Introduction to Java Servlets

Java Servlets form the backbone of many enterprise-level web applications. They are a powerful server-side technology used to create dynamic, secure, and scalable web applications in Java. This article provides a comprehensive introduction to Java Servlets, covering core concepts, architecture, lifecycle, real-world use cases, and practical code examples in a beginner-to-intermediate friendly manner.

What is a Java Servlet?

A Java Servlet is a server-side Java program that handles client requests and generates dynamic responses, usually in the form of HTML. Servlets run inside a web container (also known as a servlet container) such as Apache Tomcat, Jetty, or GlassFish.

In simple terms, a servlet acts as a bridge between:

• A client (web browser or mobile app)
• A server-side Java application

Why Use Java Servlets?

• Platform-independent and portable
• Highly scalable and robust
• Better performance compared to traditional CGI
• Seamless integration with Java technologies (JSP, JDBC, Spring)
• Secure and reliable for enterprise applications

Use Cases of Java Servlets

Java Servlets are widely used in real-world applications such as:

• User authentication systems (login and registration)
• E-commerce platforms (shopping cart, order processing)
• Banking and financial applications
• RESTful APIs and backend services
• Content management systems

Java Servlet Architecture

The servlet architecture follows a request-response model:

• Client sends an HTTP request
• Web server forwards the request to the servlet container
• Servlet container invokes the appropriate servlet
• Servlet processes the request and generates a response
• Response is sent back to the client

 Components of Servlet Architecture

Servlet Container and Its Role

A servlet container is responsible for:

• Loading and initializing servlets
• Managing servlet lifecycle
• Handling multithreading
• Providing security and session management

Popular servlet containers include Apache Tomcat, Jetty, and GlassFish.

Servlet Lifecycle

The lifecycle of a Java Servlet is controlled by the servlet container and consists of three main phases:

1. Loading and Initialization

The servlet container loads the servlet class and calls the init() method once.

2. Request Handling

For every client request, the container calls the service() method, which delegates requests to doGet(), doPost(), and other HTTP methods.

3. Destruction

Before removing the servlet, the container calls the destroy() method.

Types of Java Servlets

• GenericServlet
• HttpServlet

Most modern applications use HttpServlet because it supports HTTP-specific methods.

Creating Your First Java Servlet

Example: Simple Hello World Servlet

📄

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;

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>");
    }
}

Explanation of the Code

• The servlet extends HttpServlet
• doGet() handles HTTP GET requests
• HttpServletRequest retrieves client data
• HttpServletResponse sends data back to the client

Servlet Mapping

Servlet mapping connects a URL to a servlet.

Using web.xml

📄

<servlet>
    <servlet-name>HelloServlet</servlet-name>
    <servlet-class>HelloServlet</servlet-class>
</servlet>

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

Using Annotations

📄

@WebServlet("/hello")
public class HelloServlet extends HttpServlet {
}

Handling Form Data Using Servlets

Example: Processing User Input

📄

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

    String username = request.getParameter("username");
    response.getWriter().println("Welcome " + username);
}

This example demonstrates how servlets handle user input from HTML forms.

Session Management in Java Servlets

Servlets support session tracking using:

• HTTP Sessions
• Cookies
• URL Rewriting

Example: HTTP Session

📄

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

Advantages and Limitations of Java Servlets

Advantages

• High performance
• Secure and scalable
• Reusable components

Limitations

• Requires knowledge of Java
• HTML code mixed with Java logic

Servlets vs JSP

Java Servlets are a fundamental technology for building dynamic web applications using Java. Understanding servlet architecture, lifecycle, and core concepts is essential for mastering Java web development. With real-world use cases and strong integration capabilities, servlets continue to be a reliable backend solution.

Frequently Asked Questions (FAQs)

1. What is the main purpose of Java Servlets?

Java Servlets are used to handle client requests, process business logic, and generate dynamic web responses.

2. Do Java Servlets replace JSP?

No. Servlets and JSP complement each other. Servlets handle logic, while JSP focuses on presentation.

3. Which server is best for running servlets?

Apache Tomcat is the most popular and beginner-friendly servlet container.

4. Are Java Servlets still relevant?

Yes. They form the foundation of frameworks like Spring MVC and are widely used in enterprise systems.

5. Can servlets handle multiple users?

Yes. Servlets are multithreaded and efficiently handle concurrent client requests.