1. Resolver.php

<?php

class DbResolver implements ResolverInterface
{
    protected $adapter;

    public function __construct(AuthAdapter $adapter)
    {
        $this->adapter = $adapter;
    }
}

2. Factory.php

<?php

namespace MyModule\Factory;

use Zend\ServiceManager\FactoryInterface;
use Zend\ServiceManager\ServiceLocatorInterface;

class AuthAdapterFactory implements FactoryInterface
{
    public function createService(ServiceLocatorInterface $sl)
    {
        $db = $sl->get('Zend\Db\Adapter\Adapter');

        $adapter = new AuthAdapter($db,
          'users',
          'user_login',
          'user_password',
          'MD5(?)'
      );

      return $adapter;
    }
}

3. module.config.php

'db' => array(
    'driver'         => 'Pdo',
    'dsn'            => 'mysql:dbname=zf2tutorial;host=localhost',
),
'service_manager' => array(
    'factories' => array(
        'AuthAdapter' => 'MyModule\Factory\AuthAdapterFactory',
        'Zend\Db\Adapter\Adapter' => 'Zend\Db\Adapter\AdapterServiceFactory',
    ),
),