php - Zend form and annotations validation

I'm trying using annotatnions to build and validate zend forms. But currently I recive an error when I open showformAction:

"Fatal error: Uncaught exception 'Zend\Form\Exception\InvalidElementException' with message 'No element by the name of [username] found in form' ..."

So below is my code. What I doing wrong ?

Entity\User.php

namespace Application\Model;
use Zend\Form\Annotation;

/**
 * @Annotation\Hydrator("Zend\Stdlib\Hydrator\ObjectProperty")
 * @Annotation\Name("user")
 */
class User
{
    /**
     * @Annotation\Attributes({"type":"text" })
     * @Annotation\Validator({"type":"Regex","options":{"regex":"/^[a-zA-Z][a-zA-Z0-9_-]{1,19}/"}})
     * @Annotation\Options({"label":"Username:"})
     */
    public $username;

}
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Controller\ProductsController.php

namespace Application\Controller;

use Zend\Mvc\Controller\AbstractActionController;
use Zend\Json\Json;
use Zend\View\Model\JsonModel;
use Zend\View\Model\ViewModel;
use Zend\Debug\Debug;
use Application\Entity\Products;
use Application\Entity\Category;
use Application\Form\ProductsForm;
use Doctrine\ORM\EntityManager;
use Application\Model\User;
use Zend\Form\Annotation\AnnotationBuilder;

class ProductsController extends AbstractActionController {

  protected $albumTable;
  protected $em;
  protected $form;

 public function savetodb($data) {
    //code save to db ....
  }

  protected function getForm() {
    $entity = new User();
    $builder = new AnnotationBuilder();
    $this->form = $builder->createForm($entity);

    return $this->form;
  }

  public function showformAction() {
    $viewmodel = new ViewModel();
    $form = $this->getForm();
    $request = $this->getRequest();

    //disable layout if request by Ajax
    $viewmodel->setTerminal($request->isXmlHttpRequest());

    $is_xmlhttprequest = 1;
    if (!$request->isXmlHttpRequest()) {
      //if NOT using Ajax
      $is_xmlhttprequest = 0;
      if ($request->isPost()) {
        $form->setData($request->getPost());
        if ($form->isValid()) {
          //save to db <span title=";)">;)</span>
          $this->savetodb($form->getData());
        }
      }
    }

    $viewmodel->setVariables(array(
      'form' => $form,
      // is_xmlhttprequest is needed for check this form is in modal dialog or not
      // in view
      'is_xmlhttprequest' => $is_xmlhttprequest
    ));

    return $viewmodel;
  }

  public function validatepostajaxAction() {
    $form = $this->getForm();
    $request = $this->getRequest();
    $response = $this->getResponse();

    $messages = array();
    if ($request->isPost()) {
      $form->setData($request->getPost());
      if (!$form->isValid()) {
        $errors = $form->getMessages();
        foreach ($errors as $key => $row) {
          if (!empty($row) && $key != 'submit') {
            foreach ($row as $keyer => $rower) {
              //save error(s) per-element that
              //needed by Javascript
              $messages[$key][] = $rower;
            }
          }
        }
      }

      if (!empty($messages)) {
        $response->setContent(\Zend\Json\Json::encode($messages));
      } else {
        //save to db <span title=";)">;)</span>
        $this->savetodb($form->getData());
        $response->setContent(\Zend\Json\Json::encode(array('success' => 1)));
      }
    }

    return $response;
  }

}
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