Allow BeanRegistrar to access already registered bean definitions

[DanielLiu1123]

Description

As a framework developer, I want BeanRegistrar to provide access to already registered bean definitions, so that I can implement more flexible registration logic based on the current context.

Background

I attempted to migrate the automatic registration of HttpExchange interfaces in httpexchange-spring-boot-starter from BeanDefinitionRegistryPostProcessor to BeanRegistrar. During this process, I discovered an issue:
When using BeanDefinitionRegistryPostProcessor, I was able to see which bean definitions were already registered. This allowed me to skip the automatic registration for a given type if the user had already provided their own bean.

This capability is particularly important when users are not satisfied with the automatically registered beans or wish to customize them further. If BeanRegistrar could offer similar functionality, it would enable a putIfAbsent-like behavior, providing a better balance between user customization and automatic registration.

[sdeleuze]

I understand this kind of need, but the current design does not allow this kind of feature, see all the discussion on #21497 for more context.

[DanielLiu1123]

I believe this feature is very important. If programmatic bean registration cannot provide the functionality of @ConditionalOnMissingBean, it would be quite awkward and would hinder the adoption of programmatic bean registration.

I hope the Spring team can revisit this issue. I truly wish this capability could be introduced in Spring 7.

[sdeleuze]

Yeah that's clearly something a lot of people are going to ask, just we are not sure yet how we can implement that so that won't be available in 7.0. Feel free to add you +1 to #21497 and subscribe to it to follow our progresses.