How Does a Web Server Work in Spring Boot? (Request–Response Flow Explained)

A Complete Request–Response Flow Explained

When we build REST APIs using Spring Boot, we often focus on writing controllers and services. But have you ever wondered what actually happens when a client hits an API endpoint like /api/users?

Behind the scenes, Spring Boot performs a series of well-orchestrated steps to handle the HTTP request and send back a response.
In this blog, we’ll explore how a web server works in Spring Boot, starting from application startup to the final response returned to the client.

What Is a Web Server in Spring Boot?

A web server is responsible for:

• Listening for incoming HTTP requests
• Forwarding them to the application
• Sending responses back to the client

Spring Boot simplifies this by providing an embedded web server, meaning you don’t need to install or configure an external server separately.

Common Embedded Servers in Spring Boot

• Tomcat (default)
• Jetty
• Undertow

When you run a Spring Boot application, it automatically starts an embedded web server (usually Tomcat) on port 8080.

What Happens When You Start a Spring Boot Web Application?

Before any HTTP request is handled, Spring Boot prepares the application environment.

Here’s what happens internally:

• JVM starts and executes the main() method
• SpringApplication.run() is invoked
• Spring creates the ApplicationContext
• Beans are initialized and dependency injection occurs
• Embedded web server (Tomcat) is created and started
• Server starts listening on a port (default: 8080)

At this point, your application is ready to receive HTTP requests.

High-Level Request–Response Flow

Let’s look at the overall picture before diving into details:

1. - Client sends an HTTP request
2. - Embedded server receives the request
3. - Spring MVC processes the request
4. - Controller executes business logic
5. - Response is returned to the client

Now, let’s break this down step by step.

Step-by-Step: How Spring Boot Handles an HTTP Request

Step 1: Client Sends an HTTP Request

A client (browser, Postman, frontend app) sends a request like:

GET /api/users

This request contains:

• HTTP method (GET, POST, etc.)
• URL
• Headers
• Request body (if applicable)

Step 2: Embedded Server Receives the Request

• The embedded Tomcat server listens on port 8080
• It receives the HTTP request
• The request is forwarded to the Spring framework The server itself does not know about your controllers. It simply passes the request to Spring.

Step 3: DispatcherServlet – The Front Controller

Every Spring MVC application has a central component called DispatcherServlet.

• It acts as the front controller
• All incoming HTTP requests pass through it
• It controls the entire request flow

Responsibilities of DispatcherServlet:

• Identify the correct controller
• Delegate request handling
• Manage response generation

This follows the Front Controller Design Pattern.

Step 4: Handler Mapping Finds the Controller

Spring uses **HandlerMapping **to map URLs to controller methods.

Example controller:

@RestController
@RequestMapping("/api")
public class UserController {

    @GetMapping("/users")
    public List<User> getUsers() {
        return userService.getAllUsers();
    }
}

Spring maps:

/api/users  →  getUsers()

Step 5: Controller, Service, and Repository Layers

Once the controller is identified:

• Controller handles HTTP-specific logic
• Service contains business logic
• Repository interacts with the database

This layered architecture ensures:

• Separation of concerns
• Better maintainability
• Easier testing

Step 6: Response Conversion

After execution, the controller returns:

• A Java object
• A String
• Or a ResponseEntity

Spring then:

• Uses HttpMessageConverters
• Converts Java objects into JSON (using Jackson)

Example response:

[
  {
    "id": 1,
    "name": "Abhishek"
  }
]

Step 7: Response Is Sent Back to the Client

• DispatcherServlet sends the response back to Tomcat
• Tomcat sends the HTTP response to the client
• The request–response cycle completes

Why DispatcherServlet Is So Important

The **DispatcherServlet **is the backbone of Spring MVC because it:

• Centralizes request handling
• Enables loose coupling
• Makes applications extensible using filters and interceptors

Interview-ready statement:

DispatcherServlet acts as the traffic controller of a Spring Boot web application.

Summary

• Spring Boot starts an embedded web server automatically
• All HTTP requests are handled through DispatcherServlet
• HandlerMapping identifies the correct controller
• Business logic executes in service layers
• Responses are converted and returned to the client

Conclusion

Understanding how a web server works in Spring Boot gives you deeper insight into:

• Debugging production issues
• Designing scalable APIs
• Answering Spring MVC interview questions confidently

Once you understand this flow, Spring Boot stops feeling like magic—and starts making complete sense.