Servlets

1
Servlets

• Servlets are modules that extend the
  functionality of a java-enabled web-server
• They normally generate HTML code and web content
  dynamically. This is sent to the browser which
  displays it.
• For example, they send a query to a database
  based on parameters sent by the browser and send
  the results to the browser in html format

2
Development Environments

• There are many good development environments
  which help to write and test the servlets
• They include an editor and a java-enabled sever
• They also include all the necessary jar files an
  import statements
• Some of them are Eclipse (need to download
  plugins) and Netbeans (which also has full j2ee
  support

3
Anatomy of a Servlet

• A new servlet can be written by extending the
  HttpServlet class which has the following
  pre-defined methods
• init()
• is called when the servlet is uploaded the
  first time (this can vary depending on the
  server)
• doGet(HttpServletRequest req, HttpServletResponse
  res) throws ServletException, IOException
• is called every time the servlet is contacted by
  a GET request (which is the default way)
• doPost(HttpServletRequest req, HttpServletResponse
  res) throws ServletException, IOException
• is called when the client contacted the servlet
  with a POST request

4
import java.io. import javax.servlet. import
javax.servlet.http. public class MyServlet
extends HttpServlet public void init()
//Sobreescribir para que haga lo que
queramos public void doGet (
HttpServletRequest request,
HttpServletResponse response) throws
ServletException, IOException
//Sobreescribir para que haga lo que queramos
public void doPost( HttpServletRequest
request,
HttpServletResponse response)
throws ServletException, IOException
//Sobreescribir para que haga lo que queramos

5
The HttpServletRequest Parameter

• HttpServletRequest is the class of the first
  parameter the server uses to calls doGet and
  doPost. It gives access to
• Information about the client, for example,
  parameters passed, protocol used, clients host,
  etc.
• The input stream, ServletInputStream is used by
  the servlet to receive data from the client when
  the method POST or PUT has been used.

6
The HttpServletResponse parameter

• HttpServletResponse is the class of the second
  argument.
• Provides methods for
• Declaring the MIME type of the answer that will
  be sent to the client
• Getting the output stream ServletOutputStream and
  a Writer through which the servlet can send
  dinamically generated html code to the browser.
• Sending other information to the browser
  (cookies, refreshment time, etc)

7
Example 1

• import java.io.
• import javax.servlet.
• import javax.servlet.http.
• import java.util.Date
• public class SimpleServlet extends HttpServlet
  
• public void doGet ( HttpServletRequest request,
  
• HttpServletResponse
  response) throws
• ServletException, IOException
• // set content type
• response.setContentType("text/html")
• // open print writer to browser
• PrintWriter out response.getWriter()
• //send data
• out.println("ltHTMLgt")
• out.println("ltH1gt Mi Primer Servlet lt/H1gt")
• out.println("ltBRgt ltH2gtFecha y hora "(new
  Date())"ltH2gt")
• out.println("lt/HTMLgt")

8
Example 1

• import java.io.
• import javax.servlet.
• import javax.servlet.http.
• import java.util.Date
• public class SimpleServlet extends HttpServlet
  
• public void doGet ( HttpServletRequest request,
  
• HttpServletResponse
  response) throws
• ServletException, IOException
• // set content type
• response.setContentType("text/html")
• // open print writer to browser
• PrintWriter out response.getWriter()
• //send data
• out.println("ltHTMLgt")
• out.println("ltH1gt Mi Primer Servlet lt/H1gt")
• out.println("ltBRgt ltH2gtFecha y hora "(new
  Date())"ltH2gt")
• out.println("lt/HTMLgt")

Imports necessary classes
This is for the Date class
9
Example 1

• import java.io.
• import javax.servlet.
• import javax.servlet.http.
• import java.util.Date
• public class SimpleServlet extends HttpServlet
  
• public void doGet ( HttpServletRequest request,
  
• HttpServletResponse
  response) throws
• ServletException, IOException
• // set content type
• response.setContentType("text/html")
• // open print writer to browser
• PrintWriter out response.getWriter()
• //send data
• out.println("ltHTMLgt")
• out.println("ltH1gt Mi Primer Servlet lt/H1gt")
• out.println("ltBRgt ltH2gtFecha y hora "(new
  Date())"ltH2gt")
• out.println("lt/HTMLgt")

Every servlet extends HttpServlet
Overwrites doGet method
10
Example 1

• import java.io.
• import javax.servlet.
• import javax.servlet.http.
• import java.util.Date
• public class SimpleServlet extends HttpServlet
  
• public void doGet ( HttpServletRequest request,
  
• HttpServletResponse
  response) throws
• ServletException, IOException
• // set content type
• response.setContentType("text/html")
• // open print writer to browser
• PrintWriter out response.getWriter()
• //send data
• out.println("ltHTMLgt")
• out.println("ltH1gt Mi Primer Servlet lt/H1gt")
• out.println("ltBRgt ltH2gtFecha y hora "(new
  Date())"ltH2gt")
• out.println("lt/HTMLgt")

Tells the browser the content type of the answer
Gets writer to browser from response parameter
11
Example 1

• import java.io.
• import javax.servlet.
• import javax.servlet.http.
• import java.util.Date
• public class SimpleServlet extends HttpServlet
  
• public void doGet ( HttpServletRequest request,
  
• HttpServletResponse
  response) throws
• ServletException, IOException
• // set content type
• response.setContentType("text/html")
• // open print writer to browser
• PrintWriter out response.getWriter()
• //send data
• out.println("ltHTMLgt")
• out.println("ltH1gt Mi Primer Servlet
  lt/H1gt")
• out.println("ltBRgt ltH2gtFecha y hora "(new
  Date())"ltH2gt")
• out.println("lt/HTMLgt")

Print data to browser
Get date and time from system
Close connection to browser
12
Running the first example

• Writing a servlet with Netbeans is very easy
• Also the deployment is done automatically
• Open netbeans
• Create a web project (this will create a lot of
  directories for putting the different kind of
  files)
• Create a servlet
• Copy the code of SimpleServlet.java
• Run the file

13
A second example

• Implementing a web counter
• It will count how many times an object of this
  class has been creates
• It will show the Address of the computer that
  contacted the servlet
• It will show a

14
import java.io. import javax.servlet.
import javax.servlet.http. public class Count
extends HttpServlet int count 0
// a counter starts in 0 public
void doGet ( HttpServletRequest req,
HttpServletResponse res) throws
ServletException, IOException
count PrintWriter out
res.getWriter() res.setContentType("text/
html") out.println("ltH1gt A web page
counter lt/H1gt") out.println("ltHRgt")
out.println("This servlet was accessed "count"
time(s)") out.println("ltHRgt")
out.println("Your computer is "req.getRemoteHost
()) out.println("ltHRgt")
out.close()
15
import java.io. import javax.servlet.
import javax.servlet.http. public class Count
extends HttpServlet int count 0
// a counter starts in 0 public
void doGet ( HttpServletRequest req,
HttpServletResponse res) throws
ServletException, IOException
count PrintWriter out
res.getWriter() res.setContentType("text/
html") out.println("ltH1gt A web page
counter lt/H1gt") out.println("ltHRgt")
out.println("This servlet was accessed "count"
time(s)") out.println("ltHRgt")
out.println("Your computer is "req.getRemoteHost
()) out.println("ltHRgt")
out.close()
16
import java.io. import javax.servlet.
import javax.servlet.http. public class Count
extends HttpServlet int count 0
// a counter starts in 0 public
void doGet ( HttpServletRequest req,
HttpServletResponse res) throws
ServletException, IOException
count PrintWriter out
res.getWriter() res.setContentType("text/
html") out.println("ltH1gt A web page
counter lt/H1gt") out.println("ltHRgt")
out.println("This servlet was accessed "count"
time(s)") out.println("ltHRgt")
out.println("Your computer is "req.getRemoteHost
()) out.println("ltHRgt")
out.close()
Increments counter every time doGet is called by
the web server
17
import java.io. import javax.servlet.
import javax.servlet.http. public class Count
extends HttpServlet int count 0
// a counter starts in 0 public
void doGet ( HttpServletRequest req,
HttpServletResponse res) throws
ServletException, IOException
count PrintWriter out
res.getWriter() res.setContentType("text/
html") out.println("ltH1gt A web page
counter lt/H1gt") out.println("ltHRgt")
out.println("This servlet was accessed "count"
time(s)") out.println("ltHRgt")
out.println("Your computer is "req.getRemoteHost
()) out.println("ltHRgt")
out.close()
Sine Qua Non
18
import java.io. import javax.servlet.
import javax.servlet.http. public class Count
extends HttpServlet int count 0
// a counter starts in 0 public
void doGet ( HttpServletRequest req,
HttpServletResponse res) throws
ServletException, IOException
count PrintWriter out
res.getWriter() res.setContentType("text/
html") out.println("ltH1gt A web page
counter lt/H1gt") out.println("ltHRgt")
out.println("This servlet was accessed "count"
time(s)") out.println("ltHRgt")
out.println("Your computer is "req.getRemoteHost
()) out.println("ltHRgt")
out.close()
Print data to browser
19
What happens if the server crashes and starts
again ?

• The counter will start from 0 again
• To remember the value of the counter in cast of
  an unexpected crash, we will write the value of
  the variable in a file every time it changes.
• At the beginning, the servlet reads the initial
  value from a file, if it exists, or creates the
  file with the initial value 0

20
public class Count extends HttpServlet int
count 0 // a counter for the object
public void init() try
BufferedReader in new BufferedReader(
newFileReader(count.txt))
String l in.readLine() count
Integer.parseInt(l) catch
(FileNotFoundException e) //no need to do
anything here public
void doGet ( HttpServletRequest req,
HttpServletResponse res) throws
ServletException, IOException
count PrintWriter outFile new
PrintWriter( new Filereader(count.txt))
outFile.println(count)
outFile.close() PrintWriter outBrowser
res.getWriter() res.setContentType("text/
html") outBrowser.println("ltH1gt A web
page counter lt/H1gt") outBrowser.println("
ltHRgt") ..... .....
21
public class Count extends HttpServlet int
count 0 // a counter for the object
public void init() try
BufferedReader in new BufferedReader(
newFileReader(count.txt))
String l in.readLine() count
Integer.parseInt(l) catch
(FileNotFoundException e) //no need to do
anything here public
void doGet ( HttpServletRequest req,
HttpServletResponse res) throws
ServletException, IOException
count PrintWriter outFile new
PrintWriter( new Filereader(count.txt))
outFile.println(count)
outFile.close() PrintWriter outBrowser
res.getWriter() res.setContentType("text/
html") outBrowser.println("ltH1gt A web
page counter lt/H1gt") outBrowser.println("
ltHRgt") ..... .....
Try to open the file when the servlet is
called the first time
22
public class Count extends HttpServlet int
count 0 // a counter for the object
public void init() try
BufferedReader in new BufferedReader(
newFileReader(count.txt))
String l in.readLine() count
Integer.parseInt(l) catch
(FileNotFoundException e) //no need to do
anything here public
void doGet ( HttpServletRequest req,
HttpServletResponse res) throws
ServletException, IOException
count PrintWriter outFile new
PrintWriter( new Filereader(count.txt))
outFile.println(count)
outFile.close() PrintWriter outBrowser
res.getWriter() res.setContentType("text/
html") outBrowser.println("ltH1gt A web
page counter lt/H1gt") outBrowser.println("
ltHRgt") ..... .....
Read the line and convert the content to its
integer value
23
public class Count extends HttpServlet int
count 0 // a counter for the object
public void init() try
BufferedReader in new BufferedReader(
newFileReader(count.txt))
String l in.readLine() count
Integer.parseInt(l) catch
(FileNotFoundException e) //no need to do
anything here public
void doGet ( HttpServletRequest req,
HttpServletResponse res) throws
ServletException, IOException
count PrintWriter outFile new
PrintWriter( new Filereader(count.txt))
outFile.println(count)
outFile.close() PrintWriter outBrowser
res.getWriter() res.setContentType("text/
html") outBrowser.println("ltH1gt A web
page counter lt/H1gt") outBrowser.println("
ltHRgt") ..... .....
24
public class Count extends HttpServlet int
count 0 // a counter for the object
public void init() try
BufferedReader in new BufferedReader(
newFileReader(count.txt))
String l in.readLine() count
Integer.parseInt(l) catch
(FileNotFoundException e) //no need to do
anything here public void
doGet ( HttpServletRequest req,
HttpServletResponse res) throws
ServletException, IOException
count PrintWriter outFile new
PrintWriter( new Filereader(count.txt))
outFile.println(count)
outFile.close() PrintWriter outBrowser
res.getWriter() res.setContentType("text/
html") outBrowser.println("ltH1gt A web
page counter lt/H1gt") outBrowser.println("
ltHRgt") ..... .....
After count is incremented, open the file to
write (overwrite), write the new number and
close file
25
public class Count extends HttpServlet int
count 0 // a counter for the object
public void init() try
BufferedReader in new BufferedReader(
newFileReader(count.txt))
String l in.readLine() count
Integer.parseInt(l) catch
(FileNotFoundException e) //no need to do
anything here public
void doGet ( HttpServletRequest req,
HttpServletResponse res) throws
ServletException, IOException
count PrintWriter outFile new
PrintWriter( new Filereader(count.txt))
outFile.println(count)
outFile.close() PrintWriter outBrowser
res.getWriter() res.setContentType("text/
html") outBrowser.println("ltH1gt A web
page counter lt/H1gt") outBrowser.println("
ltHRgt") ..... .....
The rest is the same
26
Exercise

• Escriba un servlet que genere un número aleatorio
  entre 1 y 100 cada vez que es contactado y lo
  muestra al cliente junto con la IP del computador
  del cliente
• Además muestra el número más grande generado
  hasta ahora y la IP del computador para el cual
  lo generó
• La dirección del computador del cliente puede ser
  obtenida
• String s request.getRemoteHost()
• returns the IP number (like 133.8.109.158
• String s request.getRemoteAddress()
• returns the name if it can be retrieved (like
  www.waseda.jp)
• Un número aleatorio entre 1 y 100 se genera con
  la siguiente instrucción en Java
• int numero (int)(1 Math.random()100)

27
(No Transcript)
28
import java.io. import javax.servlet.
import javax.servlet.http. public class
NewServlet extends HttpServlet int
maxNumber 0 String maxIP ""
public void doGet(HttpServletRequest request,
HttpServletResponse response) throws
IOException, ServletException
int aleatorio (int)(1 Math.random()100)
String ip request.getRemoteAddr() if
(aleatorio gt maxNumber) maxNumber
aleatorio maxIP ip
response.setContentType("text/html")
PrintWriter out response.getWriter()
out.println("lth1gt Bienvenido lt/h1gtltbrgt")
out.println("lth2gt Su dirección IP es " ip
"ltbrgt") out.println("Su número es
"aleatorio"ltbrgtltbrgt") out.println("El
numero mayor generado es "maxNumber)
out.println("ltbrgtFue generado para "maxIP)
out.close()
29
import java.io. import javax.servlet.
import javax.servlet.http. public class
NewServlet extends HttpServlet int
maxNumber 0 String maxIP ""
public void doGet(HttpServletRequest request,
HttpServletResponse response) throws
IOException, ServletException
int aleatorio (int)(1 Math.random()100)
String ip request.getRemoteAddr() if
(aleatorio gt maxNumber) maxNumber
aleatorio maxIP ip
response.setContentType("text/html")
PrintWriter out response.getWriter()
out.println("lth1gt Bienvenido lt/h1gtltbrgt")
out.println("lth2gt Su dirección IP es " ip
"ltbrgt") out.println("Su número es
"aleatorio"ltbrgtltbrgt") out.println("El
numero mayor generado es "maxNumber)
out.println("ltbrgtFue generado para "maxIP)
out.close()
Inicializacion variables para guardar La IP y el
valor del máximo
30
import java.io. import javax.servlet.
import javax.servlet.http. public class
NewServlet extends HttpServlet int
maxNumber 0 String maxIP ""
public void doGet(HttpServletRequest request,
HttpServletResponse response) throws
IOException, ServletException
int aleatorio (int)(1 Math.random()100)
String ip request.getRemoteAddr()
if (aleatorio gt maxNumber)
maxNumber aleatorio maxIP ip
response.setContentType("text/html")
PrintWriter out response.getWriter()
out.println("lth1gt Bienvenido lt/h1gtltbrgt")
out.println("lth2gt Su dirección IP es " ip
"ltbrgt") out.println("Su número es
"aleatorio"ltbrgtltbrgt") out.println("El
numero mayor generado es "maxNumber)
out.println("ltbrgtFue generado para "maxIP)
out.close()
Generación número aleatorio
Obtencion ip del cliente
31
import java.io. import javax.servlet.
import javax.servlet.http. public class
NewServlet extends HttpServlet int
maxNumber 0 String maxIP ""
public void doGet(HttpServletRequest request,
HttpServletResponse response) throws
IOException, ServletException
int aleatorio (int)(1 Math.random()100)
String ip request.getRemoteAddr()
if (aleatorio gt maxNumber)
maxNumber aleatorio maxIP ip
response.setContentType("text/html")
PrintWriter out response.getWriter()
out.println("lth1gt Bienvenido lt/h1gtltbrgt")
out.println("lth2gt Su dirección IP es " ip
"ltbrgt") out.println("Su número es
"aleatorio"ltbrgtltbrgt") out.println("El
numero mayor generado es "maxNumber)
out.println("ltbrgtFue generado para "maxIP)
out.close()
Actualización de los datos del máximo generado
32
import java.io. import javax.servlet.
import javax.servlet.http. public class
NewServlet extends HttpServlet int
maxNumber 0 String maxIP ""
public void doGet(HttpServletRequest request,
HttpServletResponse response) throws
IOException, ServletException
int aleatorio (int)(1 Math.random()100)
String ip request.getRemoteAddr() if
(aleatorio gt maxNumber) maxNumber
aleatorio maxIP ip
response.setContentType("text/html")
PrintWriter out response.getWriter()
out.println("lth1gt Bienvenido lt/h1gtltbrgt")
out.println("lth2gt Su dirección IP es " ip
"ltbrgt") out.println("Su número es
"aleatorio"ltbrgtltbrgt") out.println("El
numero mayor generado es "maxNumber)
out.println("ltbrgtFue generado para "maxIP)
out.close()
Enviar info al cliente