Unit 3: Web Application development using JSP & Servlets

Servlet Overview

A servlet is a server-side Java program used to create dynamic web applications. It runs on a web server or application server and handles requests from clients (browsers) and sends responses back.

Why Servlets are Used

• To process user requests
• To generate dynamic web content
• To handle form data
• To interact with databases
• To build secure and scalable web applications

Advantages of Servlets

Advantage	Description
Platform independent	Runs on JVM
Efficient	Uses multithreading
Secure	Server-side execution
Persistent	Loaded once, reused

Servlet Architecture

Servlet architecture explains how a servlet works inside a web server.

Main Components

1. Client (Browser) – Sends request
2. Web Server – Receives request
3. Servlet Container – Manages servlet lifecycle
4. Servlet – Processes request
5. Response – Sent back to client

Request–Response Flow

1. Client sends HTTP request
2. Web server forwards request to servlet container
3. Container loads servlet (if not loaded)
4. Servlet processes request
5. Response is sent back to browser

Diagram Description (Exam Writing)

Client → Web Server → Servlet Container → Servlet
Client ← Response ← Servlet Container ← Servlet

Servlet Interface

The Servlet interface is part of javax.servlet package.
All servlets must implement this interface directly or indirectly.

Methods of Servlet Interface

Method	Purpose
init()	Initializes servlet
service()	Handles requests
destroy()	Cleans up resources
getServletConfig()	Returns configuration
getServletInfo()	Returns information

Example

public class DemoServlet implements Servlet {
    public void init(ServletConfig sc) {}
    public void service(ServletRequest req, ServletResponse res) {}
    public void destroy() {}
    public ServletConfig getServletConfig() { return null; }
    public String getServletInfo() { return null; }
}

In practice, servlets usually extend HttpServlet instead of implementing Servlet directly.

Servlet Life Cycle

The Servlet Life Cycle describes the stages a servlet goes through from creation to destruction.

Life Cycle Phases

1. Loading and Instantiation
2. Initialization (init())
3. Request Handling (service())
4. Destruction (destroy())

Life Cycle Diagram (Exam Format)

Load → init() → service() → destroy()

Explanation

Phase	Description
init()	Executed once
service()	Executed for each request
destroy()	Executed once before removal

Handling HTTP GET Requests

The GET method is used to request data from the server.
Data is sent through the URL.

Characteristics of GET

• Data visible in URL
• Limited data size
• Less secure
• Used for fetching data

Method Used

doGet(HttpServletRequest request, HttpServletResponse response)

Example

protected void doGet(HttpServletRequest request, HttpServletResponse response)
throws IOException {
    response.setContentType("text/html");
    PrintWriter out = response.getWriter();
    out.println("This is GET request");
}

Handling HTTP POST Requests

The POST method is used to send data securely to the server.

Characteristics of POST

• Data not visible in URL
• No size limitation
• More secure
• Used for form submission

Method Used

doPost(HttpServletRequest request, HttpServletResponse response)

Example

protected void doPost(HttpServletRequest request, HttpServletResponse response)
throws IOException {
    String name = request.getParameter("username");
    response.getWriter().println("Welcome " + name);
}

GET vs POST Comparison

Feature	GET	POST
Data visibility	Visible in URL	Hidden
Security	Less	More
Data size	Limited	Unlimited
Usage	Fetch data	Submit data

Summary Table

Topic	Key Point
Servlet	Server-side Java
Architecture	Client–Server model
Life Cycle	init → service → destroy
GET	Retrieve data
POST	Submit data

Exam Tips

• Write servlet life cycle steps clearly
• Always mention HttpServlet
• Draw request–response flow diagram
• Compare GET and POST in table form

Redirecting Requests to Other Resources

Redirecting means sending a client request from one resource to another (Servlet, JSP, or HTML page).

Types of Redirection

1. Request Dispatching (Forwarding)

• Done on the server side
• URL in browser does not change

Method Used

RequestDispatcher rd = request.getRequestDispatcher("welcome.jsp");
rd.forward(request, response);

2. Response Redirection

• Done on the client side
• URL changes

Method Used

response.sendRedirect("login.jsp");

Comparison Table

Forward	Redirect
Server side	Client side
URL unchanged	URL changes
Faster	Slower

Session Tracking

Session Tracking is used to maintain user information across multiple requests.

Why Session Tracking is Needed

• HTTP is a stateless protocol
• Server does not remember previous requests

Session Tracking Techniques

Technique	Description
Cookies	Stored on client
URL Rewriting	Data in URL
Hidden Fields	Hidden form data
HttpSession	Server-side session

Cookies

A cookie is a small piece of data stored on the client’s browser.

Types of Cookies

• Persistent cookies
• Non-persistent cookies

Creating a Cookie

Cookie c = new Cookie("username", "Jay");
response.addCookie(c);

Reading a Cookie

Cookie[] cookies = request.getCookies();

Advantages and Limitations

Advantages	Limitations
Simple	Less secure
Saves user preferences	Limited size

Session Tracking with HttpSession

HttpSession stores user data on the server side.

Creating a Session

HttpSession session = request.getSession();
session.setAttribute("user", "Jay");

Retrieving Session Data

String user = (String) session.getAttribute("user");

Invalidating Session

session.invalidate();

Advantages

• More secure
• Can store complex objects
• Widely used in real applications

Introduction to Java Server Pages (JSP)

JSP is a server-side technology used to create dynamic web pages using HTML and Java code.

JSP Features

• Easier than Servlets
• HTML-centric
• Platform independent
• Supports MVC architecture

Java Server Pages Overview

How JSP Works

1. Browser sends request
2. JSP is translated into Servlet
3. Servlet is compiled
4. Response sent to browser

JSP Architecture Flow

Client → JSP → Servlet → Response

A First Java Server Page Example

Example: Hello JSP

<html>
<body>
<h2>Welcome to JSP</h2>
<% out.println("Hello User"); %>
</body>
</html>

Explanation

• <% %> → Scriptlet tag
• out → Implicit object

Implicit Objects in JSP

Implicit objects are predefined objects available in JSP.

List of Implicit Objects

Object	Purpose
request	Client request
response	Server response
out	Output stream
session	Session data
application	Application context
config	Servlet config
page	JSP page
pageContext	Context info
exception	Error handling

Scripting in JSP

JSP scripting elements allow Java code in JSP.

Types of Scripting Elements

Element	Syntax	Use
Scriptlet	<% %>	Java code
Expression	<%= %>	Output
Declaration	<%! %>	Variables, methods

Standard Actions in JSP

Standard actions are predefined JSP tags.

Common JSP Standard Actions

Action	Purpose
<jsp:include>	Include resource
<jsp:forward>	Forward request
<jsp:useBean>	Create bean
<jsp:setProperty>	Set bean
<jsp:getProperty>	Get bean

Directives in JSP

Directives provide instructions to the JSP container.

Types of Directives

Directive	Purpose
page	Page-level settings
include	Include files
taglib	Custom tags

Example

<%@ page language="java" %>

Custom Tag Libraries

Custom tags allow developers to create their own JSP tags.

Advantages

• Reduces Java code in JSP
• Improves readability
• Encourages reusability

Components

• Tag handler class
• TLD file

Example Usage

<mytag:welcome />

Summary Table

Topic	Key Concept
Cookies	Client-side tracking
HttpSession	Server-side tracking
JSP	Dynamic pages
Implicit Objects	Predefined objects
Custom Tags	Reusable logic

Exam Tips

• Always mention HTTP is stateless
• Compare Cookies and HttpSession
• Write JSP lifecycle steps
• Use tables and diagrams