What is the operating principle of Spring’s @Configuration?

The @Configuration annotation in Spring is used to define configuration classes. It works by marking a class with @Configuration as a configuration class and registering the objects returned by methods in that class annotated with @Bean into the Spring container.

When the Spring container starts, it scans all classes annotated with @Configuration and creates corresponding Beans based on objects returned by methods annotated with @Bean. These Beans are registered in the Spring container and can be injected into other Beans for use through @Autowired annotation or configuration files.

The running principle of the @Configuration annotation can be divided into the following steps:

1. When the Spring container starts up, it will scan all classes that have been marked with the @Configuration annotation.
2. For every class annotated with @Configuration, Spring will instantiate the class and mark it as a configuration class.
3. Spring will scan the configuration class for methods decorated with the @Bean annotation, and use them to create corresponding bean objects by invoking these methods.
4. The objects returned by methods annotated with @Bean will be registered in the Spring container and can be injected into other beans for use either through @Autowired annotation or through configuration files.

By combining the @Configuration and @Bean annotations, Spring offers a Java-based configuration approach that can replace traditional XML configuration files. This method provides advantages such as type safety, ease of maintenance and refactoring, and better support for version control and code reuse.