Servlets

Unit Outcomes
 Topics and Sub-topics
6.1 LifeCycle of Servlet

6.2 Creating Simple Servlet: The Servlet API,

javax.servlet package, Servlet Interface, ServletConfig
Interface, ServletContext Interface, ServletRequest
Interface, ServletResponse Interface, Generic Servlet Class

6.3 The javax.servlet.http package: httpServletRequest

Interface, httpServletResponse Interface, httpSession
Interface, Cookie Class, HttpServlet Class,
httpSessionEvent Class, httpSessionBindingEvent Class,

6.4 Handling HTTP Requests and Responses , Handling

HTTP GET Requests, Handling HTTP POST Request
 Introduction to Servlet
 Basics of how web browsers and servers cooperate to provide

content to user
 Example of Static Web Page:

 User enters URL in browser (https://msbte.org.in/)

 Browser generates an HTTP Request to appropriate Server

(msbte server)
 Web server maps this request to a specific file (index..html)

 File is return in an HTTP Response (in the form of HTML) to the

browser.
 Header in the response indicates type of content(html/text).

MIME (Multipurpose Internet Mail Extension) are used for this

purpose.
Introduction to Servlet
Example of Dynamic Web Page:

Online store uses database to store its

information.
The data could be price, products for sale and it

must be updated.
User access this information via web page.

The content of such pages must be dynamically

generated to reflect the latest information

Introduction to Servlet
Existing Problems:

Earlier days, server could dynamically

construct a page by creating a separate

process for each client.
That process open a connection with database.

Communication takes place via Common

Gateway Interface(CGI).
C,C++ and Perl used to create CGI programs.
Introduction to Servlet
Problems with CGI:

Serious performance problems

Expensive in terms of processor and memory

resources required to create separate process

for each client request.
Also expensive to open and close database

connection
CGI programs are not platform-independent.
Introduction to Servlet
Problems with CGI:

Serious performance problems

Expensive in terms of processor and memory

resources required to create separate process

for each client request.
Also expensive to open and close database

connection
CGI programs are not platform-independent.
Introduction to Servlet
 Advantages of Servlet:

Performance better

Executes within the address space of a web server

Not necessary to create a separate process for each client

request
Platform independent as written in Java

Provide security for resources on server

Full functionality of Java class libraries is available to

Servlet
Communicate with Socket
Servlet
LIFE CYCLE OF SERVLET
 Three methods are

central to the life cycle

of a servlet
1. init()
2. service()
3. destroy()
 Implemented by every

servlet
 Invoked at specific time.
LIFE CYCLE OF SERVLET
First Step
User enters URL in browser
Browser generates an HTTP Request to appropriate
Server
Second Step
The server received request and maps this request to a
specific Servlet.
The Servlet is dynamically retrieved and loaded into
server space.
 Third Step
Server invoke init() method
init() method is invoked only when the servlet is first
loaded into memory(only once).
We can pass initialization parameters to the servlet for
their configuration
LIFE CYCLE OF SERVLET
Fouth Step
 Server invoke service() method of Servlet
 This method is invoked to process the HTTP request.
 Servlet can read the data provided in HTTP request
 Also we formulate HTTP Response for the client request
 Servlet remains in address space of Server to process
any other HTTP Requests from other clients
 The service() method is called for each HTTP Request.
Last Step
 Server may decide to unload Servlet from its memory
 At this time server calls destroy() method to relinquish
any resources such as file handles.
Requirement for Servlet
Programs
We need Servlet Container or Server

Popular Servers

Glassfish, Apache Tomcat

 Apache Tomcat (Downloads:

https://tomcat.apache.org/)
Is an open source product

Maintained by Apache Software Foundation

Can be used byn NetBeans IDE

 Creating Simple Servlet
 Once you installed Tomcat the servlet-api.jar file contained all

the classes and interfaces needed to create servlets.

C:\Apache Software Foundation\Tomcat 9.0\lib
 To make servlet-api.jar accessible, update the CLASSPATH

environment variable

CLASSPATH=C:\Apache Software Foundation\Tomcat

9.0\lib
Creating Simple Servlet
 Important Directories in Tomcat Folder

 webapps (main directory where we create web

application)
 examples

 WEB-INF

 classes
o put your classfile here
 web.xml (the important file where we write deployment
descriptor for Servlet)
Creating Simple Servlet
 Web.xml

 For a Java servlet to be accessible from a browser, you must tell the

servlet container what servlets to deploy, and what URL's to map the
servlets to.
 This is done in the web.xml file of your Java web application.

 First you configure the servlet. This is done using the <servlet>

element. Here you give the servlet a name, and writes the class name of
the servlet.
 Second, you map the servlet to a URL or URL pattern. This is done in

the <servlet-mapping> element. In the above example, all URL's

Creating Simple Servlet
 Web.xml
<servlet>
<servlet-name>Test</servlet-name>
<servlet-class>First</servlet-class>
</servlet>
Deployment
<servlet-mapping> Descriptor
<servlet-name>Test</servlet-name>
<url-pattern>TestApp</url-pattern>
</servlet-mapping>
Creating Simple
Servlet
Respons
Client Servlet
e Request
Browse 1
r Servlet
2Server
Servlet
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Servlet
4

Web.xm
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First Example
import java.io.*;
import javax.servlet.*; / / Servlet API
public class First extends GenericServlet
{
public void service(ServletRequest req,ServletResponse
res) throws IOException,ServletException
{
res.setContentType("text/html");
PrintWriter out=res.getWriter();
out.print("<html><body>");
out.print("<b>hello generic servlet</b>");
out.print("</body></html>");
}
}
 Example
 javax.servlet

 This package contains the classes and interfaces required to build servlets

 First

 Subclass of GenericServlet class which provides the functionality that simplifies the creation of
servlet
 Don’t need to write init() and destroy() method of Servlet Lifecyle
 Only service() method need to overwrite

 public void service() method

 This method handles the request from a client

 ServletRequest argument enables to read the data that is provided in request
 ServletResponse object in argument enables a servlet to formulate a response for the client

 throws IOException,ServletException

 res.setContentType("text/html"): set the response content type

Servlet
API
Two packages contains classes and interfaces that are
required

to build servlets.

1. javax.servlet

2. javax.servlet.http
javax.servlet
Package
 Following table summarizes several key interfaces provided in
this package.
 The most significant of these is Servlet interface
Servlet
Interface
 All servlets must implements the Servlet interface.
 It declares the init(), service() and destroy() methods that

are called by the server during the lifecycle of a servlet.

 The getServletConfig() method is called by the servlet to

obtain initialization parameters.

 The getServletInfo() method provide a string of useful

information about the servlet (example version info)

Servlet
Interface
ServletConfig
Interface
 Allows servlet to obtain configuration data when it is
loaded
ServletContext Interface
 Enables servlets to obtain information about their environment.
ServletRequest
Interface
 Enables servlets to obtain information about client
request.
ServletResponse
Interface
 Enables servlets to formulate a response for
client.
javax.servlet
Package
 Following table summarizes several key classes provided in
this package.
GenericServlet
Class
 Provides implementation of basic lifecycle methods for a
servlet.
 This class implements Servlet and ServletConfig
Interfaces
 In addition the following methods are used to append a

string to the server log file.

 void log(String s)

 void log(String s,Throwable e)

ServletInputStream
Class
 Extends InputStream Class
 Provides an input stream that a servlet developer can use

to read the data from a client request.

 Additional methods are

 int readLine(byte b[], int offset, int size) throws

IOException
ServletIOutputStream
Class
 Extends OutputStream Class

 Provides an input stream that a servlet developer can use

to write the data from a client response.

ServletException
Class
Javax.servlet defines two exceptions


 ServletException: Indicates a servlet problem has

occurred

 UnavailableException: It extends ServletException and

indicates that a servlet is unavailable

 Steps to create
1. Servlet
Create and compile import
import javax.servlet.*;
java.io.*;
the Servlet Source
public class First extends
GenericServlet{ public void
code
service(ServletRequest
req,ServletResponse res)
2. Copy the class file throws IOException,ServletException{

(First.class) to proper res.setContentType("text/html");

directory PrintWriter out=res.getWriter();
out.print("<html><body>");
out.print("<b>hello generic
C:\Apache Software servlet</b>");
}out.print("</body></html>");
Foundation\Tomcat 9.0\ }
webapps\examples\WEB-
INF\classes
 Steps to create
Servlet
3. Create deployment
descriptor for servlet.To do this
update the web.xml file
Example:
 Steps to create
Servlet
4. Start Tomcat
5. Start a web browser and request the
servlet
localhost:9090/examples/TestApp
The javax.servlet.http
Package
• The javax.servlet.http package contains a number of

interfaces and classes that are commonly used by servlet

developers.

• You will see that its functionality makes it easy to build

servlets that work with HTTP requests and responses.

The javax.servlet.http
Package: interfaces
HttpServletRequest Interface
Methods
HttpServletRequest Interface
Methods
The HttpServletResponse
Interface
• The HttpServletResponse interface enables a servlet to
formulate an HTTP response to a client. Several
constants are defined.
• These correspond to the different status codes that can be
assigned to an HTTP response.
• For example, SC_OK indicates that the HTTP request
succeeded, and SC_NOT_FOUND indicates that the
requested resource is not available.
HttpServletResponse Interface
Methods
HttpServletResponse Interface
Methods
The HttpSession Interface
The javax.servlet.http
Package: CLASSES
Cookie Class

• The Cookie class encapsulates a cookie. A cookie is

stored on a client and contains state information.
• Cookies are valuable for tracking user activities.
• For example, assume that auser visits an online store. A
cookie can save the user’s name, address, and other
information. The user does not need to enter this data
each time he or she visits the store.
Cookie Class
• HttpServletResponse contains a method addCookie(), which
can be used by a servlet to write a cookie to a user’s machine.
• This data for that cookie is then included in the header of the
HTTP response that is sent to the browser.
• The names and values of cookies are stored on the user’s
machine.
• Some of the information that is saved for each cookie
includes the following:
• The name of the cookie
• The value of the cookie
• The expiration date of the cookie
• The domain and path of the cookie
Cookie Class: Constructor
• There is one constructor for Cookie. It has the signature
shown here:
Cookie(String name, String value)
• Here, the name and value of the cookie are supplied as
arguments to the constructor.
Cookie Class: Method
HttpServlet Class

• The HttpServlet class extends GenericServlet.

• It is commonly used when developing servlets that receive
and process HTTP requests.
HttpServlet Class
Handling HTTP Requests and Responses
 The HttpServlet class provides specialized methods that handle
the various types of HTTP requests.
 A servlet developer typically overrides one of these methods.
 These methods are :
 doDelete( )
 doGet( )
 doHead( )
 doOptions( )
 doPost( )
 doPut( ),
 doTrace( ).

 However, the GET and POST requests are commonly used when
handling form input.
doGet() vs doPost()
Program
import java.io.*;
import javax.servlet.*;
import javax.servlet.http.*;
public class MyHttpServlet extends HttpServlet
{
public void doGet(HttpServletRequest req, HttpServletResponse res)
{
try{
res.setContentType("text/html");
PrintWriter pw=res.getWriter();
pw.println("<B>This is HTTP Servlet");
pw.close();
}catch(Exception e){}
}
}
Output

Deployment Descriptor written in

web.xml
Cookie Example: 1 File
<html>
<body>
<center>
<form name="Form1"
method="post"
action="http://localhost:8080/examples/servlet/AddCookieServlet">
<B>Enter a value for MyCookie:</B>
<input type=textbox name="data" size=25 value="">
<input type=submit value="Submit">
</form>
</body>
</html>
Cookie Example: 2 File
import java.io.*;
import javax.servlet.*;
import javax.servlet.http.*;
public class AddCookieServlet extends HttpServlet {
public void doPost(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException
{
// Get parameter from HTTP request.
String data = request.getParameter("data");
// Create cookie.
Cookie cookie = new Cookie("MyCookie", data);
// Add cookie to HTTP response.
response.addCookie(cookie);
// Write output to browser.
response.setContentType("text/html");
PrintWriter pw = response.getWriter();
pw.println("<B>MyCookie has been set to");
pw.println(data);
pw.close();
}
}
Cookie Example: 3 File
import java.io.*;
import javax.servlet.*;
import javax.servlet.http.*;
public class GetCookiesServlet extends HttpServlet {
public void doGet(HttpServletRequest request,
HttpServletResponse response)
throws ServletException, IOException {
// Get cookies from header of HTTP request.
Cookie[] cookies = request.getCookies();
// Display these cookies.
response.setContentType("text/html");
PrintWriter pw = response.getWriter();
pw.println("<B>");
for(int i = 0; i < cookies.length; i++) {
String name = cookies[i].getName();
String value = cookies[i].getValue();
pw.println("name = " + name +
"; value = " + value);
}
pw.close();
}
}
Session Tracking
 HTTP is a stateless protocol.

 Each request is independent of the previous one.

 However, in some applications, it is necessary to save state

information so that information can be collected from

several interactions between a browser and a server.

 Sessions provide such a mechanism.

Session Tracking
 A session can be created via the getSession() method of
HttpServletRequest. An HttpSession object is returned.
 This object can store a set of bindings that associate names
with objects.
 The setAttribute(), getAttribute(), getAttributeNames(), and
removeAttribute() methods of HttpSession manage these
bindings.
 It is important to note that session state is shared among all
the servlets that are associated with a particular client.
Program
import java.io.*;
import java.util.*;
import javax.servlet.*;
import javax.servlet.http.*;
public class DateServlet extends HttpServlet {
public void doGet(HttpServletRequest request, HttpServletResponse response) throws
ServletException, IOException
// Get the HttpSession object.
HttpSession hs = request.getSession(true);
// Get writer.
response.setContentType("text/html");
PrintWriter pw = response.getWriter();
pw.print("<B>");
// Display date/time of last access.
Date date = (Date)hs.getAttribute("date");
if(date != null) {
pw.print("Last access: " + date + "<br>");
}
// Display current date/time.
date = new Date();
hs.setAttribute("date", date);
pw.println("Current date: " + date);
}
}
Example
 The getSession( ) method gets the current session. A new
session is created if one does not already exist.
 The getAttribute( ) method is called to obtain the object that
is bound to the name “date”. That object is a Date object
that encapsulates the date and time when this page was last
accessed. A Date object encapsulating the current date and
time is then created.
 The setAttribute( ) method is called to bind the name “date”
to this object.
Referenc
es
 Java The Complete Reference By Herbert
Schildt
 Images from Internet