Servlet 01

Introduction to Servlets

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Objectives

In this lesson, you will learn to:

Identify the characteristics of a Servlet

Create a Servlet

Deploy a Servlet

Introduction to Servlets

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Need for Servlets

Servlets are required to:

Reduce the overhead on the server and network

To take care of processing data on the Web server

Introduction to Servlets

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Servlets are:

Java programs that can be deployed on a Java enabled Web server

Used to extend the functionality of a Web server

Used to add dynamic content to Web pages

Introduction to Servlets

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Characteristics of Servlets

Servlets are:

Efficient

Initialization code for a servlet is executed only once

Robust

Provide all powerful features of Java

Introduction to Servlets

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Portable across Web servers

Persistent

Increase the performance of a system by preventing frequent disk access

Introduction to Servlets

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Comparison between Servlets and Applets

Applets:

Are embedded in Web pages

Require the browser to be Java-enabled

Take a lot of time to be downloaded

Introduction to Servlets

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Servlets:

Execute on the Web server, thus help overcome problems with download time

Do not require the browser to be Java-enabled

Introduction to Servlets

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Comparison between Servlets and other Server-Side Scripting Technologies

Common Gateway Interface (CGI) scripts, JSP, and ASP are alternatives to servlets

CGI Scripts:

Are programs written in C, C++, or Perl

Get executed in a server

Run in separate processes for each client access

Require the interpreter to be loaded on the server

A JSP file is automatically converted to a servlet before it is executed

Introduction to Servlets

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Comparison between Servlets and other Server- Side Scriptings

Active Server Pages (ASP):

Is a server-side scripting language developed by Microsoft

Enables a developer to combine HTML and a scripting language in the same Web page

Are not compatible with all Web servers

Introduction to Servlets

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Working of Servlets

Client browser passes requests to a servlet using the following methods:

GET

1.Uses a query string to send additional information to the server

2.Query string is displayed in the client browser

Introduction to Servlets

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POST

1.Sends the data as packets to the server through a separate socket connection

2.Complete transaction is invisible to the client

3.Slower compared to the GET method

Introduction to Servlets

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The javax.servlet Package

Hierarchy of classes that are used to create a servlet

Class java.lang.Object

Class javax.servlet.GenericServlet

Class javax.servlet.HttpServlet

Interface javax.servlet.Servlet

Interface javax.servlet.ServletConfig

Interfacejavax.io.Serializable

Introduction to Servlets

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The javax.servlet Package (Contd.)

A brief description of the classes and interfaces

Class/Interface Description

HTTPServlet class

Provides a HTTP specific implementation of the Servlet interface.

HTTPServletRequest interface

Provides methods to process requests from the clients.

Introduction to Servlets

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HTTPServlet Response interface

Response to the client is sent in the form of a HTML page through an object of the HTTPServletResponse class.

ServletConfig class

Used to store the servlets startup configuration values and the initialization parameters.

Introduction to Servlets

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Life Cycle of a Servlet

Life cycle of a servlet:

Client (Browser)

init()

service()

destroy()

Request

Response

Introduction to Servlets

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Life Cycle of a Servlet (Contd.)

The following table describes few methods that are used in creating a servlet:

Method name Functionality

Servlet.init (ServletConfig config) throws ServletException

Contains all initialization code for the servlet.

Servlet. service()

Receives all requests from clients, identifies the type of the request, and dispatches them to the doGet() or doPost() methods for processing.

Introduction to Servlets

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Life Cycle of a Servlet (Contd.)

Method name Functionality

Servlet. destroy()

Executes only once when the servlet is removed from server.

HTTPServlet Response. getWriter()

Returns a reference to a PrintWriter object.

HTTPServlet Response. setContentType (String type)

Sets the type of content that is to be sent as response to the client browser.

Introduction to Servlets

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Deploying a Servlet

A servlet can be deployed in:

Java Web Server (JWS)

JRun

Apache

Java 2 Enterprise Edition (J2EE) server

Introduction to Servlets

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Identify the mechanism to record thenumber of hits on a Web site

Client browsers that are used to access the Web site run on different machines

Hence, the client (browser) cannot keep track of the hit count data

This data has to be captured on the server’s side

Thus, a technique that can be used to write a server-side program is necessary to solve this problem

Introduction to Servlets

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Identify the classes to be used

A class that extends the HttpServlet class must be coded to solve the problem

Introduction to Servlets

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Identify the methods to be used

The init() method needs to be coded in the hitcountServlet class to initialize the hitcounter to zero

The doGet() method needs to be coded to increment the hit counter whenever a client browser makes a request for the site home page

Introduction to Servlets

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Write and compile the servlet

Set the values of PATH and CLASSPATH variables

Code the servlet program

Compile the program by using the command javac hitcountServlet.java

Introduction to Servlets

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Execute the servlet

Type http://127.0.0.1:8000/hitcountwebcontext/hitcounter in the address bar of the browser to execute the servlet

Introduction to Servlets

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Problem for you….

In the URL that is provided, identify the Web context name and the alias name of the servlet

http://127.0.0.1:8000/name/execute

Introduction to Servlets

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Summary

In this lesson, you learned that:

Servlets are server-side Java programs that can be deployed on a Web Server

The Servlet interface provides the basic framework for coding servlets

Introduction to Servlets

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Servlets are :

Portable

Extensible

Persistent

Robust

Introduction to Servlets

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Summary (Contd.)

The life cycle of a servlet is composed of the init(), service(), and the destroy() methods

Servlets can be deployed in the J2EE server and JWS

The deploytool command is used to deploy a servlet on a J2EE server