Spring Boot

Spring Boot (built on Spring) — full car

• Spring Boot is a framework — a framework built on top of the Spring framework to simplify building applications
• Uses Spring internally, adds:
  • auto-configuration
  • embedded嵌入式 server (Tomcat) — No need to install Tomcat manually, run app → server starts automatically.
  • starter dependencies — don’t add many dependencies, just use spring-boot-starter-web
  • default conventions

With Spring Boot:

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

Spring Boot starts everything, and you just define components.

How Spring Boot starts internally

when you run:

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

This single line triggers a full startup pipeline.

1. Create Spring Application

   SpringApplication.run(App.class, args);

   • Create a SpringApplication object
   • Detect: Web app (Spring MVC)? Reactive app?
   • Prepare environment

2. Load configuration

   Spring loads:

   • application.yml / application.properties
   • environment variables
   • JVM parameters

3. Create IoC Container — this is where all Beans live

4. Component Scan (VERY IMPORTANT)

   Triggered by @SpringBootApplication, which includes @ComponentScan. Finds annotations and creates Beans for them.

5. Auto Configuration (Spring Boot magic)

   Triggered by @EnableAutoConfiguration. Spring Boot checks dependencies.

6. Bean Creation & Injection

   Spring:

   1. Creates objects
   2. Injects dependencies (@Autowired)
   3. Applies AOP (transaction, async, etc.)

7. Start Embedded Server

   Spring Boot starts:

   • Tomcat (default)

   Now your app is listening on:

   http://localhost:8080

What @SpringBootApplication really is = @Configuration + @EnableAutoConfiguration + @ComponentScan