Hi,
Concerning JSR303, I cannot think of better tutorials than those of Emmanuel Bernard, who is the spec leader of this JSR. You can find them at
Bean Validation Tutorials.
Also, the
Java EE tutorial has a chapter on Bean Validation:
Java EE tutorial.
JSR303 annotations can be part of your JPA entities, and bean validation can be triggered at any layer of your application, from the presentation layer to the data access layer.
Good reading