Unraveling the Core: How Spring Boot Works Internally

How Spring Boot Works Internally, HTTP Requests to REST Controls

In the software development world, Spring Boot has emerged as a trailblazer, making it simpler and faster for developers to create stand-alone, production-grade Spring-based applications. While most developers enjoy the convenience and efficiency that Spring Boot provides, understanding how it works internally can lead to more effective and optimized applications. In this article, we will dissect the inner workings of Spring Boot, focusing on its orchestration of servlets, interceptors, controllers, and the handling of REST calls and other aspects when an HTTP request comes in. How Spring Boot Works Internally, From HTTP Requests to REST Controls

Deep Dive: How Spring Boot Works Internally, From HTTP Requests to REST Controls

Simplifying Spring: The Concept of Spring Boot

Spring Boot simplifies the development process by eliminating boilerplate configuration, streamlining component integration, and providing embedded HTTP servers (like Tomcat, Jetty, or Undertow) for immediate web application deployment. It stands on the Spring framework and inherits its features, like dependency injection and transaction management, while providing a simpler approach to configuration.

// Necessary import statements
import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;
import org.springframework.web.bind.annotation.GetMapping;
import org.springframework.web.bind.annotation.RestController;

@SpringBootApplication // Marks this class as a Spring Boot Application
@RestController // Marks this class as a Controller
public class SimpleApplication {

    public static void main(String[] args) {
        // Running the Spring Boot application
        SpringApplication.run(SimpleApplication.class, args);

    // A simple REST endpoint returning a welcome message
    public String home() {
        return "Welcome to the simplified world of Spring Boot!";

In this code:

  1. The @SpringBootApplication annotation is an example of Spring Boot’s approach to eliminating boilerplate code. It’s a convenience annotation that encapsulates @Configuration (indicating that the class can use Spring IoC to generate bean definitions and service requests), @EnableAutoConfiguration (enabling the automatic configuration of the Spring application context), and @ComponentScan (enabling scanning of components included within the base package).
  2. The @RestController annotation simplifies the creation of RESTful web services. It’s a stereotype annotation that combines @ResponseBody and @Controller, meaning the returned value of the methods can be bound to the web response body.
  3. The SpringApplication.run() method launches the application. It starts the embedded server (default is Tomcat), meaning the deployment step common in traditional applications is unnecessary. It simplifies web application deployment significantly.
  4. The home() method is mapped to the root URL ("/") of the application. When you visit this URL, the method gets called, and it returns a simple string response. This simplicity is possible because of Spring Boot’s automatic configuration of an embedded server, defaulting to Tomcat, which enables quick testing and development without needing a separate server setup.

This example highlights the streamlined nature of Spring Boot, making it an excellent choice for microservices and web applications. The embedded server, automatic configuration, and simplified project setup allow developers to get an application up and running with minimal fuss, focusing on business logic rather than complex configuration and setup procedures. How Spring Boot Works Internally, HTTP Requests to REST Controls

Deep Dive: How Spring Boot Works Internally, From HTTP Requests to REST Controls

The Journey of an HTTP Request

When an HTTP request hits a Spring Boot application, it undergoes a series of steps. These steps are crucial in understanding how Spring Boot works internally:

a. Embedded Servlet Container: The request first arrives at the embedded servlet container. Unlike traditional applications where you deploy your app to a server, Spring Boot comes with an embedded server, enhancing portability and simplifying deployment. Internally, Spring Boot automatically configures the embedded server, setting up the servlet web application context and the dispatcher servlet that controls the routing of requests.

b. DispatcherServlet: As the gatekeeper, the DispatcherServlet receives the HTTP request and dispatches it to the appropriate controller. It’s integral to the Spring MVC framework, directing requests to handler methods and ensuring that the responses get returned correctly. Spring Boot auto-configures these steps, relieving developers from the minutiae of manual setup.

The journey of an HTTP request through a Spring Boot application is an automated process handled by the framework’s internal components. While we don’t typically write the code for these internal operations, it’s crucial to understand what happens behind the scenes. Below is a conceptual representation of these steps, followed by a simple Spring Boot application that listens for HTTP requests:

  1. Embedded Servlet Container Setup:
    In a traditional Spring MVC application, you would need to manually set up a servlet container or server. However, Spring Boot simplifies this by including an embedded server (Tomcat by default). The server starts when we run our Spring Boot application, and the servlet container (like Tomcat) hosts our application.
  2. DispatcherServlet:
    The DispatcherServlet acts as the front controller for Spring-based web applications. It receives all HTTP requests and dispatches them to the appropriate handlers. Spring Boot auto-configures the DispatcherServlet to ensure it’s set up with sensible defaults.

Here is a basic Spring Boot application that demonstrates these concepts:

import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;
import org.springframework.context.annotation.Bean;
import org.springframework.web.bind.annotation.GetMapping;
import org.springframework.web.bind.annotation.RequestParam;
import org.springframework.web.bind.annotation.RestController;
import org.springframework.web.servlet.config.annotation.EnableWebMvc;

@SpringBootApplication  // Indicates a Spring Boot application
@EnableWebMvc  // Explicitly enables Web MVC (usually optional)
@RestController  // Indicates that this class will have request handling methods
public class HttpRequestJourneyApplication {

    public static void main(String[] args) {
        // Starts the application, which includes starting the embedded server
        SpringApplication.run(HttpRequestJourneyApplication.class, args);

    // Simple handler method to demonstrate request handling
    public String greeting(@RequestParam(name = "name", defaultValue = "World") String name) {
        return String.format("Hello, %s!", name);

    // In a real-world scenario, the DispatcherServlet is already set up by Spring Boot's
    // auto-configuration and doesn't require explicit bean creation.
    // The method below is just for demonstration purposes to show what happens internally.
    public DispatcherServlet dispatcherServlet() {
        // The DispatcherServlet that handles all incoming requests and dispatches them to handlers
        return new DispatcherServlet();

In this code:

  • The @SpringBootApplication annotation initializes the Spring Boot application, including the configuration of the embedded servlet container.
  • The @EnableWebMvc annotation (though typically optional, since it’s included in @SpringBootApplication) is explicitly enabled here to highlight that we’re using Spring MVC. It ensures that we have a DispatcherServlet set up to handle incoming HTTP requests.
  • The @RestController marks the class as a controller capable of handling HTTP requests in the MVC architecture.
  • The greeting method demonstrates a simple HTTP request handler. When a GET request is sent to “/greeting,” this method will respond with a simple text greeting. The @RequestParam allows for dynamic input to be included in the response.
  • The dispatcherServlet bean method is purely illustrative. In typical Spring Boot applications, you would not need to manually define the DispatcherServlet bean, as Spring Boot’s auto-configuration takes care of this setup. This method is here to visually represent the automatic configuration performed by Spring Boot.

Through this code, we observe the simplicity of setting up a Spring Boot application capable of handling HTTP requests while understanding the internal automatic orchestration undertaken by the framework when an HTTP request comes in. How Spring Boot Works Internally, HTTP Requests to REST Controls

Deep Dive: How Spring Boot Works Internally, From HTTP Requests to REST Controls

Interceptors and Controllers: Managing HTTP Requests

Understanding interceptors and controllers is vital in dissecting how Spring Boot works internally, particularly in handling HTTP requests:

a. Interceptors: Before reaching the controller, the request may pass through one or several interceptors (HandlerInterceptor instances). Interceptors are used to execute code before or after certain stages of the request processing cycle, enabling common concerns like logging, security, and transformations to be handled outside business logic.

  - Pre-Handle phase: Before passing the request to the controller, Spring Boot allows pre-processing, determining whether the handler should execute.
  - Post-Handle phase: After the handler executes, further modifications can happen before sending the response back to the client.
  - After Completion phase: After the view renders the response, any resource cleanup or log actions take place.

b. Controllers: The controller, annotated with @Controller or @RestController in Spring Boot, receives the processed request, invoking the required business logic. Controllers are responsible for processing incoming REST API requests, preparing a model map with data and selecting a view for rendering the response.

Let’s explore how Interceptors and Controllers work within a Spring Boot application through code examples. How Spring Boot Works Internally, HTTP Requests to REST Controls


Interceptors provide a mechanism to intercept an HTTP request and response. Here’s a simple implementation of an interceptor:

import org.springframework.stereotype.Component;
import org.springframework.web.servlet.HandlerInterceptor;
import org.springframework.web.servlet.ModelAndView;

import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

public class CustomInterceptor implements HandlerInterceptor {

    public boolean preHandle(HttpServletRequest request, HttpServletResponse response, Object handler) throws Exception {
        // Log or modify information in the pre-handle stage
        System.out.println("Pre Handle method is Calling");
        return true; // return false to reject the execution

    public void postHandle(HttpServletRequest request, HttpServletResponse response, Object handler, ModelAndView modelAndView) throws Exception {
        // Executed after the handler method returns a ModelAndView
        System.out.println("Post Handle method is Calling");

    public void afterCompletion(HttpServletRequest request, HttpServletResponse response, Object handler, Exception exception) throws Exception {
        // Cleanup or log actions after the request is complete
        System.out.println("Request and Response are completed");

To make our interceptor effective, we need to register it:

import org.springframework.context.annotation.Configuration;
import org.springframework.web.servlet.config.annotation.InterceptorRegistry;
import org.springframework.web.servlet.config.annotation.WebMvcConfigurer;

public class WebConfig implements WebMvcConfigurer {

    private final CustomInterceptor customInterceptor;

    public WebConfig(CustomInterceptor customInterceptor) {
        this.customInterceptor = customInterceptor;

    public void addInterceptors(InterceptorRegistry registry) {


Controllers handle incoming HTTP requests and route them to the appropriate service layer. Below is an example of a simple controller:

import org.springframework.web.bind.annotation.GetMapping;
import org.springframework.web.bind.annotation.RequestParam;
import org.springframework.web.bind.annotation.RestController;

public class SimpleController {

    public String hello(@RequestParam(name = "name", defaultValue = "World") String name) {
        return String.format("Hello, %s!", name);

In this setup, the CustomInterceptor will intercept any request handling done by SimpleController. It will print messages at different stages: before the request is handled (preHandle), after the handler is executed (postHandle), and after the request is completed (afterCompletion). The SimpleController is a basic example of how requests are handled on the server-side.

By combining controllers and interceptors, you can handle requests and simultaneously perform actions at various stages of the request lifecycle, keeping the concerns properly separated. This way, the business logic remains untainted by cross-cutting concerns, making the codebase cleaner and more maintainable.

REST Calls: Simplifying Backend Operations

Spring Boot’s internal handling of REST calls is where its magic becomes evident. With the simplification of HTTP responses and the direct mapping of requests to Java objects, Spring Boot allows effortless RESTful service creation.

  • RequestMapping: Spring Boot handles different HTTP requests by using annotations to map requests to controller methods (e.g., @GetMapping, @PostMapping). It interprets the REST API calls and routes them to the appropriate service methods.
  • Data Transformation: It automatically handles data conversion for requests and responses, typically converting JSON or XML data into Java objects (and vice versa) using HTTPMessageConverter instances.
  • Exception Handling: Spring Boot provides a centralized exception handling mechanism across all REST controls, ensuring clean, maintainable error-handling code.
  1. Beyond the Basics: Additional Spring Boot Features

Spring Boot also internalizes several auto-configurations and advanced features to streamline development further:

  • AutoConfiguration: Spring Boot auto-detects the classes on the classpath and automatically configures beans and properties accordingly. This feature is central to the framework’s ease of use, as it attempts to configure the most common settings compatible with most applications.
  • Starter Templates: These pre-configured project templates (starters) help set up the necessary dependencies, reducing the risk of version conflicts and configuration errors.
  • Actuators: For production-grade systems, Spring Boot provides actuators that offer insights into application metrics, health checks, and other information.

To demonstrate the features above outlined, here is a comprehensive example that touches upon the various aspects of REST calls in Spring Boot, such as request mapping, data transformation, exception handling, and more.

Let’s start by creating a simple REST controller that uses request mappings, followed by examples of data transformation and exception handling.

import org.springframework.web.bind.annotation.*;

public class ProductController {

    private final ProductService productService;

    public ProductController(ProductService productService) {
        this.productService = productService;

    // Mapping for GET requests to retrieve a product
    public Product getProduct(@PathVariable Long id) {
        return productService.getProductById(id);

    // Mapping for POST requests to create a new product
    public Product createProduct(@RequestBody Product product) {
        return productService.saveProduct(product);

    // ... other request mappings like @PutMapping and @DeleteMapping

This controller handles incoming requests for products and uses a service to perform operations. Notice the @RequestMapping at the class level and specific request mappings like @GetMapping and @PostMapping at the method level. How Spring Boot Works Internally, HTTP Requests to REST Controls

Data Transformation

Spring Boot automatically converts incoming JSON requests to Java objects and Java objects to JSON responses. This is achieved through the @RequestBody and @ResponseBody annotations, and it’s handled behind the scenes by the HttpMessageConverter classes.

Exception Handling

You can handle exceptions using a controller-advice class that handles exceptions for all controllers. Here’s an example:

import org.springframework.http.HttpStatus;
import org.springframework.web.bind.annotation.*;

public class GlobalExceptionHandler {

    public @ResponseBody ErrorResponse handleResourceNotFound(ResourceNotFoundException ex) {
        ErrorResponse response = new ErrorResponse();
        return response;

    // Other exception handlers for different types of exceptions

In this example, ResourceNotFoundException is a custom exception that should be thrown by your service classes when a resource isn’t found.

Beyond the Basics: Additional Features

1. AutoConfiguration: Spring Boot’s @EnableAutoConfiguration annotation guesses the configuration based on the libraries on the classpath. It simplifies the configuration process, often reducing it to no configuration at all for a simple application.

import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.EnableAutoConfiguration;
import org.springframework.context.annotation.ComponentScan;

public class Application {
    public static void main(String[] args) {
        SpringApplication.run(Application.class, args);

2. Starter Templates: Spring Initializr (start.spring.io) is an example of a tool used to create pre-configured templates. By selecting dependencies, you get a project with all the necessary dependencies in your pom.xml or build.gradle file.

3. Actuators: Actuators provide production-ready features to monitor and manage your application.

To enable actuators, you need to add the dependency in your build configuration file:

<!-- In your pom.xml -->

After including this dependency, various endpoints become available (like /actuator/health for health status and /actuator/metrics for metrics).

These code snippets and explanations showcase the different layers and internal workings of a Spring Boot application, highlighting how the framework simplifies many complex aspects of web development.


The internal mechanics of Spring Boot coalesce to provide a seamless, developer-friendly experience from handling HTTP requests to managing RESTful services. By understanding what happens under the hood, developers can better leverage the framework’s capabilities, enhancing efficiency, and solidifying the robustness of application architecture. In essence, Spring Boot abstracts complex configurations and empowers developers to steer their focus back to building high-quality, innovative applications. Whether you’re intercepting HTTP requests, routing with controllers, or setting up REST calls, Spring Boot handles the heavy lifting, making enterprise-level development more approachable and manageable than ever before. How Spring Boot Works Internally, HTTP Requests to REST Controls

For more information on related topics, check out the following articles: Best Practices for Java Architects on GitHub

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