How does Spring Boot's auto-configuration mechanism actually work?

Spring Boot Fundamentals

Auto-configuration classes are ordinary @Configuration classes bundled inside spring-boot-autoconfigure, each guarded by @Conditional annotations (like @ConditionalOnClass, @ConditionalOnMissingBean) that only let it register its beans if certain conditions hold — typically, that a relevant library is present on the classpath and the developer hasn't already defined a conflicting bean themselves. @EnableAutoConfiguration triggers Spring Boot to evaluate every candidate auto-configuration class listed in a metadata file and apply the ones whose conditions pass.