Why use spring.factories for Spring Boot auto-configuration instead of annotations?

Question

The documentation states:

Developing auto-configuration and using conditions

If you work in a company that develops shared libraries, or if you work on an open-source or commercial library, you might want to develop your own auto-configuration. Auto-configuration classes can be bundled in external jars and still be picked-up by Spring Boot.

If I have annotations for everything else (even @AutoConfigureAfter or @AutoConfigureBefore annotations),

Why maintain a properties file to point to a class with an annotation?

Answer 1

Because we are not going to scan the world to figure out what auto-configuration classes exist in your project. For one, an auto-configuration is just a regular @Configuration class.

The way a Spring component is found is via explicit declaration or component scan but we need to know the list of auto-configuration classes way before we actually start the context.