@RunWith(MockitoJUnitRunner.class)
public class UniqueConstraintValidatorTest  {
    @Mock
    private EntityManager entityManager;

    @InjectMocks
    private UniqueConstraintValidator validator;

    @Test
    testMethod() {

@Stateless
public class UniqueConstraintValidator implements ConstraintValidator<UniqueName, String> {

    //@Inject
    public EntityManager entityManager;


    @Override
    public void initialize(UniqueName constraintAnnotation) {}

    @Override
    public boolean isValid(String value, ConstraintValidatorContext context) {
        QClient client = QClient.client;
        JPAQuery query = new JPAQuery(entityManager);
        query.from(client);
        query.where(client.name.eq(value));
        List results = query.createQuery().getResultList();
 
        if (results.size() > 0) {
            return false;
        }
        return true;
    }

}

entityManager.setFlushMode(FlushModeType.COMMIT);