Provide support for post-processing a LocalValidatorFactoryBean's validator Configuration without requiring sub-classing

[wilkinsona]

Affects: 5.3

LocalValidatorFactoryBean currently allows the validator Configuration to be post-processed but this requires a custom sub-class and an override of postProcessConfiguration. Given that Framework itself already provides a sub-class (OptionalValidatorFactoryBean) this is a little awkward as you have to know which class to sub-class or maybe sub-class both depending on circumstances.

A Spring Boot user has proposed an enhancement to make it easier to register ValueExtractors with the Configuration. We'd like to do this via composition with some sort of ConfigurationPostProcessor or ConfigurationCustomizer callback. We could create our own LocalValidatorFactoryBean sub-class that enables this customization but that functionality would then be lost by anyone who's using OptionalValidatorFactoryBean instead.

We wondered if Framework could add a callback-based mechanism for post-processing the Configuration instead. We could then auto-configure the LocalValidatorFactoryBean with any post-processors/customizers found in the context and users could implement one to register their ValueExtractor without resorting to inheritance.

[mdeinum]

I've seen more and more different customizers pop up in Spring and Spring Boot code. Things like the CacheManagerCustomizer, CodecCustomizer, and EntityManagerFactoryBuilderCustomizer. Each serving the purpose of customizing a specific type of bean. Each of them is also invoked in similar ways (some ordered, some not, and there are some slight differences).

But isn't this also the task of a BeanPostProcessor, and isn't this a specialized version of a BeanPostProcessor with an implementation of the BeanPostProcessor.postProcessBeforeInitialization for a specific type of bean? Or maybe Spring is in need of an additional way of customizing beans. Now it looks like that for each type of bean that requires customization the wheel is (somewhat) reinvented or at least copy/pasted with sometimes some minor differences (or those differences aren't known).

I do get that the BeanPostProcessor doesn't apply to this specific case as it is an internally created instance instead of a managed bean, but it does apply to certain other cases.

[quaff]

BeanPostProcessor cannot change state of immutable bean, Customizer is applied to their Builder.

[mdeinum]

Not in all cases, the EntityManagerFactoryBuilder has a customizer which could also be done by a BeanPostProcessor, the same applies to the CacheManagerCustomizer. As I stated in my comment it won't work for the internal beans here, but for those beans that are already exposed as beans.

I pointed out that there seems to be an emerging pattern for customizing beans outside of the regular BeanPostProcessor which might be better suited for a general solution instead of reinventing/implementing the wheel each time. It has been duplicated in Spring Boot several times, and in other projects as well.

[jhoeller]

@wilkinsona would it be sufficient to let postProcessConfiguration delegate to a setter-specified Consumer<Configuration> instance - or possibly a custom ConfigurationCustomizer type, or a vararg of such a type? I assume you would build your own detection mechanism on top and then just pass such customizers, or an adapter around Boot's customizer bean type, to that new LocalValidatorFactoryBean setter method?

The main question for me is the use of a special customizer type. Would you want to reuse it in Boot? If you'll be doing a custom customizer type in any case, we could also simply go with Consumer<Configuration> at the core level so that there's no conflict of customizer types on the common Boot classpath.