1、public\index.php中调用了bootstrap\app.php：

$app = require_once __DIR__.'/../bootstrap/app.php';

2、bootstrap\app.php中调用了vendor\laravel\framework\src\Illuminate\Foundation\Application.php：

$app = new Illuminate\Foundation\Application(    realpath(__DIR__.'/../'));

　　然后再vendor\laravel\framework\src\Illuminate\Foundation\Application.php构造函数中调用了registerCoreContainerAliases方法设置了别名，如'db' => [\Illuminate\Database\DatabaseManager::class]：

public function __construct($basePath = null)    {        if ($basePath) {            $this->setBasePath($basePath);        }        $this->registerBaseBindings();        $this->registerBaseServiceProviders();        $this->registerCoreContainerAliases();    }

3、bootstrap\app.php中调用了singleton函数将类App\Http\Kernel绑定到Illuminate\Contracts\Http\Kernel中：

$app->singleton(    Illuminate\Contracts\Http\Kernel::class,    App\Http\Kernel::class);

4、public\index.php中调用了make方法生成类App\Http\Kernel的实例：

$kernel = $app->make(Illuminate\Contracts\Http\Kernel::class);

5、public\index.php中调用了类App\Http\Kernel实例的父类Illuminate\Foundation\Http\Kernel的handle方法：

$response = $kernel->handle(    $request = Illuminate\Http\Request::capture());

　　在handle方法中调用了sendRequestThroughRouter方法：

public function handle($request)    {        try {            $request->enableHttpMethodParameterOverride();            $response = $this->sendRequestThroughRouter($request);        } catch (Exception $e) {            $this->reportException($e);            $response = $this->renderException($request, $e);        } catch (Throwable $e) {            $this->reportException($e = new FatalThrowableError($e));            $response = $this->renderException($request, $e);        }        $this->app['events']->dispatch(            new Events\RequestHandled($request, $response)        );        return $response;    }

　　在sendRequestThroughRouter方法中调用了bootstrap方法：

protected function sendRequestThroughRouter($request)    {        $this->app->instance('request', $request);        Facade::clearResolvedInstance('request');        $this->bootstrap();        return (new Pipeline($this->app))                    ->send($request)                    ->through($this->app->shouldSkipMiddleware() ? [] : $this->middleware)                    ->then($this->dispatchToRouter());    }

　　在bootstrap方法中调用了容器vendor\laravel\framework\src\Illuminate\Foundation\Application.php中的bootstrapWith方法执行了$bootstrappers中所有类的bootstrap方法，其中包含\Illuminate\Foundation\Bootstrap\RegisterFacades类：

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,    ];    public function bootstrap()    {        if (! $this->app->hasBeenBootstrapped()) {            $this->app->bootstrapWith($this->bootstrappers());        }    }    protected function bootstrappers()    {        return $this->bootstrappers;    }

　　

public function bootstrapWith(array $bootstrappers)    {        $this->hasBeenBootstrapped = true;        foreach ($bootstrappers as $bootstrapper) {            $this['events']->fire('bootstrapping: '.$bootstrapper, [$this]);            $this->make($bootstrapper)->bootstrap($this);            $this['events']->fire('bootstrapped: '.$bootstrapper, [$this]);        }    }

6、vendor\laravel\framework\src\Illuminate\Foundation\Bootstrap\RegisterFacades.php中的bootstrap方法中setFacadeApplication方法设置容器信息：

public function bootstrap(Application $app)    {        Facade::clearResolvedInstances();        Facade::setFacadeApplication($app);        AliasLoader::getInstance($app->make('config')->get('app.aliases', []))->register();    }

7、在控制器中调用DB对应的方法时：

use Illuminate\Support\Facades\DB;$data = DB::connection('数据库连接名')->select('sql语句');

　　由于没有相应的静态函数，首先会调用DB的父类Illuminate\Support\Facades\Facade中的__callStatic方法：

public static function __callStatic($method, $args)    {        $instance = static::getFacadeRoot();        if (! $instance) {            throw new RuntimeException('A facade root has not been set.');        }        return $instance->$method(...$args);    }

　　在__callStatic方法中通过static::getFacadeRoot()方法实例化Illuminate\Support\Facades\DB::getFacadeAccessor()别名对应的类，然后通过$instance->$method(...$args)调用实例中对应的方法：

protected static function getFacadeAccessor()    {        return 'db';    }