How Does Spring Boot Work Internally? (Explained Step-by-Step)

Support this post with a reaction:

Spring Boot starts with @SpringBootApplication, loads required dependencies using starters, configures them automatically using @EnableAutoConfiguration, and scans for your components using @ComponentScan all this with minimal setup!

But there are lots of things happens behind the scenes to simplify development. Here’s a breakdown of the internal working:

1. @SpringBootApplication and main() – Entry Point

  • This is the starting point of every Spring Boot application.
  • It is a meta-annotation that combines:
    • @Configuration – Marks the class as a source of bean definitions.
    • @EnableAutoConfiguration – Enables Spring Boot’s auto-configuration feature.
    • @ComponentScan – Scans for Spring components (like @Component, @Service, etc.) in the current and sub-packages.
  • The main() method runs SpringApplication.run(), which starts the embedded server and initializes the application context.

2. Starter Dependencies

  • These are pre-configured sets of dependencies (like web, JPA, security) provided by Spring Boot.
  • When you add a starter (e.g., spring-boot-starter-web), Spring Boot knows what components to auto-configure.
  • You don’t need to worry about adding and managing individual library versions.

3. @EnableAutoConfiguration – Smart Configuration

  • This annotation tells Spring Boot to automatically configure your application based on the libraries on the classpath.
  • For example:
    • If you add spring-boot-starter-data-jpa, Spring Boot will try to configure JPA and a DataSource.
    • If no database is configured manually, it might auto-configure an H2 in-memory database.
  • It reduces the need for manual configuration in XML or Java.

4. @ComponentScan – Bean Discovery

  • It automatically scans and registers Spring-managed beans (like Controllers, Services, Repositories, etc.) in the package where the main class is located and all its sub-packages.
  • This makes dependency injection work without explicitly defining beans.

Leave a Comment

You must be to post a comment.

Similar Reads

Hi, Welcome back!
Forgot Password?
Don't have an account?  Register Now