Empty validatedBy in @Constraint

Question

I noticed that all built-in constraints have an empty value for the validatedBy parameter in @Constraint. i.e. @Constraint(validatedBy = {})

First, why are they allowed to have an empty value for validatedBy? I thought you can leave it blank only for constraint composition that does not need addtional validation?

Also, note that the Hibernate Validator can still find a validator implementation class for each built-in constraint, despite the validatedBy is empty, but if I leave the validatedBy blank for my constraint, my custom validator never gets picked up. Why is that?

Thanks.

Answer 1

Those built-in are treated in special implementation-specific way and their validators are configured programmatically.

For Hibernate Validator it's done in ConstraintHelper.java. I think you can't achieve the same for your custom constraints.

Answer 2

Maybe have a look at this answer:

How to avoid cross dependency between layers because of @Constraint validatedBy?

And those two links:

Adding constraints programmatically: http://docs.jboss.org/hibernate/stable/validator/reference/en-US/html/validator-specifics.html#section-programmatic-api

Adding constraints per xml: http://docs.jboss.org/hibernate/validator/4.1/reference/en-US/html/validator-xmlconfiguration.html