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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 runsSpringApplication.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.
- If you add
- 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.