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.