Mastering Grails: Mock testing with Grails

In this article, you’ll learn how the mocking capabilities of GrailsUnitTestCase and ControllerUnitTestCase make it easy to unit test Grails artifacts that you’d otherwise assume need to be tested using an integration test. The mockForConstraintsTests(), mockDomain(), and mockLogging() methods all leverage Groovy’s metaprogramming magic to make testing your domain classes, services, and controllers a breeze.
(Link: Mastering Grails: Mock testing with Grails)