上次说了provider，那么这次来说说facade

首先是启动的源头，从laravel的kernel类中的$bootstrappers 数组，我们可以看到它的一些系统引导方法，其中的RegisterFacades便是用来注册facade门面类的了。

    protected $bootstrappers = [
        \Illuminate\Foundation\Bootstrap\LoadEnvironmentVariables::class,
        \Illuminate\Foundation\Bootstrap\LoadConfiguration::class,
        \Illuminate\Foundation\Bootstrap\HandleExceptions::class,
        \Illuminate\Foundation\Bootstrap\RegisterFacades::class,
        \Illuminate\Foundation\Bootstrap\RegisterProviders::class,
        \Illuminate\Foundation\Bootstrap\BootProviders::class,
    ];

 

 同样是有一个register类，通过这个类进行别名等注册操作

namespace Illuminate\Foundation\Bootstrap;

use Illuminate\Foundation\AliasLoader;
use Illuminate\Support\Facades\Facade;
use Illuminate\Foundation\PackageManifest;
use Illuminate\Contracts\Foundation\Application;

class RegisterFacades
{
    /**
     * Bootstrap the given application.
     *
     * @param  \Illuminate\Contracts\Foundation\Application  $app
     * @return void
     */
    public function bootstrap(Application $app)
    {
        //清除facade所有已绑定实例
        Facade::clearResolvedInstances();
        //将app存入对象
        Facade::setFacadeApplication($app);
        //将这些别名数组通过构造函数存入对象中
        AliasLoader::getInstance(array_merge(
            //通过Illuminate\Config\Repository获取config/app.php中的aliases数组
            $app->make('config')->get('app.aliases', []),
            //还记得最开始的博文中提到的那个包清单吗，这里也从中获取了它的aliases
            $app->make(PackageManifest::class)->aliases()
        //通过spl_autoload_register函数注册另一种自动加载函数
        ))->register();
    }
}

    public static function getInstance(array $aliases = [])
    {
        //注册时的实例是空的，会通过构造方法把刚刚传入的别名数组存入对象
        if (is_null(static::$instance)) {
            return static::$instance = new static($aliases);
        }

        $aliases = array_merge(static::$instance->getAliases(), $aliases);

        static::$instance->setAliases($aliases);

        return static::$instance;
    }

    private function __construct($aliases)
    {
        $this->aliases = $aliases;
    }

    public function register()
    {
        if (! $this->registered) {
            $this->prependToLoaderStack();

            $this->registered = true;
        }
    }

    /**
     * Prepend the load method to the auto-loader stack.
     *
     * @return void
     */
    protected function prependToLoaderStack()
    {
        //最后通过自动加载函数将本对象中的load方法放入自动加载队列的前端，在我们通过类名调用方法时，会触发自动加载函数队列，会优先触发这个函数，查找到对应文件的路径然后加载相应文件
        spl_autoload_register([$this, 'load'], true, true);
    }

    public function load($alias)
    {
        //若传入的命名空间不为当前facade数组内才会通过这个方法加载
        if (static::$facadeNamespace && strpos($alias, static::$facadeNamespace) === 0) {
            $this->loadFacade($alias);

            return true;
        }
        //否则直接返回其别名
        if (isset($this->aliases[$alias])) {
            return class_alias($this->aliases[$alias], $alias);
        }
    }

    /**
     * Load a real-time facade for the given alias.
     *
     * @param  string  $alias
     * @return void
     */
    protected function loadFacade($alias)
    {
        require $this->ensureFacadeExists($alias);
    }

    /**
     * Ensure that the given alias has an existing real-time facade class.
     *
     * @param  string  $alias
     * @return string
     */
    protected function ensureFacadeExists($alias)
    {
        //从缓存中返回路径
        if (file_exists($path = storage_path('framework/cache/facade-'.sha1($alias).'.php'))) {
            return $path;
        }
        
        file_put_contents($path, $this->formatFacadeSt