symfony: [Semantical Error] The annotation does not exist, or could not be auto-loaded.

This model

<?php

use Doctrine\ORM\Mapping as ORM;
use Symfony\Component\Validator\Constraints as Assert;

/**
 * @ORM\Entity
 */
class Order
{
    /**
     * @ORM\Id
     * @ORM\Column(type="integer")
     * @ORM\GeneratedValue(strategy="AUTO")
     */
    protected $id;

    /**
     * @Assert\NotBlank()
     */
    public $date;

    public function setId($id)
    {
        $this->id = $id;
    }

    public function getId()
    {
        return $this->id;
    }
}

raise this error

[Semantical Error] The annotation "@Symfony\Component\Validator\Constraints\NotBlank" in property Order::$date does not exist, or could not be auto-loaded.

This is because DocParser->autoloadAnnotations is false by default. How can I fix this?