Understanding Java Servlets

Java Servlets are a fundamental technology in Java-based web development. 

Before modern frameworks such as Spring MVC and Spring Boot became popular, servlets formed the backbone of server-side Java web applications. 

Even today, understanding servlets is essential because many higher-level frameworks are built on top of servlet technology.

1. What Is a Servlet?

A Servlet is a Java class that runs on a web server and handles client requests, usually from a web browser. 

Servlets are used to process HTTP requests and generate dynamic responses, such as HTML pages or JSON data.

Servlets operate inside a Servlet Container (also known as a web container), which manages their lifecycle, request handling, and security. 

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

In simple terms:

• The browser sends an HTTP request

• The servlet processes the request

• The servlet generates a response

• The server sends the response back to the client

2. Why Use Servlets? (Purpose and Role)

Before servlets, web servers mainly served static HTML pages. 

Servlets introduced dynamic content generation, allowing web applications to:

• Process form data

• Access databases

• Manage user sessions

• Generate dynamic HTML or API responses

Servlets bridge the gap between HTTP requests and Java business logic.

3. Advantages of Java Servlets

1. Platform Independence

Servlets are written in Java, making them platform-independent. 

They can run on any operating system that supports a Java Virtual Machine (JVM).

2. High Performance

Unlike CGI scripts that create a new process for each request, servlets use a multi-threaded approach, making them faster and more efficient.

3. Robust and Secure

Servlets benefit from Java’s built-in security features, such as memory management and exception handling.

4. Scalability

Servlet containers efficiently manage multiple requests, allowing applications to scale to handle many users.

5. Integration with Java Ecosystem

Servlets integrate easily with databases, APIs, and enterprise Java libraries.

4. Disadvantages of Java Servlets

1. Verbose Code

Servlet code can become lengthy and difficult to manage, especially when generating HTML directly.

2. Poor Separation of Concerns

Mixing business logic and presentation logic in servlets can make applications harder to maintain.

3. Steep Learning Curve

Beginners may find servlet APIs complex compared to modern frameworks.

4. Limited UI Capabilities

Servlets are not designed for frontend rendering, which is why technologies like JSP and templating engines are often used alongside them.

5. Servlet Architecture Overview

The core components of servlet architecture include:

• Client (Web Browser)

• Web Server

• Servlet Container

• Servlet

The servlet container is responsible for:

• Loading servlets

• Managing servlet lifecycle

• Handling multithreading

• Mapping URLs to servlets

6. Servlet Class Structure

Most servlets extend the HttpServlet class, which provides methods for handling HTTP requests.

6.1 Common Servlet Methods

• doGet() – Handles HTTP GET requests

• doPost() – Handles HTTP POST requests

• doPut() – Handles HTTP PUT requests

• doDelete() – Handles HTTP DELETE requests

6.2 Basic Servlet Class Structure

import jakarta.servlet.http.*;
import jakarta.servlet.annotation.WebServlet;
import java.io.IOException;

@WebServlet("/example")
public class ExampleServlet extends HttpServlet {

    protected void doGet(HttpServletRequest request, HttpServletResponse response)
            throws IOException {
        response.getWriter().println("Hello from Servlet!");
    }
}

7. Servlet Lifecycle Explained

The servlet lifecycle is managed entirely by the servlet container.

7.1 Loading and Instantiation

The servlet container loads the servlet class and creates an instance when the application starts or when the first request arrives.

7.2 Initialization (init() method)

The init() method is called once to initialize resources.

public void init() {
    System.out.println("Servlet initialized");
}

7.3 Request Handling (service() method)

The container calls the service() method for each request. This method dispatches requests to doGet(), doPost(), and other HTTP methods.

7.4 Destruction (destroy() method)

The destroy() method is called when the servlet is removed from service.

public void destroy() {
    System.out.println("Servlet destroyed");
}

8. Hands-On Servlet Programming Example

8.1 Step 1: Create a Dynamic Web Project

Use an IDE like Eclipse and configure Apache Tomcat.

8.2 Step 2: Create a Servlet

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

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

        response.setContentType("text/html");
        response.getWriter().println("<h1>Hello, Servlet World!</h1>");
    }
}

8.3 Step 3: Run the Application

Deploy the project on Tomcat and open:

http://localhost:8080/your-project-name/hello

You should see a dynamic message generated by the servlet.

9. Servlets vs JSP vs Modern Frameworks

• Servlets: Low-level, powerful, but verbose

• JSP: Better for view rendering

• Spring MVC / Spring Boot: Built on servlets, easier to use, cleaner architecture

Understanding servlets helps developers grasp how modern Java web frameworks work internally.

10. Best Practices When Using Servlets

• Avoid writing HTML directly in servlets

• Use MVC architecture

• Handle exceptions properly

• Manage resources efficiently

• Keep servlets lightweight and focused

11. Conclusion

Java servlets are the foundation of Java web development. 

Although modern frameworks have simplified development, servlets remain an essential concept for understanding how Java web applications process requests and responses.

By learning servlets, developers gain deeper insight into request handling, lifecycle management, and server-side programming principles. 

This knowledge not only strengthens Java fundamentals but also makes it easier to master advanced frameworks and enterprise-level systems.