Tomcat Server - Servlet Container

Source: Dev.to

What is a Servlet?

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

• Receives an HTTP request
• Returns an HTTP response

Typical flow:

Browser → Servlet → Response (HTML / JSON)

Example

GET /login

The servlet runs Java code and returns:

## Welcome

Note: A servlet cannot run by itself; it needs a container.

What is a Servlet Container?

A Servlet Container is software that:

• Starts your servlets
• Opens a port (e.g., 8080)
• Accepts browser requests
• Executes servlet Java code

In other words, it provides the runtime environment for servlets.

What is Tomcat?

Tomcat acts as the servlet container for Spring applications.

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

Without Tomcat, a Spring web app is just a collection of Java files that cannot be accessed via a browser.

Relationship with Spring Boot

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

What Happens When You Run a Spring Boot App?

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

When the application starts:

1. Spring Boot starts an embedded Tomcat instance.
2. Registers Spring as a servlet.
3. Tomcat listens on port 8080.
4. Incoming requests are forwarded to Spring.
5. Spring executes the appropriate controllers.

The spring-boot-starter-web dependency (included by default) brings in spring-boot-starter-tomcat, which provides the embedded Tomcat server.

Spring Boot creates a single main servlet called DispatcherServlet, which serves as Spring’s front controller.

What Is DispatcherServlet?

DispatcherServlet is a special servlet written by Spring. Since Tomcat can only talk to servlets, it forwards all incoming requests to this servlet.

Typical flow:

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

Example Mapping

One DispatcherServlet
   ├─ "/login"      → login()
   ├─ "/register"   → register()
   └─ "/products"   → products()

Thus, Spring uses a single servlet to handle thousands of routes.

What Happens Internally When You Hit a URL in Spring Boot?

Assume you browse to http://localhost:8080/hello.

1. Browser sends HTTP request

   GET /hello
   Host: localhost:8080

2. Tomcat (Web Server) receives the request

   • Listens on port 8080.
   • Forwards the request to the registered DispatcherServlet.

3. DispatcherServlet receives the request

   • Handles GET /hello.

4. HandlerMapping

   • Spring looks up the URL in its HandlerMapping registry.
   • Finds the mapping: /hello → HelloController.hello().

5. Controller execution

   • Spring invokes the corresponding controller method.

During startup, Spring builds a map of URL patterns to controller methods, e.g.:

This mapping enables Spring to route requests efficiently without needing multiple servlets.