Symfony ile tek bir class alanı için assert validation constraint yaratma

namespace Application\BackendBundle\Model\User;

use JMS\Serializer\Annotation as Serializer;
use Symfony\Component\Validator\Constraints as Assert;
use Application\BackendBundle\Validator\Constraint as BackendAssert;

class Create
{
    /**
     * @var string
     *
     * @BackendAssert\Username
     *
     * @Serializer\Type("string")
     */
    public $username;

    /**
     * @var string
     *
     * @Assert\NotBlank(message="Password is required")
     *
     * @Serializer\Type("string")
     */
    public $password;
}

namespace Application\BackendBundle\Validator\Constraint;

use Symfony\Component\Validator\Constraint;

/**
 * @Annotation
 */
class Username extends Constraint
{
    const INVALID_CONTENT = 'Username must form of only alphabetic and numeric characters.';
    const MIN_LENGTH = 'Username cannot be less than 6 characters.';
    const MAX_LENGTH = 'Username cannot be longer than 20 characters.';

    public function validatedBy()
    {
        return get_class($this).'Validator';
    }
}

namespace Application\BackendBundle\Validator\Constraint;

use Symfony\Component\Validator\Constraint;
use Symfony\Component\Validator\ConstraintValidator;

class UsernameValidator extends ConstraintValidator
{
    public function validate($value, Constraint $constraint)
    {
        if (! ctype_alnum($value)) {
            $this->context
                ->buildViolation($constraint::INVALID_CONTENT)
                ->addViolation();
        } else if (mb_strlen($value) < 6) {
            $this->context
                ->buildViolation($constraint::MIN_LENGTH)
                ->addViolation();
        } else if (mb_strlen($value) > 20) {
            $this->context
                ->buildViolation($constraint::MAX_LENGTH)
                ->addViolation();
        }
    }
}