Why @Configuration + @Bean Overrides @Component in Spring Boot and How Bean Definition Overriding Evolved

The author discovered that a @Configuration class defining a @Bean for UserManager (with a non‑null userName ) co‑existed with a @Component class of the same type (with a null userName ) in a Spring Boot 2.0.3 project. When other components injected UserManager , the injected instance was the one created by the @Configuration bean, not the @Component one.

To verify the behavior, the author debugged the application and added logging to the constructors. Only the constructor called by the @Bean method was executed, confirming that a single UserManager bean existed and it was the one from the configuration class.

The underlying reason is that during the container refresh, Spring first registers beans discovered by component scanning (including the @Component bean). Later, when processing @Configuration classes, the ConfigurationClassPostProcessor creates a ConfigurationClassBeanDefinition for each @Bean method and registers it, overriding any existing bean definition with the same name. This replacement is logged at INFO level, e.g.: 2021-10-03 20:37:33.697 INFO 13600 --- [main] o.s.b.f.s.DefaultListableBeanFactory : Overriding bean definition for bean 'userManager' with a different definition: replacing [Generic bean: class [com.lee.qsl.manager.UserManager]...] with [Root bean: class [null]; factoryBeanName=userConfig; factoryMethodName=userManager]

Spring 5.0.7 (used by Spring Boot 2.0.3) allowed this overriding silently. Starting with Spring 5.1.2 and Spring Boot 2.1.0, the framework introduced the allowBeanDefinitionOverriding property, giving developers the option to enable or disable bean overriding. By default, Spring still permits overriding, but Spring Boot disables it unless the property spring.main.allow-bean-definition-overriding=true is set.

The article concludes that the observed behavior is expected: the @Configuration + @Bean definition wins over the @Component definition, and developers can control this via the new configuration flag.