml\">" );

66



69

70 // read current survey response

71 int value =

72 Integer.parseInt( request.getParameter( "animal" ) );

73 String sql;

74

Obtain the survey response

69


Outline

SurveyServlet.java

(4 of 6)

Lines 79-80

Line 81

Line 84

Line 85

Lines 90-91

Line 92

75 // attempt to process a vote and display current results

76 try

77 {

78 // update total for current survey response

79 sql = "UPDATE surveyresults SET votes = votes + 1 " +

80 "WHERE id = " + value;

81 statement.executeUpdate( sql );

82

83 // get total of all survey responses

84 sql = "SELECT sum( votes ) FROM surveyresults";

85 ResultSet totalRS = statement.executeQuery( sql );

86 totalRS.next(); // position to first record

87 int total = totalRS.getInt( 1 );

88

89 // get results

90 sql = "SELECT surveyoption, votes, id FROM surveyresults " +

91 "ORDER BY id";

92 ResultSet resultsRS = statement.executeQuery( sql );

93 out.println( "<title>Thank you!</title>" );


95


97 out.println( "<p>Thank you for participating." );

98 out.println( "<br />Results:</p><pre>" );

99

Create sql to update total for current survey response

Execute sql statement to update total for current survey response

Create query to get total of all survey responsesExecute query to get total of

all survey responses

Create query to get survey results

Execute query to get survey results

70


Outline

SurveyServlet.java

(5 of 6)

Lines 103-111

Line 105

Line 107

100 // process results

101 int votes;

102

103 while ( resultsRS.next() )

104 {

105 out.print( resultsRS.getString( 1 ) );

106 out.print( ": " );

107 votes = resultsRS.getInt( 2 );

108 out.printf( "%.2f", ( double ) votes / total * 100 );

109 out.print( "% responses: " );

110 out.println( votes );

111 } // end while

112

113 resultsRS.close();

114

115 out.print( "Total responses: " );

116 out.print( total );

117


119 out.println( "</pre></body></html>" );

120 out.close();

121 } // end try

122 // if database exception occurs, return error page

123 catch ( SQLException sqlException )

124 {

125 sqlException.printStackTrace();

126 out.println( "<title>Error</title>" );


128 out.println( "<body><p>Database error occurred. " );

129 out.println( "Try again later.</p></body></html>" );

Loop through all records in resultsRS

Obtain the value of the first column from the current record

Obtain the value of the second column from the current record

71


Outline

SurveyServlet.java

(6 of 6)

Lines 140-141

130 out.close();

131 } // end catch

132 } // end method doPost

133

134 // close SQL statements and database when servlet terminates

135 public void destroy()

136 {

137 // attempt to close statements and database connection

138 try

139 {

140 statement.close();

141 connection.close();

142 } // end try

143 // handle database exceptions by returning error to client

144 catch ( SQLException sqlException )

145 {

146 sqlException.printStackTrace();

147 } // end catch

148 } // end method destroy

149 } // end class SurveyServlet

Close Statement and database connection

72


Outline

Survey.html

(1 of 2)




4

5 

6


8 <head>

9 <title>Survey</title>

10 </head>

11

12 <body>

13 <form method = "post" action = "/jhtp6/animalsurvey">

14 <p>What is your favorite pet?</p>

15 <p>

16 <input type = "radio" name = "animal"

17 value = "1" />Dog<br />


19 value = "2" />Cat<br />


21 value = "3" />Bird<br />


23 value = "4" />Snake<br />


25 value = "5" checked = "checked" />None

26 </p>

27 <p><input type = "submit" value = "Submit" /></p>

28 </form>

29 </body>

30 </html>

Provide a form in which the user can select an animal from a list of radio button, then click the Submit button to invoke animalsurvey

73


Outline

Survey.html

(2 of 2)

Program output

74


Fig. 26.23 | Deployment descriptor information for servlet SurveyServlet.


servlet element

servlet-name animalsurvey

description Connecting to a database from a servlet.

servlet-class com.deitel.jhtp6.servlets.SurveyServlet

init-param

param-name databaseDriver

param-value com.mysql.jdbc.Driver

init-param

param-name databaseName

param-value jdbc:mysql://localhost/animalsurvey

init-param

param-name username

param-value jhtp6

init-param

param-name password

param-value jhtp6


servlet-name animalsurvey

url-pattern /animalsurvey

75


26.9 Welcome Files

• Welcome files– Ordered list of files

• HTML, JSP documents

– Loaded when the request URL is not mapped to a servlet– Defined using the welcome-file-list element

• Contain one or more welcome-file elements– Specify the partial URL of a welcome file

• Without a leading or trailing /• E.g., specify index.html and index.htm as welcome files

<welcome-file-list>

<welcome-file>index.html</welcome-file>

<welcome-file>index.htm</welcome-file></welcome-file-list>

76


Outline

index.html

(1 of 2)

Lines 15-24



3

4

5 

6


8 <head>

9 <title>Welcome File</title>

10 </head>

11

12 <body>

13 <p>Click a link to test each example demonstrated in this chapter</p>

14 <p>

15 <a href = "/jhtp6/servlets/WelcomeServlet.html">

16 WelcomeServlet</a><br />

17 <a href = "/jhtp6/servlets/WelcomeServlet2.html">

18 WelcomeServlet2</a><br />

19 <a href = "/jhtp6/servlets/WelcomeServlet3.html">

20 WelcomeServlet3</a><br />

21 <a href = "/jhtp6/servlets/RedirectServlet.html">

22 RedirectServlet</a><br />

23 <a href = "/jhtp6/servlets/Survey.html">

24 SurveyServlet</a><br />

25 </p>

26 </body>

27 </html>

Provide links to test all the examples demonstrated in this chapter

77


Outline

index.html

(2 of 2)

Program output

2005 Pearson Education, Inc. All rights reserved. 1 26 Servlets.

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