Tomcat Server - Servlet Container

Tomcat is the web server that runs your Spring application and connects it to the browser.

What is a Servlet?

In Java web development, a Servlet is just a Java class that:

Receives an HTTP request and returns an HTTP response

Like this:

Browser  →  Servlet  →  Response (HTML / JSON)

Example:

GET /login

Servlet runs Java code and returns:

<h1>Welcome</h1>

But…

A servlet cannot run by itself
It needs a Servlet Container

What is a Servlet Container?

A Servlet Container is a software that:

✔ Starts your servlets
✔ Listens to HTTP requests
✔ Calls the right servlet
✔ Sends the response back
✔ Manages threads, memory, security

Servlet Container = Runtime environment for servlets

So we need something that:

• Opens port 8080
• Accepts browser requests
• Runs your servlet Java code

That “something” is called a Servlet Container

What is Tomcat?
Apache Tomcat is a Servlet Container

Tomcat’s job is:

Browser → Tomcat → Servlet → Response → Tomcat → Browser

Tomcat:

• Opens a port (like 8080)
• Understands HTTP
• Runs servlets
• Manages lifecycle of web apps

Without Tomcat, your Spring web app is just Java files — nobody can access it via browser.

Spring is a framework that helps you write Java code.

So Spring needs Tomcat to run on.

Think like this:

Spring Boot Application
    ↓
Runs inside Tomcat
    ↓
Tomcat runs inside JVM

What happens when you run a Spring Boot app?

When you click Run on:

@SpringBootApplication
public class MyApp {
    public static void main(String[] args) {
        SpringApplication.run(MyApp.class, args);
    }
}

Spring Boot:

• Starts Tomcat internally
• Registers Spring as a servlet
• Tomcat listens on port 8080
• Requests come in
• Tomcat forwards them to Spring
• Spring runs your controllers

When you create a Spring Boot Web project, this dependency is added automatically:

spring-boot-starter-web

Inside it is:

spring-boot-starter-tomcat

Spring Boot comes with Tomcat built-in by default

Spring Boot creates exactly ONE main servlet

It is called:

DispatcherServlet

This is Spring’s front controller.

What is DispatcherServlet?

It is a special servlet written by Spring.

Tomcat talks only to servlets.
So Spring creates one servlet and tells Tomcat:

“Send all requests to this servlet.”

So flow becomes:

Browser
   ↓
Tomcat
   ↓
DispatcherServlet (Spring)
   ↓
Your @Controller / @RestController

One DispatcherServlet
        |
        |-- "/login" → login()
        |-- "/register" → register()
        |-- "/products" → products()

So:

Spring has one servlet but thousands of routes

What happens internally when you hit a URL in Spring Boot?

You type this in browser

http://localhost:8080/hello

1️⃣ Browser sends HTTP request

Browser sends:

GET /hello
Host: localhost:8080

This goes to…

2️⃣ Tomcat (Web Server)
Tomcat is running because Spring Boot started it.

Tomcat:

• Is listening on port 8080
• Receives the HTTP request
• But Tomcat doesn’t know Spring controllers.

It only knows:

“I must send requests to a servlet”

Spring registered one servlet:

DispatcherServlet

So Tomcat forwards request to it.

3️⃣DispatcherServlet (Heart of Spring MVC)

DispatcherServlet receives:

GET /hello

4️⃣ HandlerMapping

Spring looks in HandlerMapping
This is just an internal routing table.

It finds:

/hello → HelloController.hello()

5️⃣ Spring calls your method

Your controller runs:

Now it asks:

“Which controller method should handle /hello?”

To answer that, Spring has built a map when app started:

/hello  →  hello()
/login  →  login()
/users  →  getUsers()

Annotation Returns
@Controller HTML page
@RestController JSON / Text / Data

@RestController = @Controller + @ResponseBody