Following the DRY (Don't Repeat Yourself) principle, Hibernate Validator let's you express your domain constraints once (and only once) and ensure their compliance at various level of your system automatically.

Annotations are a very convenient and elegant way to specify invariant constraints on the domain model implementation, the persistent classes. Hibernate Validator comes bundled with a set of common validations (@NotNull, @Email, @Max, and so on), and you can build you own validation rules very easily.