Validating data is a common task that occurs throughout all application layers, from the presentation to the persistence layer. Often the same validation logic is implemented in each layer which is time consuming and error-prone. To avoid duplication of these validations, developers often bundle validation logic directly into the domain model, cluttering domain classes with validation code which is really metadata about the class itself.
JSR 349 - Bean Validation 1.1 - defines a metadata model and API for entity and method validation. The default metadata source are annotations, with the ability to override and extend the meta-data through the use of XML. The API is not tied to a specific application tier nor programming model. It is specifically not tied to either web or persistence tier, and is available for both server-side application programming, as well as rich client Swing application developers.
Hibernate Validator is the reference implementation of this JSR 349. The implementation itself as well as the Bean Validation API and TCK are all provided and distributed under the Apache Software License 2.0.