Servlets
Introduction
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Agenda
Introduction to Servlets
Deploying a Simple Servlet
3 Servlet Life Cycle
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Objectives
At the end of this module, you will be able to:
Describe the role of HTTP Servlet in Web Programming
Describe and use the Servlet Life Cycle methods appropriately
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Introduction to Servlets
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Server-side Programming
Static HTTP transaction - Browser requests for index.html, and Server responds with HTML
file
index.html
1. Browser Request 2. Server Finds File
Reads the File
<html> ….
</html>
4. Browser Displays Page 3. Server Response
HTTP (Hyper Text Transport Protocol) is the protocol
that clients and servers use on the web to communicate
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Server-side Programming (Contd.).
Dynamic HTTP transaction - Browser requests OrderServlet.class, server runs program that
creates HTML, server returns HTML to browser
2. ExecutesWeb
server
Server program
.php /
.cgi 3. May communicate with
1. Browser Request
Database
4. Creates HTML
<html> ….
</html> 5. Server returns HTML
6. Browser Displays Page
5 Response
3
Database
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Web Server
A computer having server software installed within it which serves up web pages
A program that uses client/ server model and World Wide Web’s Hypertext Transfer
Protocol (HTTP)
Responsible for accepting HTTP requests from clients (web browsers) and serving HTTP
responses which are web pages such as HTML documents
Popular web servers
Apache HTTP Server (Apache)
Microsoft Internet Information Server (IIS)
Sun Java System Web Server
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Java server-side web components
A web component is a software entity that runs on a web server
Provides it with the capabilities needed for dynamically handling client requests and
generating web presentation content
The J2EE specification defines two types of web components
Servlets
Java Server Pages(JSPs)
Database
Application Client EJB
Web Client JSP / Servlets EJB Database
Client Tier Web Tier Business Tier EIS Tier
Client Environment J2EE Server Database Server
J2EE Application N-Tiered Architecture
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What are Servlets?
A Java class that runs on a web server and dynamically handles client requests
It extends the functionality of a web server by receiving client requests and dynamically
generating a response
Since servlets are Java-based, they are platform independent
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Uses of Servlets
Processing and/or storing data submitted by an HTML form
Example: Processing data of a login form
Providing dynamic content
Example: Returning results of a database query to the client
Managing state information on top of the stateless HTTP
Example: For an online shopping cart system which manages shopping carts for many
concurrent customers and maps every request to the right customer
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Servlet Architecture Overview
Java Servlet API are in packages javax.servlet
and javax.servlet.http
Provide interfaces and classes for writing Servlet interface
servlets
All servlets must implement Servlet interface GenericServlet class
Defines life-cycle methods
Extend GenericServlet class HttpServlet class
To implement generic services
LoginServlet
Extend HttpServlet class (a user-defined servlet)
To implement HTTP-specific services
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Deploying a Simple
Servlet
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Steps for writing and deploying simple servlet
1. Set environment variables: JAVA_HOME, CATALINA_HOME, CLASSPATH, and PATH
2. Create a directory, say “MyWebApplication” in C:\Program Files\Apache Software
Foundation\Tomcat 5.0\webapps
3. Compile your servlet say “WelcomeServlet.java” in C:\Program Files\Apache Software
Foundation\Tomcat5.0\webapps\MyWebApp\WEB-INF\classes directory
4. Create web.xml file and place it in C:\Program Files\Apache Software Foundation\Tomcat
5.0\webapps\MyWebApplication\WEB-INF folder
5. Start Tomcat server by double-clicking C:\Program Files\Apache Software Foundation\Tomcat
5.0\bin\startup.batch file OR by selecting Start -> Programs -> Apache Tomcat 5.0 -> Monitor
Tomcat
6. Open browser with URL: http://localhost:10000/ to check Tomcat is successfully installed
7. Give URL as http://localhost:10000/MyWebApplication/WelcomeServlet to test your servlet
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Directory Structure of a Web application
MyWebApplication
WEB-INF
jsp files html files
classes lib tags
web.xml
*.tld
All server-side Library All .tag files
.class files Archive files
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Demo for a Simple Servlet
A simple HTTP Servlet that displays a Welcome message on the web page
Files required:
WelcomeServlet.java
web.xml
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Demo for a Simple Servlet (Contd.).
An HTTP Servlet that displays a Welcome message
import java.io.*;
import javax.servlet.*;
import javax.servlet.http.*;
public class WelcomeServlet extends HttpServlet {
public void doGet(HttpServletRequest req,HttpServletResponse res)
throws ServletException, IOException {
res.setContentType("text/html"); // set header field first
PrintWriter pw = res.getWriter(); // then get writer & write response data
pw.println("<HTML>");
pw.println("<HEAD><TITLE>Welcome</TITLE></HEAD>");
pw.println("<BODY>");
pw.println("<H3>” + “Welcome to Java Servlet Technology!!” + "</H3>");
pw.println("</BODY>");
pw.println("</HTML>");
pw.close(); //closes the writer
}
}
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The Web Deployment Descriptor – web.xml
An XML file web.xml is a deployment descriptor that describes
mapping from URIs to application resources
initialization parameters
security constraints
registration of listeners and filters
Example: web.xml
<web-app>
<display-name>A Small Web Application</display-name>
<servlet>
<servlet-name>MyFirstServlet</servlet-name>
<servlet-class>WelcomeServlet</servlet-class>
</servlet>
<servlet-mapping>
<servlet-name>MyFirstServlet</servlet-name>
<url-pattern>/welcome.msg</url-pattern>
</servlet-mapping>
</web-app>
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Web Container
Web components and their container run on J2EE server
Provides execution environment for servlets and JSPs of a web application
Manages execution of JSP and servlet components for J2EE applications
Fig: J2EE Server and Container Types
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Role of a Web Container
Communication Support
Provides an easy way for servlets to talk to web server
Lifecycle Management
Controls lifecycle of servlets
Multithreading Support
Automatically creates a new Java thread for every servlet request it receives
Declarative Security
Enables to configure security in an XML deployment descriptor thereby avoiding hard-coding it in
servlet or any other class code
JSP Support
Does JSP processing
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How web container handles Servlet requests
Container creates 2 objects on receiving a request for a servlet: HttpServletRequest and
HttpServletResponse
Finds right servlet based on URL in the request
Creates a thread for that request
Passes request and response objects to the servlet thread
Calls servlet’s service() method
The service() method in turn calls doGet or doPost based on type of request (Assume request
was an HTTP GET)
The doGet method generates dynamic page and captures it in response object
Converts response object into an HTTP response on completion of thread
Sends this response to the client
Finally deletes the request and response objects
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Knowledge Checkpoint
1. Suppose you are a web developer working for an Online Movie Service. You want to use a
servlet called MovieServlet so that clients can access the latest film shows for the day in a
particular city from your movie database. Determine the correct sequence of following steps
carried out by MovieServlet when processing a request from a client
Sl.No Description
1 Check information included in the Http request
2 Access any necessary business components or data storage
3 Set the appropriate Http response parameters
4 Read data submitted by the client
5 Send the response to the client
6 Format the results in a response
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Servlet Life Cycle
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Life Cycle of a Servlet
An HTTP servlet's life cycle is controlled by the web container where it is deployed
Source: http://www.iam.ubc.ca/guides/javatut99/servlets/lifecycle/index.html
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Servlet interface
Provides methods that manage the servlet and its communications with clients
init(ServletConfig)
Initializes the servlet. Runs once before any requests can be serviced
service(ServletRequest, ServletResponse)
Processes a single request from the client
destroy()
This method releases all the resources
getServletConfig()
Returns a servlet config object and this object contains any initialization parameters and
startup configuration information for this servlet
getServletInfo()
Returns a string containing information about the servlet, such as its author, version, and
copyright
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Lifecycle Methods
Interaction between a web server, a servlet, and a client is controlled using the life-cycle
methods
A servlet's life cycle methods are
init()
service()
destroy()
The init() and destroy() methods will be called only once during the life time of your Servlet
The service() and it's broken down methods ( doGet(), doPost() etc ) will be called as many
times as requests are received for them by the web container
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Initializing a servlet
Web container initializes servlet after web container loads and instantiates servlet class and before it
delivers requests from clients
The init method is invoked only once during servlet's lifetime – when servlet is first created
Override init method of Servlet interface if you want the servlet to
Read persistent configuration data
Initialize resources
Perform any other one-time activities
Two versions of init method – one that takes no arguments and one that takes a ServletConfig object
as an argument
init( ) - use this when your servlet does not need any specific initialization
init(ServletConfig) - use this when your servlet needs to check specific settings before completing
initialization
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Servicing client requests
Once servlet is loaded and initialized, the servlet is able to handle client requests
Web container processes the requests in servlet’s service method
Every time the server receives an incoming request for a servlet, it generates a new thread and calls the
service method
The service method then checks the HTTP request type and calls the appropriate doXXX method
Syntax: public void service(ServletRequest req, ServletResponse res)
Role of service method
To extract information from the request
Access external resources
Populate the response based on that information
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Destroying a Servlet
Server may unload a servlet instance
If the servlet has been idle for some time (default is 30 minutes) or
If the web application is undeployed or
If server shuts down
Before it removes the servlet, it calls destroy method only once
Before servlets get destroyed, this method helps in
Cleanup operations such as closing database connections
Halting background threads
Releasing other resources such as IO streams
Syntax: public void destroy()
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Summary
In this module, you were able to:
Describe the role of HTTP Servlet in Web Programming
Describe and use the Servlet Life Cycle methods appropriately
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Quiz
1. The doGet() or doPost() method of a Servlet are invoked by --------------------
a) init() method
b) service() method
c) destroy() method
--------------- is the deployment descriptor file for Servlets
a) servlet-config.xml
b) web.xml
c) struts-config.xml
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References
1. TutorialsPoint.COM (2012). Servlets Tutorial. Retrieved April 1, 2012 from
http://www.tutorialspoint.com/servlets/index.htm
2. Stephanie Bodoff. Oracle Sun Developer Network: JavaServer Pages Technology. Retrieved April 1,
2012, from, http://java.sun.com/j2ee/tutorial/1_3-fcs/doc/Servlets.html
3. SkillSoft (2003).Developing a basic HTTP servlet. Retrieved April 10, 2012, from,
http://www.alc.amadeus.com/content/public/alw/skillsoft/cbtlib/73468/73473/eng/thin/transcript.html
4. Saravanan Sivaji (2009). Developing a Basic Servlet Program. Retrieved April 10, 2012,
from,http://saravananmtech.wordpress.co m/2009/04/
5. Javatut99 (1999). Lifecycle of a Servlet. Retrieved April 10, 2012, from,
http://www.iam.ubc.ca/guides/javatut99/servlets/lifecycle/index.html
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Thank You
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