Servlets

1
Servlets

• Chapter 26

2
Servlets

• A servlet is a Java program that resides on the
  server and sends information back to the client.
• For example, a web server sends HTML to a
  browser.
• The HTML can come from a file on the server or it
  can be sent by a program such as a Java servlet.

3
Servlets

• Packages for servlets
• javax.servlet defines GenericServlet, which is
  protocol-independent.
• javax.servlet.http defines HttpServlet, which
  uses the HTTP protocol.
• Both GenericServlet and HttpServlet implement the
  Servlet interface.

4
Methods of interface Servlet
5
HttpServlet

• The two most common requests from a client web
  browser are
• Get retrieves information from the server,
  usually an HTML document. Handled by HttpServlet
  method doGet().
• Post sends information to the server, such as
  information from a form. Handled by HttpServlet
  method doPost().

6
Other HttpServlet Methods
7
Other HTTP Interfaces

• Both doGet() and doPost() receive a response and
  request object.
• The request object implements the
  HttpServletRequest interface, and the response
  object implements the HttpServletResponse
  interface.
• These enable the servlet to get the client
  request and formulate a response.

8

• lt?xml version "1.0"?gt
• lt!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0
  Strict//EN"
• "http//www.w3.org/TR/xhtml1/DTD/xhtml1-strict.
  dtd"gt
• lt!-- Fig. 26.7 WelcomeServlet.html --gt
• lthtml xmlns "http//www.w3.org/1999/xhtml"gt
• ltheadgt
• lttitlegtHandling an HTTP Get Requestlt/titlegt
• lt/headgt
• ltbodygt
• ltform action "/jhtp6/welcome1" method
  "get"gt
• ltpgtltlabelgtClick the button to invoke the
  servlet
• ltinput type "submit" value "Get HTML
  Document" /gt
• lt/labelgtlt/pgt
• lt/formgt
• lt/bodygt
• lt/htmlgt

9

• // Fig. 26.6 WelcomeServlet.java. A simple
  servlet to process get requests.
• import javax.servlet.ServletException
• import javax.servlet.http.HttpServlet
• import javax.servlet.http.HttpServletRequest
• import javax.servlet.http.HttpServletResponse
• import java.io.IOException
• import java.io.PrintWriter
• public class WelcomeServlet extends HttpServlet
• // process "get" requests from clients
• protected void doGet( HttpServletRequest
  request,
• HttpServletResponse response )
• throws ServletException, IOException
• response.setContentType( "text/html" )
• PrintWriter out response.getWriter()
• // send XHTML page to client

10

• out.println( "lthtml xmlns
  \"http//www.w3.org/1999/xhtml\"gt" )
• // head section of document
• out.println( "ltheadgt" )
• out.println( "lttitlegtA Simple Servlet
  Examplelt/titlegt" )
• out.println( "lt/headgt" )
• // body section of document
• out.println( "ltbodygt" )
• out.println( "lth1gtWelcome to
  Servlets!lt/h1gt" )
• out.println( "lt/bodygt" )
• // end XHTML document
• out.println( "lt/htmlgt" )
• out.close() // close stream to complete
  the page
• // end method doGet
• // end class WelcomeServlet

11

• ltweb-app xmlns"http//java.sun.com/xml/ns/j2ee"
• xmlnsxsi"http//www.w3.org/2001/XMLSchema-ins
  tance"
• xsischemaLocation"http//java.sun.com/xml/ns/
  j2ee
• http//java.sun.com/xml/ns/j2ee/web-app_2_4.
  xsd"
• version"2.4"gt
• lt!-- General description of your Web
  application --gt
• ltdisplay-namegt
• Java How to Program JSP
• and Servlet Chapter Examples
• lt/display-namegt
• ltdescriptiongt
• This is the Web application in which we
• demonstrate our JSP and Servlet examples.
• lt/descriptiongt
• lt!-- Servlet definitions --gt
• ltservletgt

12
Get Requests Containing Data

• A request to the server can also contain data.
• Data comes in the form of parameters.
• Parameters are name-value pairs supplied at the
  end of the URL after a question mark.
• For example http//localhost?firstnameJon
• Use for multiple parameters.

13

• lt?xml version "1.0"?gt
• lt!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0
  Strict//EN"
• "http//www.w3.org/TR/xhtml1/DTD/xhtml1-strict.
  dtd"gt
• lt!-- Fig. 26.13 WelcomeServlet2.html --gt
• lthtml xmlns "http//www.w3.org/1999/xhtml"gt
• ltheadgt
• lttitlegtProcessing get requests with
  datalt/titlegt
• lt/headgt
• ltbodygt
• ltform action "/jhtp6/welcome2" method
  "get"gt
• ltpgtltlabelgt
• Type your first name and press the
  Submit button
• ltbr /gtltinput type "text" name
  "firstname" /gt
• ltinput type "submit" value "Submit"
  /gt
• lt/pgtlt/labelgt
• lt/formgt

14
// Fig. 26.12 WelcomeServlet2.java. Processing
HTTP get requests containing data. import
javax.servlet.ServletException import
javax.servlet.http.HttpServlet import
javax.servlet.http.HttpServletRequest import
javax.servlet.http.HttpServletResponse import
java.io.IOException import java.io.PrintWriter
public class WelcomeServlet2 extends HttpServlet
// process "get" request from client
protected void doGet( HttpServletRequest request,
HttpServletResponse response )
throws ServletException, IOException
String firstName request.getParameter(
"firstname" ) response.setContentTyp
e( "text/html" ) PrintWriter out
response.getWriter() // send
XHTML document to client // start XHTML
document out.println( "lt?xml version
\"1.0\"?gt" )
15
out.printf( "sss", "lt!DOCTYPE html
PUBLIC", " \"-//W3C//DTD XHTML 1.0
Strict//EN\"", " \"http//www.w3.org/TR/x
html1/DTD/xhtml1-strict.dtd\"gt\n" )
out.println( "lthtml xmlns \"http//www.w3.org/19
99/xhtml\"gt" ) // head section of
document out.println( "ltheadgt" )
out.println( "lttitlegtProcessing get requests with
datalt/titlegt" ) out.println( "lt/headgt" )
// body section of document
out.println( "ltbodygt" ) out.println(
"lth1gtHello " firstName ",ltbr /gt" )
out.println( "Welcome to Servlets!lt/h1gt" )
out.println( "lt/bodygt" ) // end
XHTML document out.println( "lt/htmlgt" )
out.close() // close stream to complete the
page // end method doGet // end class
WelcomeServlet2
16
Post Requests

• The next example shows how to handle a post
  request.
• The example is identical to before, except for
  the doPost() instead of doGet().
• This is because the classes are hiding some of
  the differences between these two kinds of
  requests.

17

• lt?xml version "1.0"?gt
• lt!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0
  Strict//EN"
• "http//www.w3.org/TR/xhtml1/DTD/xhtml1-strict.
  dtd"gt
• lt!-- Fig. 26.16 WelcomeServlet3.html --gt
• lthtml xmlns "http//www.w3.org/1999/xhtml"gt
• ltheadgt
• lttitlegtHandling an HTTP Post Request with
  Datalt/titlegt
• lt/headgt
• ltbodygt
• ltform action "/jhtp6/welcome3" method
  "post"gt
• ltpgtltlabelgt
• Type your first name and press the
  Submit button
• ltbr /gtltinput type "text" name
  "firstname" /gt
• ltinput type "submit" value "Submit"
  /gt
• lt/labelgtlt/pgt
• lt/formgt

18
// Fig. 26.15 WelcomeServlet3.java. Processing
post requests containing data. import
javax.servlet.ServletException import
javax.servlet.http.HttpServlet import
javax.servlet.http.HttpServletRequest import
javax.servlet.http.HttpServletResponse import
java.io.IOException import java.io.PrintWriter
public class WelcomeServlet3 extends
HttpServlet // process "post" request
from client protected void doPost(
HttpServletRequest request, HttpServletResponse
response ) throws ServletException,
IOException String firstName
request.getParameter( "firstname" )
response.setContentType( "text/html" )
PrintWriter out response.getWriter()
// send XHTML page to client // start
XHTML document out.println( "lt?xml version
\"1.0\"?gt" )
19
out.printf( "sss", "lt!DOCTYPE html
PUBLIC", " \"-//W3C//DTD XHTML 1.0
Strict//EN\"", " \"http//www.w3.org/TR/x
html1/DTD/xhtml1-strict.dtd\"gt\n" )
out.println( "lthtml xmlns \"http//www.w3.org/19
99/xhtml\"gt" ) // head section of
document out.println( "ltheadgt" )
out.println( "lttitlegtProcessing post requests
with datalt/titlegt" ) out.println(
"lt/headgt" ) // body section of
document out.println( "ltbodygt" )
out.println( "lth1gtHello " firstName ",ltbr /gt"
) out.println( "Welcome to Servlets!lt/h1gt"
) out.println( "lt/bodygt" )
// end XHTML document out.println(
"lt/htmlgt" ) out.close() // close stream
to complete the page // end method doPost
// end class WelcomeServlet3
20
Redirecting Requests

• Sometimes a request is redirected to another
  place, such as to a different server or web page.
• The response objects sendRedirect() method
  accomplishes this.
• The next example illustrates this

21

• lt?xml version "1.0"?gt
• lt!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0
  Strict//EN"
• "http//www.w3.org/TR/xhtml1/DTD/xhtml1-strict.
  dtd"gt
• lt!-- Fig. 26.19 RedirectServlet.html --gt
• lthtml xmlns "http//www.w3.org/1999/xhtml"gt
• ltheadgt
• lttitlegtRedirecting a Request to Another
  Sitelt/titlegt
• lt/headgt
• ltbodygt
• ltpgtClick a link to be redirected to the
  appropriate pagelt/pgt
• ltpgt
• lta href "/jhtp6/redirect?pagedeitel"gt
• www.deitel.comlt/agtltbr /gt
• lta href "/jhtp6/redirect?pagewelcome1"gt
• Welcome servletlt/agt
• lt/pgt

22
// Fig. 26.18 RedirectServlet.java. Redirecting
a user to a different Web page. import
javax.servlet.ServletException import
javax.servlet.http.HttpServlet import
javax.servlet.http.HttpServletRequest import
javax.servlet.http.HttpServletResponse import
java.io.IOException import java.io.PrintWriter
public class RedirectServlet extends HttpServlet
// process "get" request from client
protected void doGet( HttpServletRequest request,
HttpServletResponse response ) throws
ServletException, IOException String
location request.getParameter( "page" )
if ( location ! null ) if (
location.equals( "deitel" ) )
response.sendRedirect( "http//www.deitel.com"
) else if ( location.equals( "welcome1"
) ) response.sendRedirect( "welcome1"
) // end if
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// code that executes only if this servlet
does not redirect the user to another page
response.setContentType( "text/html" )
PrintWriter out response.getWriter()
// start XHTML document out.println( "lt?xml
version \"1.0\"?gt" ) out.printf(
"sss", "lt!DOCTYPE html PUBLIC", "
\"-//W3C//DTD XHTML 1.0 Strict//EN\"", "
\"http//www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dt
d\"gt\n" ) out.println( "lthtml xmlns
\"http//www.w3.org/1999/xhtml\"gt" ) //
head section of document out.println(
"ltheadgt" ) out.println( "lttitlegtInvalid
pagelt/titlegt" ) out.println( "lt/headgt" )
24

• // body section of document
• out.println( "ltbodygt" )
• out.println( "lth1gtInvalid page
  requestedlt/h1gt" )
• out.println( "ltpgtlta href "
• "\"servlets/RedirectServlet.html\"gt" )
• out.println( "Click here to choose
  againlt/agtlt/pgt" )
• out.println( "lt/bodygt" )
• // end XHTML document
• out.println( "lt/htmlgt" )
• out.close() // close stream to complete
  the page
• // end method doGet
• // end class RedirectServlet

25
Two-tier Architecture

• A program may consist of a GUI interface, logic
  to process requests, and a database to store
  requests.
• Traditionally, these might be constructed as
  two-tiers.
• The GUI and logic are in one tier, the database
  is in the other

DB
Client
Fat client
26
Three-tier Architecture

• If the client is broken into two pieces, then the
  architecture becomes three-tier.
• One piece is a thin client, which means a client
  with very little logic, such as a web browser,
  mostly responsible for presentation.
• The other piece is the application logic which
  resides on a server.

DB
Client
Logic
Thin client
27
JDBC with Servlets

• A web page interacting with a servlet that uses
  JDBC to interact with a database can act as a
  three-tier architecture.
• The next example illustrates this

28

• lt?xml version "1.0"?gt
• lt!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0
  Strict//EN"
• "http//www.w3.org/TR/xhtml1/DTD/xhtml1-strict.
  dtd"gt
• lt!-- Fig 26.22 Survey.html --gt
• lthtml xmlns "http//www.w3.org/1999/xhtml"gt
• ltheadgt
• lttitlegtSurveylt/titlegt
• lt/headgt
• ltbodygt
• ltform method "post" action "/jhtp6/animalsurve
  y"gt
• ltpgtWhat is your favorite pet?lt/pgt
• ltpgt
• ltinput type "radio" name "animal"
• value "1" /gtDogltbr /gt
• ltinput type "radio" name "animal"
• value "2" /gtCatltbr /gt

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// Fig. 26.21 SurveyServlet.java // A Web-based
survey that uses JDBC from a servlet. package
com.deitel.jhtp6.servlets import
java.io.PrintWriter import java.io.IOException i
mport java.sql.Connection import
java.sql.DriverManager import java.sql.Statement
import java.sql.ResultSet import
java.sql.SQLException import javax.servlet.Servle
tConfig import javax.servlet.ServletException im
port javax.servlet.UnavailableException import
javax.servlet.http.HttpServlet import
javax.servlet.http.HttpServletRequest import
javax.servlet.http.HttpServletResponse public
class SurveyServlet extends HttpServlet
private Connection connection private
Statement statement
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// set up database connection and create SQL
statement public void init( ServletConfig
config ) throws ServletException //
attempt database connection and create Statement
try Class.forName(
config.getInitParameter( "databaseDriver" ) )
connection DriverManager.getConnection(
config.getInitParameter(
"databaseName" ), config.getInitParame
ter( "username" ), config.getInitParam
eter( "password" ) ) // create
Statement to query database statement
connection.createStatement() // end try
// 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() )
// end catch // end method init
Provided to server
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// process survey response protected void
doPost( HttpServletRequest request,
HttpServletResponse response ) throws
ServletException, IOException // set
up response to client response.setContentTyp
e( "text/html" ) PrintWriter out
response.getWriter() // start XHTML
document out.println( "lt?xml version
\"1.0\"?gt" ) out.printf( "sss",
"lt!DOCTYPE html PUBLIC", "
\"-//W3C//DTD XHTML 1.0 Strict//EN\"", "
\"http//www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dt
d\"gt\n" ) out.println( "lthtml xmlns
\"http//www.w3.org/1999/xhtml\"gt" ) //
head section of document out.println(
"ltheadgt" ) // read current survey
response int value Integer.parseInt(
request.getParameter( "animal" ) ) String
sql
32
// attempt to process a vote and display
current results try //
update total for current survey response
sql "UPDATE surveyresults SET votes votes 1
" "WHERE id " value
statement.executeUpdate( sql ) // get
total of all survey responses sql
"SELECT sum( votes ) FROM surveyresults"
ResultSet totalRS statement.executeQuery( sql
) totalRS.next() // position to first
record int total totalRS.getInt( 1
) // get results sql
"SELECT surveyoption, votes, id FROM
surveyresults " "ORDER BY id"
ResultSet resultsRS statement.executeQuery( sql
) out.println( "lttitlegtThank
you!lt/titlegt" ) out.println( "lt/headgt"
) out.println( "ltbodygt" )
out.println( "ltpgtThank you for
participating." ) out.println( "ltbr
/gtResultslt/pgtltpregt" )
33
// process results int votes
while ( resultsRS.next() )
out.print( resultsRS.getString(
1 ) ) out.print( " " )
votes resultsRS.getInt( 2 )
out.printf( ".2f", ( double ) votes / total
100 ) out.print( " responses "
) out.println( votes )
// end while resultsRS.close()
out.print( "Total responses " )
out.print( total ) // end
XHTML document out.println(
"lt/pregtlt/bodygtlt/htmlgt" )
out.close() // end try
34
// if database exception occurs, return
error page catch ( SQLException
sqlException )
sqlException.printStackTrace()
out.println( "lttitlegtErrorlt/titlegt" )
out.println( "lt/headgt" ) out.println(
"ltbodygtltpgtDatabase error occurred. " )
out.println( "Try again later.lt/pgtlt/bodygtlt/htmlgt"
) out.close() // end catch
// end method doPost
35

• // close SQL statements and database when
  servlet terminates
• public void destroy()
• // attempt to close statements and database
  connection
• try
• statement.close()
• connection.close()
• // end try
• // handle database exceptions by returning
  error to client
• catch( SQLException sqlException )
• sqlException.printStackTrace()
• // end catch
• // end method destroy
• // end class SurveyServlet

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End of Slides