Servlet

A Servlet is a Java class used to handle HTTP requests and generate dynamic web content on the server side. It extends the capabilities of a web server by providing a mechanism to process client requests, usually from a web browser or API client.

• Key Role: Servlets process user requests, interact with databases, and return responses (HTML, JSON, XML, etc.).
• Part of: Java EE (Jakarta EE) for building web applications.
• Alternative to: Older CGI scripts and now often replaced by modern Spring Boot, REST APIs, and frameworks like JSP, JSF, and Spring MVC.

How they work

1. Client (Browser or API) Sends a Request → Servlet Container receives it.
2. Servlet Processes the Request (e.g., retrieves data from a database).
3. Servlet Generates a Response (HTML page, JSON data, etc.).
4. Server Sends Response to the Client.

• Runs inside a Servlet Container (like Apache Tomcat, Jetty, WildFly).
• Uses the HttpServlet class for handling HTTP requests (GET, POST, etc.).

Lifecycle

A Servlet follows this lifecycle managed by the container:

1. Initialization (init()) – Executed once when the servlet is first loaded.
2. Request Handling (service()) – Handles HTTP requests (doGet(), doPost()).
3. Destruction (destroy()) – Called before removing the servlet from memory.

Example: Basic Java Servlet

import java.io.IOException;
import java.io.PrintWriter;
import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
 
@WebServlet("/hello")  // Defines URL mapping
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, World!</h1>");
    }
}

• @WebServlet("/hello") → Maps this servlet to http://localhost:8080/app/hello.
• doGet() → Handles GET requests, sends back an HTML response.
• PrintWriter → Writes the response output.

Advanced Features

• Session Management: Track user sessions using HttpSession.
• Servlet Filters: Intercept and modify requests/responses (e.g., authentication, logging).
• Servlet Listeners: Monitor lifecycle events in web applications.
• Asynchronous Processing: Handle long-running tasks without blocking threads.

Modern Alternatives

While Servlets remain foundational in Java web development, many modern frameworks abstract servlets to simplify development:

• Spring Boot (Spring MVC, REST APIs): Replaces low-level servlet coding.
• JAX-RS (Jakarta RESTful Web Services): For building RESTful APIs.
• Microservices (Spring Cloud, Quarkus, Micronaut): Serverless and cloud-native development.