In the previous Time for action section, we learned how to use standard JSR-303 bean validation annotations to validate the fields of our domain object. This works great for simple validations, but sometimes, we need to validate some custom rules that aren't available in standard annotations. For example, what if we need to validate that the newly added product ID is not the same as any of the existing product IDs? To accomplish such kinds of validations, we can use custom validation annotations.