🚀 What Happens When You Start a Spring Boot Project – Step-by-Step

Have you ever wondered what actually happens under the hood when you run a Spring Boot application? It looks simple on the surface, but a lot is going on behind the scenes.

In this post, I’ll walk you through each step in the Spring Boot startup process—from hitting "Run" to handling web requests.

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🟢 1. You Hit Run or Execute SpringApplication.run()

Your entry point looks like this:

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

Here’s what kicks off the magic.

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⚙️ 2. Spring Boot Creates a SpringApplication Instance

Internally, Spring Boot creates an instance of SpringApplication.

SpringApplication app = new SpringApplication(MyApp.class);

It also sets up:

• Environment

• ApplicationContext

• Banner

• Listeners

• Initializers

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🏗️ 3. Environment Is Prepared

Spring Boot prepares the environment by:

• Loading system properties

• Reading application.properties or application.yml

• Setting up profiles (like dev, prod, etc.)

• Merging environment variables

This is handled by ConfigFileApplicationListener.

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🔍 4. ApplicationContext Is Created

Depending on the project type, one of the following is created:

• AnnotationConfigApplicationContext (non-web)

• AnnotationConfigServletWebServerApplicationContext (web apps)

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🔎 5. Component Scanning & Bean Definition

Spring Boot uses:

• @ComponentScan to find @Component, @Service, @Repository, etc.

• @Configuration to load custom configs

These classes are registered as Spring Beans in the ApplicationContext.

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🔧 6. Auto-Configuration via @EnableAutoConfiguration

Spring Boot checks all entries in:

META-INF/spring.factories

Example:

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

It loads only the relevant configurations based on classpath dependencies.

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🧱 7. Embedded Server (like Tomcat) Starts

If it’s a web project, Spring Boot:

• Auto-configures Tomcat (or Jetty, Undertow)

• Registers DispatcherServlet

• Binds server port (default 8080)

• Starts the HTTP listener

No need to deploy a WAR file — it runs as a standalone JAR!

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🔄 8. ApplicationRunner or CommandLineRunner Executes

If your project has logic like this:

@Component
public class MyRunner implements CommandLineRunner {
    public void run(String... args) {
        System.out.println("App started!");
    }
}

Spring Boot will run it after startup.

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🖥️ 9. Application Is Up and Ready!

You’ll see:

Started MyApp in 3.456 seconds
Tomcat started on port(s): 8080 (http)

🎉 Your app is now live at http://localhost:8080

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🌐 10. Request Handling Begins

From now on:

• DispatcherServlet routes incoming HTTP requests

• Controllers like @RestController respond to them

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

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🧠 Recap: Spring Boot Startup Flow

@SpringBootApplication
       ↓
SpringApplication.run()
       ↓
Prepare Environment & Context
       ↓
Load application.properties
       ↓
Component Scanning
       ↓
Auto-Configuration
       ↓
Start Embedded Server
       ↓
Initialize Beans
       ↓
Run ApplicationRunner / CommandLineRunner
       ↓
App is READY to Handle Requests

Happy Learning :)