How Java Web Apps Work (HTTP and Servlets)

When you open a website, your browser sends a request and a server sends back a response. In Java, the code that answers those requests is a servlet. Even if you later use Spring Boot, every request still passes through a servlet under the hood (Spring’s DispatcherServlet). Understanding servlets makes everything else make sense.

The restaurant analogy#

• You (the browser) place an order: “GET me the menu page”.
• The waiter (the web server / servlet container, e.g. Tomcat) takes the order to the right cook.
• The cook (your servlet) prepares the dish (HTML or JSON).
• The waiter brings back the response with a status: 200 “here you go”, 404 “we don’t have that”, 500 “the kitchen had a problem”.

HTTP in 2 minutes#

A request has a method, a path, headers and sometimes a body:

GET /courses?level=beginner HTTP/1.1
Host: vitalstack.co.in
Accept: text/html
Cookie: JSESSIONID=A1B2C3

A response has a status code, headers and a body:

HTTP/1.1 200 OK
Content-Type: text/html; charset=UTF-8

<html><body><h1>Beginner courses</h1>...</body></html>

Methods#

Method	Meaning	Example
GET	read something	open a page, list courses
POST	create / submit	sign-up form, place an order
PUT	replace / update	update a profile
DELETE	remove	delete a comment

Status codes#

Code	Meaning
200 OK	success
201 Created	something new was created
301/302	redirect to another URL
400 Bad Request	invalid input
401 / 403	not signed in / not allowed
404 Not Found	no such page
500 Internal Server Error	bug on the server

HTTP is stateless: each request is independent, and the server doesn’t remember you between requests. Sessions and cookies (lesson 5) add memory.

What is a servlet?#

A servlet is a Java class that extends HttpServlet and handles requests for certain URLs:

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

@WebServlet("/hello")
public class HelloServlet extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws IOException {
        resp.setContentType("text/html;charset=UTF-8");
        resp.getWriter().println("<h1>Hello from a Java servlet!</h1>");
    }
}

When a browser visits /hello, Tomcat calls doGet and sends back whatever you write to the response.

The servlet container#

You don’t run servlets with main. A servlet container does it:

• Apache Tomcat: the most popular (Spring Boot embeds it by default).
• Jetty, Undertow: lighter alternatives.
• Full Jakarta EE servers: WildFly, Payara, Open Liberty.

The container listens on a port (8080), parses HTTP, creates HttpServletRequest/HttpServletResponse objects, picks the servlet by URL, manages threads (one request = one thread from a pool), sessions and security.

javax vs jakarta#

Java EE was handed to the Eclipse Foundation and renamed Jakarta EE. From Jakarta EE 9, packages changed from javax.servlet.* to jakarta.servlet.*.

Tomcat	Package	Use with
9.x	javax.servlet	older projects
10.1 / 11	jakarta.servlet	new projects, Spring Boot 3

This course uses **Tomcat 10.1+ and jakarta.servlet**. If you copy old tutorials that use javax, change the imports.

Where servlets fit today#

Browser ──HTTP──▶ Tomcat ──▶ Filters ──▶ Servlet ──▶ your code ──▶ Database
                                         ▲
                     Spring Boot: DispatcherServlet routes to @Controller methods

Modern teams rarely write raw servlets for big apps; they use Spring Boot. But interviews ask about the servlet lifecycle, filters, sessions and the request flow, and debugging Spring often means understanding this layer.

Exercise#

1. In DevTools, find one request with status 200, one 304 and (if you can) a 404. What’s in their headers?
2. Write the HTTP request line your browser sends for https://example.com/search?q=java.
3. Which HTTP method fits each action: view a product, add to cart, change quantity, remove an item?

Hint for 3

GET, POST, PUT (or PATCH), DELETE.

Interview corner#

• What is a servlet? A Java class that handles HTTP requests inside a servlet container, typically by extending HttpServlet.
• What is a servlet container? Software (like Tomcat) that manages servlet lifecycle, maps URLs, handles threads, sessions and the HTTP protocol.
• Is HTTP stateful? No, it’s stateless; sessions/cookies add state.
• GET vs POST? GET reads data, parameters go in the URL, and it should be safe and idempotent; POST sends data in the body to create/change something.

Summary#

• HTTP = request (method, path, headers, body) and response (status, headers, body).
• A servlet handles requests for URLs; a container like Tomcat runs it.
• New projects use jakarta.servlet with Tomcat 10.1+.
• Spring Boot’s DispatcherServlet is itself a servlet.

Next: set up Tomcat and Maven and run your first servlet.