Introduction to Java Servlets
A Servlet is a Java program that runs on a server and handles HTTP requests and responses.
Servlets are part of Java EE (now Jakarta EE) and are the foundation of many Java web technologies like:
JSP (JavaServer Pages)
Spring MVC (built on top of Servlet API)
They execute inside a Servlet Container such as:
🧩 What Does a Servlet Do?
A servlet:
Receives an HTTP request from the browser
Processes the request (business logic, accessing DB, etc.)
Sends back an HTTP response (HTML, JSON, text, etc.)
It’s essentially the controller in early Java web applications.
Servlets go through 3 main phases:
1️⃣ Initialization
Executed once when the servlet loads.
public void init() { }
2️⃣ Service
Handles every request.
public void service(HttpServletRequest req, HttpServletResponse resp)
3️⃣ Destroy
Called when the server shuts down or servlet is removed.
public void destroy() { }
📌 Common HTTP Methods in Servlets
You typically override:
doGet() → Handle GET requests
doPost() → Handle POST form submissions
doPut()
doDelete()
Example:
protected void doGet(HttpServletRequest req, HttpServletResponse resp)
throws ServletException, IOException {
resp.getWriter().write("Hello from Servlet!");
}
🧪 Basic Servlet Example
1. Create a Servlet
import jakarta.servlet.*;
import jakarta.servlet.http.*;
import java.io.IOException;
public class HelloServlet extends HttpServlet {
@Override
protected void doGet(HttpServletRequest req, HttpServletResponse resp)
throws ServletException, IOException {
resp.setContentType("text/plain");
resp.getWriter().write("Hello, world! This is a Java Servlet.");
}
}
2. Configure the Servlet (Jakarta EE 9+)
You can configure servlets using annotations:
import jakarta.servlet.annotation.WebServlet;
@WebServlet("/hello")
public class HelloServlet extends HttpServlet { ... }
This maps the servlet to:
http://localhost:8080/your-app/hello
3. Deploy to a Servlet Container
Most common container → Apache Tomcat
Steps:
Deploy WAR to Tomcat’s webapps/ directory
Access via browser
📦 Servlet Request & Response Objects
HttpServletRequest
Provides data from the client:
Query parameters (req.getParameter("name"))
Headers (req.getHeader("User-Agent"))
Session management
Request body
HttpServletResponse
Allows you to:
Write output
Set status codes
Add headers
Send redirects (resp.sendRedirect("/login"))
Set cookies
🍪 Cookies & Sessions in Servlets
Servlets make session tracking easy via:
Setting a session attribute:
HttpSession session = req.getSession();
session.setAttribute("user", "John");
Retrieving a session attribute:
String user = (String) session.getAttribute("user");
Cookies are also supported via the Cookie class.
🧮 Forwarding vs Redirect
Forward (server-side)
req.getRequestDispatcher("home.jsp").forward(req, resp);
URL doesn’t change
Faster (no extra trip to client)
Redirect (client-side)
resp.sendRedirect("/login");
URL changes
Used for form resubmissions, login flows
🧰 Why Servlets Are Still Important Today
Even though frameworks like Spring Boot dominate modern development, servlets remain essential because:
Spring MVC and other frameworks are built on top of the Servlet API
Understanding servlets helps you understand how Java web requests work
Many legacy enterprise systems still use Servlets
Good interview topic for Java backend roles
🔚 Summary
Servlets are the foundation of Java web development.
They:
Run inside servlet containers
Handle HTTP requests
Produce dynamic responses
Manage sessions, parameters, cookies
Form the core of modern frameworks like Spring
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