我有这个类用于注册表类：

namespace App\Core;

final class Registry {
    private $data = array();

    /**
     * @param   string  $key
     * 
     * @return  mixed
     */
    public function get($key) {
        return (isset($this->data[$key]) ? $this->data[$key] : null);
    }

    /**
     * @param   string  $key
     * @param   string  $value
     */ 
    public function set($key, $value) {
        $this->data[$key] = $value;
    }

    /**
     * @param   string  $key
     *
     * @return  bool
     */
    public function has($key) {
        return isset($this->data[$key]);
    }

}

而这个主控制器：

namespace App\Core;
use App\Core\Registry;

abstract class Controller {
    protected $registry;

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

    public function __get($key) {
        return $this->registry->get($key); //error line
    }

    public function __set($key, $value) {
        $this->registry->set($key, $value);
    }
}

并需要在索引控制器中获取值：

namespace App\Catalog\Controller;

class IndexController extends \App\Core\Controller {

    public function __construct()
    {
    }

    public function index(){
        $this->db->query();
    }
}

在db类中：

namespace App\Core;

class Db {

    public function __construct()
    {
    }

    public function query(){
        echo 'Im query';
    }
}

现在在主索引中我添加：

require '../vendor/autoload.php';

$registry = new App\Core\Registry();
use App\Core\Db;
$registry->set('db', new Db());

在行动中，我需要从 Db 类打印 Im query 但我看到这个错误：

致命错误：未捕获错误：在/Applications/MAMP/htdocs/mvc/application/Core/Controller.php:16中调用null上的成员函数get（）堆栈跟踪：＃0 / Applications / MAMP / htdocs / mvc / application / Catalog / Controller / IndexController.php（13）：App \ Core \ Controller - > __ get（'db'）＃1 [内部函数]：App \ Catalog \ Controller \ IndexController-> index（）＃2 / Applications / MAMP / htdocs / mvc / application / Core / System / Route.php（649）：call_user_func_array（Array，Array）＃3 /Applications/MAMP/htdocs/mvc/application/Core/System/Route.php(599):Apple \ Core \ System \ Route-> callback（Array，Array）＃4 /Applications/MAMP/htdocs/mvc/public/index.php(63）：App \ Core \ System \ Route-> end（）＃5 在第16行的/Applications/MAMP/htdocs/mvc/application/Core/Controller.php中抛出

注意：我使用composer PSR-4加载所有类，如下所示：

"autoload": {
    "psr-4": {"App\\": "application/"}
  }

怎么办可以解决这个错误？！

php