How it works...

Our User class uses three of the new constraints introduced by Bean Validation 2.0:

• @NotBlank: Assures that the value is not null, empty, or an empty string (it trims the value before evaluation, to make sure there aren't spaces).
• @Email: Allows only a valid email format. Forget those crazy JavaScript functions!
• @NotEmpty: Ensures that a list has at least one item.
• @PositiveOrZero: Guarantees that a number is equal or greater than zero.

Then we create a test class (using JUnit) to test our validations. It first instantiates Validator:

@BeforeClass
public static void setUpClass() {
    validator = Validation.buildDefaultValidatorFactory().getValidator();
}

Validator is an API that validates beans according to the constraints defined for them.

Our first test method tests a valid user, which is a User object that has:

• Name not empty
• Valid email
• profileId list only with integers greater than zero:

User user = new User(
   "elder", 
   "elder@eldermoraes.com", 
   asList(1,2));

And finally, the validation:

Set<ConstraintViolation<User>> cv = validator.validate(user);

The validate() method from Validator returns a set of constraint violations found, if any, or an empty set if there are no violations at all.

So, for a valid user it should return an empty set:

assertTrue(cv.isEmpty());

And the other methods work with variations around this model:

• invalidName(): Uses an empty name
• invalidEmail(): Uses a malformed email
• invalidId(): Adds some negative numbers to the list

Note that the invalidId() method adds two negative numbers to the list:

asList(-1,-2,1,2));

So, we expect two constraint violations:

assertEquals(2, cv.size());

In other words, Validator checks not only the constraints violated, but how many times they are violated.