How Spring Boot Works Internally – Step-by-Step with Real Example

Spring Boot makes it easy to create stand-alone, production-grade Spring applications with minimal configuration. But what really happens when you run a Spring Boot app?

Let’s break it down step-by-step with code and an easy-to-understand flow.

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1. Application Entry Point – @SpringBootApplication

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

• @SpringBootApplication is a meta-annotation that includes:

  • @Configuration – Marks this class as a source of bean definitions

  • @EnableAutoConfiguration – Tells Spring Boot to auto-configure beans

  • @ComponentScan – Enables scanning for components (@Component, @Service, etc.)

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2. Bootstrapping with SpringApplication.run()

When SpringApplication.run(MyApp.class, args) is called:

• It creates a SpringApplication instance

• Prepares the environment and application context

• Loads application.properties or application.yml

• Registers listeners and initializers

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3. Auto-Configuration Magic

Spring Boot looks into the META-INF/spring.factories file and loads @Configuration classes based on dependencies.

Example:

org.springframework.boot.autoconfigure.EnableAutoConfiguration=\
org.springframework.boot.autoconfigure.web.servlet.WebMvcAutoConfiguration,\
org.springframework.boot.autoconfigure.jdbc.DataSourceAutoConfiguration

• If spring-web is present → WebMvcAutoConfiguration kicks in

• If spring-boot-starter-data-jpa is present → JPA configurations are loaded

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4. Embedded Server Starts Automatically

If it's a web app:

• Tomcat is auto-configured and started by Spring Boot

• No need for external WAR deployment

TomcatServletWebServerFactory factory = new TomcatServletWebServerFactory();

App runs on http://localhost:8080 by default

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5. Bean Scanning and Registration

Spring Boot scans for annotations like:

• @Component

• @Service

• @Repository

• @RestController

And registers them as beans in the ApplicationContext.

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6. Request Dispatching with DispatcherServlet

Spring Boot registers DispatcherServlet automatically for web apps.

Example Controller:

@RestController
public class HelloController {
    @GetMapping("/hello")
    public String hello() {
        return "Hello from Spring Boot!";
    }
}

Request to /hello is handled by this method.

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7. Application Ready

Once the context is loaded, Spring Boot triggers:

• CommandLineRunner or ApplicationRunner if present

• Prints a log like:

  Started MyApp in 3.456 seconds
  

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Quick Example: Complete App

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

@RestController
class DemoController {
    @GetMapping("/")
    public String home() {
        return "Welcome to my blog!";
    }
}

Run it, and your app is live at http://localhost:8080/

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Spring Boot Internals – Summary Flow

@SpringBootApplication
       ↓
SpringApplication.run()
       ↓
Load Properties & Beans
       ↓
Auto-Configuration via spring.factories
       ↓
Embedded Tomcat/Jetty Starts
       ↓
Component Scanning & Bean Registration
       ↓
DispatcherServlet Routes Requests
       ↓
App is Ready to Serve