如何在CakePHP框架中使用表单验证？

<?= $this->Form->create($user) ?>
    <?= $this->Form->input('username') ?>
    <?= $this->Form->input('password') ?>
    <?= $this->Form->button(__('Submit')) ?>
<?= $this->Form->end() ?>

<?php
namespace AppModelEntity;

use CakeORMEntity;
use CakeAuthDefaultPasswordHasher;

class User extends Entity
{
    protected $_accessible = [
        '*' => true,
        'id' => false
    ];

    protected function _setPassword($password)
    {
        return (new DefaultPasswordHasher)->hash($password);
    }

    public function validationDefault(Validator $validator)
    {
        $validator
            ->notEmptyString('username', __('Username cannot be empty'))
            ->maxLength('username', 50, __('Username cannot be longer than 50 characters'))
            ->notEmptyString('password', __('Password cannot be empty'))
            ->maxLength('password', 255, __('Password cannot be longer than 255 characters'));

        return $validator;
    }
}

public function add()
{
    $user = $this->Users->newEntity();
    if ($this->request->is('post')) {
        $user = $this->Users->patchEntity($user, $this->request->getData());
        if ($this->Users->save($user)) {
            $this->Flash->success(__('The user has been saved.'));

            return $this->redirect(['action' => 'index']);
        }
        $this->Flash->error(__('The user could not be saved. Please, try again.'));
    }
    $this->set(compact('user'));
    $this->viewBuilder()->setLayout('login');
}