Should Spring Boot auto-configuration classes use the @Configuration annotation?

Question

Based on my testing, I've found no obvious difference when Spring Boot auto-configuration classes have or don't have the @Configuration annotation - if they are configured in spring.factories correctly they are loaded regardless of the @Configuration annotation.

However, it seems like every custom auto-configuration example and demo uses the @Configuration annotation. Is there a reason all these examples use @Configuration (or is it just convention)? Is there any impact to not using @Configuration?

Some examples that use @Configuration on auto-configuration classes:

• https://docs.spring.io/spring-boot/docs/current/reference/html/features.html#features.developing-auto-configuration.condition-annotations.class-conditions
• https://www.baeldung.com/spring-boot-custom-auto-configuration#creating-a-custom-auto-configuration
• https://www.baeldung.com/spring-boot-custom-starter#1-auto-configuration-classes
• https://www.javadevjournal.com/spring-boot/spring-boot-custom-starter/
• https://billykorando.com/2019/12/30/building-a-custom-spring-boot-starter/
• http://www.masterspringboot.com/getting-started-with-spring-boot/spring-boot-quickstarts/how-to-build-a-custom-spring-boot-starter-in-no-time/
• https://github.com/snicoll/spring-boot-master-auto-configuration/blob/main/hornetq-spring-boot-autoconfigure/src/main/java/hornetq/autoconfigure/HornetQAutoConfiguration.java

Answer 1

Yes, they should be. The documentation states that an auto-configuration class should be annotated with @Configuration:

Under the hood, auto-configuration is implemented with standard @Configuration classes. Additional @Conditional annotations are used to constrain when the auto-configuration should apply. Usually, auto-configuration classes use @ConditionalOnClass and @ConditionalOnMissingBean annotations.

If you do not annotate them with @Configuration you are relying upon Spring Framework’s “lite” @Bean mode. This changes the behaviour of the auto-configuration classes. It is not guaranteed to work in the future as the implementation does not meet the documented requirements.