JBoss Enterprise Application Platform 4.3
Introduction
Annotations are a very convenient and elegant way to specify invariant constraints for a domain model. You can, for example, express that a property should never be null, that the account balance should be strictly positive, etc. These domain model constraints are declared in the bean itself by annotating its properties. A validator can then read them and check for constraint violations. The validation mechanism can be executed in different layers in your application without having to duplicate any of these rules (presentation layer, data access layer). Following the DRY principle, Hibernate Validator has been designed for that purpose.
Hibernate Validator works at two levels. First, it is able to check in-memory instances of a class forconstr aint violations. Second, it can apply the constraints to the Hibernate metamodel and incorporate them into the generated database schema.