Introduction to Java Servlets

Last Updated : 1 Sep, 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 client sends an HTTP request.
The web server or Servlet container receives the request.
The container identifies the servlet mapped to the requested URL.
The container invokes the appropriate servlet method, such as doGet() or doPost().
The servlet processes the request and generates a response.
The container sends the response back to the client.

Need of Server-Side Extensions

Server-side technologies are used when a web application needs to process requests on the server and generate dynamic responses.

They allow applications to process user requests.
They can interact with databases and other services.
They can generate dynamic HTML or other HTTP responses.
They allow business logic to be executed on the server.
Java Servlets provide a standard server-side API for Java web applications.

Servlet Container

A Servlet container, also called a Servlet engine, provides the runtime environment for servlets. It manages servlet lifecycle, request processing, response generation, sessions, security, and other web application services.

Apache Tomcat is a commonly used Servlet container.

Services Provided by a Servlet Container

Servlet lifecycle management: Loads, initializes, executes, and destroys servlets.
Request and response handling: Creates request and response objects and passes them to the servlet.
URL mapping: Maps incoming URLs to the appropriate servlet.
Session management: Provides APIs for maintaining client sessions, commonly through HttpSession.
Security: Supports authentication, authorization, and security constraints.
Resource management: Manages web application resources and servlet components.
Concurrency management: Handles multiple requests and invokes servlets as required.

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 17 or later(Java Development Kit)
Install Apache Tomcat server 10.1
Install an IDE such as Eclipse or IntelliJ IDEA.
Use the Jakarta Servlet API corresponding to the selected Tomcat version.

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.

Step 3: Run the Application

Start the Tomcat server and open:

http://localhost:8080/HelloWorldServlet/hello

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 Use Java Servlet over other Technologies?

Servlets provide a standard API for handling HTTP requests and responses in Java web applications.

They provide a standard server-side programming model.
The Servlet container manages their lifecycle.
They support HTTP-specific request handling.
They can maintain sessions using HttpSession.
They can work with databases and other backend services.
They can be integrated with filters, listeners, JSP, and other Jakarta EE technologies.

Java Servlets Vs 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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