What does the spring annotation @ConditionalOnMissingBean do?

The @ConditionalOnMissingBean annotation is used to load a bean only if a given bean is missing:

@Bean
@ConditionalOnMissingBean(SomeBean.class)
public SomeBean otherBean(){
    return new SomeBean();
}

The above bean will get loaded by Spring only if there is no other bean of this type present in the context. On the other hand, if there is already a bean of the type SomeBean present in the application context, the above bean will not be created.

Some use cases where this annotation comes in handy are:

• Specifying a fallback bean which gets only loaded as a backup if there is no bean of the same type present (for example: using an in-memory database if there is no real database configured)
• Specifying a default bean which allows being overridden in the case that a more specific bean of the same type is present in the context (for example: using a default authentication mechanism unless someone decides to replace it with his own custom authentication)

Reference: https://docs.spring.io/spring-boot/docs/current/reference/html/boot-features-developing-auto-configuration.html