Introduction to Java Servlets

Last Updated : 24 Apr, 2026

Java Servlet is a Java program that runs on a Java-enabled web server or application server. It handles client requests, processes them and generates responses dynamically. Servlets are the backbone of many server-side Java applications due to their efficiency and scalability.

Work on the server-side to manage request-response lifecycle.
Capable of handling multiple client requests efficiently.
Suitable for building robust and scalable enterprise applications.

Servlets Architecture

Servlet architecture defines how client requests are processed by the server using servlets. It follows a request-response model where the web container manages execution.

Jsp-servlet-architecture — Servlet Architecture

Servlet Architecture Workflow

Execution of Servlets basically involves Six basic steps:

The Clients send the request to the Web Server.
The Web Server receives the request.
The Web Server passes the request to the corresponding servlet.
The Servlet processes the request and generates the response in the form of output.
The Servlet sends the response back to the webserver.
The Web Server sends the response back to the client and the client browser displays it on the screen.

Need of Server-Side Extensions

Server-side extensions allow dynamic web page generation by running programs on the server.
Web servers provide APIs to help developers build these applications.
Java Servlets (part of Jakarta EE) are a key API for Java-based web development.

Servlet Container

Servlet container, also known as Servlet engine, is an integrated set of objects that provide a run time environment for Java Servlet components. It is a system that manages Java Servlet components on top of the Web server to handle the Web client requests.

Services provided by the Servlet container:

Network Services: Loads a Servlet class. The loading may be from a local file system, a remote file system or other network services. The Servlet container provides the network services over which the request and response are sent.
Decode and Encode MIME-based messages: Provides the service of decoding and encoding MIME-based messages.
Manage Servlet container: Manages the lifecycle of a Servlet.
Resource management: Manages the static and dynamic resources, such as HTML files, Servlets and JSP pages.
Security Service: Handles authorization and authentication of resource access.
Session Management: Maintains a session by appending a session ID to the URL path

Steps to implementation of creating a basic servlet program

Below are the basic steps to create and run a simple servlet program in Java

Prerequisites

Install JDK (Java Development Kit)
Install Apache Tomcat server (e.g., version 9 or 10)
Install an IDE like Eclipse or IntelliJ IDEA
Download the Servlet API JAR (already included in Tomcat)

Step 1: Create a Dynamic Web Project (in Eclipse)

Open Eclipse -> File ->New ->Dynamic Web Project
Name the project (e.g., HelloWorldServlet)
Target runtime -> Select Apache Tomcat
Click Finish

Step 2: Create Servlet class

Right-click on src -> New-> Servlet
Name it HelloWorldServlet and click Finish

HelloWorldServlet.java

Java

import java.io.*;
import jakarta.servlet.*;
import jakarta.servlet.http.*;

public class HelloWorldServlet extends HttpServlet {
    protected void doGet(HttpServletRequest request, HttpServletResponse response) 
    throws ServletException, IOException {
        response.setContentType("text/html");
        PrintWriter out = response.getWriter();
        out.println("<html><body><h1>Hello, World!</h1></body></html>");
    }
}

Explanation: This servlet handles a GET request and sends an HTML response to the client. It sets the response type to text/html, gets the PrintWriter, and prints a simple “Hello, World!” message inside HTML tags.

Configuring a Servlet

To deploy a servlet, you need to configure it in the web.xml file. This file maps URLs to servlets. For example,

XML-Based Configuration (web.xml):

XML

<web-app xmlns="http://www.oracle.com/webfolder/technetwork/jsc/xml/ns/javaee/index.html" 
         xmlns:xsi="https://www.w3.org/2001/XMLSchema-instance" 
         xsi:schemaLocation="http://www.oracle.com/webfolder/technetwork/jsc/xml/ns/javaee/index.html 
         http://www.oracle.com/webfolder/technetwork/jsc/xml/ns/javaee/index.html/web-app_3_0.xsd" 
         version="3.0">

    <servlet>
        <servlet-name>HelloWorldServlet</servlet-name>
        <servlet-class>HelloWorldServlet</servlet-class>
    </servlet>

    <servlet-mapping>
        <servlet-name>HelloWorldServlet</servlet-name>
        <url-pattern>/hello</url-pattern>
    </servlet-mapping>

</web-app>

Explanation: This is a web.xml file used for mapping URLs to servlets. So, when you visit http://localhost:8080/yourApp/hello, the servlet runs and shows "Hello, World!" in the browser.

Annotation-Based Configuration (Modern Approach)

From Servlet 3.0, servlet configuration can also be done using annotations. Instead of using web.xml, we can configure the servlet using the @WebServlet annotations.

Java

@WebServlet("/hello") 
public class HelloWorldServlet extends HttpServlet {
    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        response.setContentType("text/html");
        PrintWriter out = response.getWriter();
        out.println("<html><body><h1>Hello, World!</h1></body></html>");
    }
}

Explanation: Here we have used the @WebServlet("/hello") annotation to register the servlet directly in code (no need for web.xml). It maps the servlet to the URL /hello.

Why Choose Java Servlet over other Technologies?

Dynamic web content requires server-side technologies. While there are many options, Java Servlet stand out due to their advantages over alternatives like Common Gateway Interface (CGI)

Limitations of CGI:

Process Overhead: CGI creates and destroys a process for every client request, leading to high resource consumption.
Scalability Issues: Poor performance with increased client requests.

Difference Between Java Servlets and CGI

The table below demonstrates the difference between servlet and CGI

Servlet	CGI (Common Gateway Interface)
Servlets are portable and efficient.	CGI is not portable.
In Servlets, sharing data is possible.	In CGI, sharing data is not possible.
Servlets can directly communicate with the webserver.	CGI cannot directly communicate with the webserver.
Servlets are less expensive than CGI.	CGI is more expensive than Servlets.
Servlets can handle the cookies.	CGI cannot handle the cookies.

Key Classes and Interfaces

Various classes and interfaces present in these packages are:

Component	Type	Package
Servlet	Interface	jakarta.servlet.*
ServletRequest	Interface	jakarta.servlet.*
ServletResponse	Interface	jakarta.servlet.*
GenericServlet	Class	jakarta.servlet.*
HttpServlet	Class	jakarta.servlet.http.*
HttpServletRequest	Interface	jakarta.servlet.http.*
HttpServletResponse	Interface	jakarta.servlet.http.*
Filter	Interface	jakarta.servlet.*
ServletConfig	Interface	jakarta.servlet.*

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