关于控制器中配置中间件的疑问

代码如下

IndexController

<?php

namespace App\Http\Controllers;
use App\Http\Controllers\Test;

class IndexController extends TestController
{

    public function __construct()
    {
        parent::__construct();

        $this->middleware(function($request,$next){
            //1. echo  $this->a;
            return $next($request);
        });
        //2. echo  $this->a;
    }

    public function index()
    {

       //echo 111;
    }
}

TestController

<?php

namespace App\Http\Controllers;

class TestController extends Controller
{

    public function __construct()
    {
        $this->middleware(function($request,$next){
            $this->a = 11;
            return $next($request);
        });

    }

}

web.php

Route::get('/', 'IndexController@index');

疑问

为什么在IndexController中的构造函数的1.处能输出a值,在2.处就无法获取该属性,控制器中的中间件闭包是怎么执行的，看了看源码没看懂，懂的人希望指点一二