Understanding Bean Definition Overriding in Spring Boot 2.0.3: @Configuration vs @Component

The author encountered a situation where a class annotated with both @Configuration + @Bean and @Component produced two possible UserManager beans, but only one was injected at runtime. By examining the Spring container, the article shows that the @Configuration + @Bean definition overrides the @Component definition.

Using Spring Boot 2.0.3 (Spring Framework 5.0.7.RELEASE), the default bean scope is singleton. The author verifies the number of UserManager instances by debugging the constructors; only the constructor used by the @Configuration bean is called.

Source code inspection reveals that during component scanning, the UserManager class is first registered as a ScannedGenericBeanDefinition (from @Component). Later, when processing @Configuration classes, a ConfigurationClassBeanDefinition with the same bean name replaces the previous definition.

Spring logs an INFO‑level message about the overriding, for example:

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]; scope=singleton; ...] with [Root bean: class [null]; factoryBeanName=userConfig; factoryMethodName=userManager; ...]

In Spring 5.1.2 (Spring Boot 2.1.0) the behavior was changed: bean definition overriding can be disabled, causing the application to fail fast with an error like:

The bean 'userManager', defined in class path resource [com/lee/qsl/config/UserConfig.class], could not be registered. A bean with that name has already been defined in file [D:\qsl-project\spring-boot-bean-component\target\classes\com\lee\qsl\manager\UserManager.class] and overriding is disabled.

The new property spring.main.allow-bean-definition-overriding (or the underlying allowBeanDefinitionOverriding in DefaultListableBeanFactory) lets developers decide whether overriding is permitted. By default, Spring itself allows overriding, but Spring Boot 2.1.0 disables it unless the property is set to true.

Finally, the article summarizes that Spring 5.0.7 allows both annotations to coexist with the @Configuration bean winning, while later versions provide explicit configuration to control this behavior.