Why @Configuration @Bean Overrides @Component in Spring Boot 2.0.3

Problem Description

During development a class annotated with both @Configuration and @Component defines a @Bean method that creates a UserManager instance with a non‑null userName, while a @Component -annotated UserManager would have a null userName. The question is which bean is injected into other components.

Verification

Debugging and constructor logging reveal that only the constructor with arguments is invoked, indicating that a single UserManager instance is created by the @Configuration + @Bean definition.

Source Code Analysis

The @Configuration annotation is itself a @Component, allowing it to be discovered by component scanning. The ConfigurationClassPostProcessor processes @Configuration classes, turning them into ConfigurationClassBeanDefinition objects that replace the original ScannedGenericBeanDefinition for the same bean name.

The bean definition overriding is logged at INFO level:

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 Upgrade Optimization

Starting with Spring 5.1.2 (Spring Boot 2.1.0) the property allowBeanDefinitionOverriding can be configured. When overriding is disabled, the container throws an error:

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 [...UserManager.class] and overriding is disabled.

Spring Boot 2.1.0 introduced its own allowBeanDefinitionOverriding flag (default false) that overrides Spring’s default (which is true), giving developers explicit control.

Summary

In Spring 5.0.7 / Spring Boot 2.0.3 both @Configuration + @Bean and @Component can define the same bean type; the former overrides the latter, and an INFO log records the replacement. Later versions add a configurable property to enable or disable such overriding, making the behavior explicit for developers.