In this case, for clarity’s sake, the domain model I want to validate will be composed of only one naive, anemic class, which will be the blueprint for user objects: attribute is used for defining the message that will be displayed if the constrained field raises a violation when being validated. But you might be wondering what are the practical benefits of doing this? The class is ready to be validated, sure, but the missing piece here is having a mechanism, capable of scanning the annotations, checking the values assigned to the constrained fields, and returning the validation errors (or in JSR 303 terminology, the constraint violations).
Even better it’s possible to customize many constraints with parameters in order to express more refined criteria. And that’s exactly how Hibernate Validator works under the hood.
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Should its state be validated before being persisted or updated in a database, or when passed around to other layer(s)? Prior to Java EE 6, Java didn’t provide a standard way of validating the fields of a domain class by mean of a centralized mechanism. The Java Beans Validation specification makes it fairly easy to selectively validate class fields (and even entire classes) by using constraints declared with a few intuitive annotations.
At the time of this writing, JSR 303 has only two compliant implementations that you can pick up out there, Apache BVal and Hibernate Validator.
Conversely, if the arguments in question are injected from an outer layer (a user interface layer is a good example of this), the validation process can be cumbersome and tedious, in most cases leading to having chunks of boilerplate code scattered across multiple application layers.
In the end, everything boils down to this simple yet fundamental question: What makes a domain object valid? So no matter what approach you use to decide whether your domain objects are valid, Java Beans Validation will make the process easier.
For instance, we could develop a basic web application, similar to the one I built in the Servlet API tutorial, and validate user data submitted through an HTML form in an effective and performant manner.
As building such web application is definitively out of this post’s scope, the task will be left as homework for you, in case that you want to start pulling the reins of Hibernate Validator in the Web terrain.
Finally, the Finally, validating a user object looks exactly the same as when using the default email validator. Of course, I’m not saying that you’ll always need to mess up your life by creating custom constraints and validators, as this would downgrade the functionality of Java Beans Validation to nearly zero.
Nevertheless, from a design point of view it is useful to know that the standard offers a decent level of customization.
It’s possible, however, to get the same result by using a standard instances here and there, and validate the constrained classes in multiple places, even in the wrong ones!