What Is a Servlet in Java? Beginner’s Guide to Java Web Development

Introduction

When you browse a website, you send requests to a server, and the server sends responses back to your browser.
In Java web development, this communication is handled using Servlets.

If you are new to Java web technologies, don’t worry—this guide starts from zero, explains everything in simple language, and gradually moves toward technical concepts with clear examples.

What Is a Servlet?

Definition:

A Servlet is a Java program that runs on a web server and handles client requests (usually from a web browser) and returns responses.

In simple words:

A Servlet acts as a bridge between a user’s browser and Java code running on a server.

Why Are Servlets Used in Web Applications?

Before Servlets, Java programs were mostly standalone applications. But web applications need to:

Receive data from users (forms, URLs)
Process that data using business logic
Generate dynamic responses (HTML, JSON, etc.)

Servlets are used because they:

Handle HTTP requests and responses
Generate dynamic web content
Are platform-independent
Are secure, fast, and scalable
Integrate well with Java frameworks like JSP, Spring, Hibernate

Almost every Java web framework internally uses Servlets.

Where Do Servlets Fit in Java Web Architecture?

Java Web Application Architecture

Browser (Client)
     ↓
 Web Server (Tomcat)
     ↓
   Servlet
     ↓
 Business Logic (Java Classes)
     ↓
 Database

Role of Servlets:

Act as the controller
Receive user input
Decide what logic to execute
Send the response back to the browser

This follows the MVC (Model–View–Controller) design pattern.

How Do Servlets Work? (Request–Response Lifecycle)

Servlets follow a request–response model.

Step-by-Step Lifecycle:

User enters a URL or submits a form
Browser sends an HTTP request
Web server forwards the request to the Servlet
Servlet processes the request
Servlet generates a response
Server sends the response back to the browser

Text-Based Diagram Explanation

User Browser
     |
     | HTTP Request
     ↓
 Web Server (Tomcat)
     |
     | Calls Servlet
     ↓
   Servlet
     |
     | Processes Data
     ↓
 HTTP Response (HTML/Text)
     |
     ↓
User Browser

Real-Life Analogy

Think of a Restaurant

Customer → User (Browser)
Waiter → Servlet
Kitchen → Business Logic
Food → Response

The customer never goes into the kitchen.
The waiter (Servlet) takes the order, talks to the kitchen, and delivers food.

Servlet = Waiter of a web application

Basic Servlet Flow

Client sends request
Servlet receives request
Servlet processes request
Servlet sends response

This flow is the core foundation of Java web development.

Beginner-Friendly Servlet Code Example

Step 1: Import Required Packages

import java.io.IOException;
import java.io.PrintWriter;
import jakarta.servlet.ServletException;
import jakarta.servlet.http.HttpServlet;
import jakarta.servlet.http.HttpServletRequest;
import jakarta.servlet.http.HttpServletResponse;

Step 2: Create a Servlet Class

public class HelloServlet extends HttpServlet {

    protected void doGet(HttpServletRequest request,
                         HttpServletResponse response)
                         throws ServletException, IOException {

        response.setContentType("text/html");
        PrintWriter out = response.getWriter();

        out.println("<h1>Hello, Welcome to Java Servlets!</h1>");
    }
}

Step-by-Step Explanation

HttpServlet → Base class for all Servlets
doGet() → Handles GET requests
HttpServletRequest → Contains request data
HttpServletResponse → Used to send response
PrintWriter → Writes output to browser

When the browser hits this servlet URL, it prints a message on the screen.

Servlet Container

Servlets do not run alone. They need a Servlet Container like:

Apache Tomcat
Jetty
GlassFish

What the Container Does:

Loads the servlet
Manages lifecycle
Handles threading
Provides security
Manages request/response objects

You never manually create or destroy servlets—the container handles it.

Best Practices for Beginners

Keep Servlets small and focused
Use Servlets for control logic only
Use JSP for UI (View layer)
Follow MVC architecture
Handle exceptions properly
Use meaningful class names