Hi,
I don't think that's a spec problem.
The @NotNull constraint should be validated, no matter wether Foo is an interface or an abstract class. Generally constraints on overridden/implemented methods are to be considered, you only may not add any more parameter constraints in subclasses/implementations.
Are you using the Hibernate Validator CDI portable extension? Which container do you use? Could you create an entry with the complete stack trace and a detailed description of your environment to
https://forum.hibernate.org/viewforum.php?f=9?
--Gunnar