In Java web development, regular Java classses cannot be directly accessed via a browser. To enable HTTP-based interaction, Java provides Servlets—specialized classes that act as dynamic resources within a web application and respond to client requests through URLs.

It's important to distinguish related terms:

• Service: Refers to business logic layers.
• Server: The runtime environment (e.g., Tomcat) hosting the application.
• Servlet: A Java component handling dynamic web requests.

Steps to Implement a Servlet

1. Implemant the javax.servlet.Servlet Interface

Create a class that implements the Servlet interface from the servlet-api.jar. This requires overriding all declared methods.

2. Override Required Methods

The following example demonstrates a basic implementation with lifecycle and utility methods:

import javax.servlet.*;
import java.io.IOException;

public class ExampleServlet implements Servlet {

    private ServletConfig config;

    // Called once after instantiation by the servlet container
    @Override
    public void init(ServletConfig config) throws ServletException {
        this.config = config;
        System.out.println("ExampleServlet initialized.");
    }

    // Returns the ServletConfig object
    @Override
    public ServletConfig getServletConfig() {
        return this.config;
    }

    // Handles each client request
    @Override
    public void service(ServletRequest req, ServletResponse res)
            throws ServletException, IOException {
        System.out.println("Processing request in ExampleServlet...");
    }

    // Provides metadata about the servlet
    @Override
    public String getServletInfo() {
        return "A sample servlet for demonstration.";
    }

    // Invoked before the servlet is removed from service
    @Override
    public void destroy() {
        System.out.println("ExampleServlet is being destroyed.");
    }
}

3. Configure the Servlet in web.xml

Map the servlet class to a URL pattern inside WEB-INF/web.xml:

<?xml version="1.0" encoding="UTF-8"?>
<web-app xmlns="http://xmlns.jcp.org/xml/ns/javaee"
         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
         xsi:schemaLocation="http://xmlns.jcp.org/xml/ns/javaee 
         http://xmlns.jcp.org/xml/ns/javaee/web-app_4_0.xsd"
         version="4.0">

    <servlet>
        <servlet-name>ExampleServlet</servlet-name>
        <servlet-class>com.example.ExampleServlet</servlet-class>
    </servlet>

    <servlet-mapping>
        <servlet-name>ExampleServlet</servlet-name>
        <url-pattern>/example</url-pattern>
    </servlet-mapping>

</web-app>

Here, accessing /example in the browser triggers ExampleServlet.

4. Deploy and Run

Package the application as a WAR file or place it in the webapps directory of a servlet container like Apache Tomcat, then start the server. The servlet becomes accessible via the configured URL path.