如果你对一件工具的使用原理了如指掌，那么你在用这件工具的时候会充满信心！
一旦用户（浏览器）发送了一个HTTP请求，我们的apache或者nginx一般都转到index.php，因此，之后的一系列步骤都是从index.php开始的，我们先来看一看这个文件代码。
<?php
require __DIR__.‘/../bootstrap/autoload.php‘;
$app = require_once __DIR__.‘/../bootstrap/app.php‘;
/*
|--------------------------------------------------------------------------
| Run The Application
|--------------------------------------------------------------------------
|
| Once we have the application, we can handle the incoming request
| through the kernel, and send the associated response back to
| the client‘s browser allowing them to enjoy the creative
| and wonderful application we have prepared for them.
|
*/
$kernel = $app->make(Illuminate\Contracts\Http\Kernel::class);
$response = $kernel->handle(
$request = Illuminate\Http\Request::capture()
);
$response->send();
$kernel->terminate($request, $response);
作者在注释里谈了$kernel的作用，$kernel处理来访的请求，并且发送相应返回给用户浏览器。
这里又涉及到了一个$app对象，所以附上$app对象的源码，这份源码是\bootstrap\app.php
<?php
/*
|--------------------------------------------------------------------------
| Create The Application
|--------------------------------------------------------------------------
|
| The first thing we will do is create a new Laravel application instance
| which serves as the "glue" for all the components of Laravel, and is
| the IoC container for the system binding all of the various parts.
|
*/
$app = new Illuminate\Foundation\Application(
realpath(__DIR__.‘/../‘)
);
/*
|--------------------------------------------------------------------------
| Bind Important Interfaces
|--------------------------------------------------------------------------
|
| Next, we need to bind some important interfaces into the container so
| we will be able to resolve them when needed. The kernels serve the
| incoming requests to this application from both the web and CLI.
|
*/
$app->singleton(
Illuminate\Contracts\Http\Kernel::class,
App\Http\Kernel::class
);
$app->singleton(
Illuminate\Contracts\Console\Kernel::class,
App\Console\Kernel::class
);
$app->singleton(
Illuminate\Contracts\Debug\ExceptionHandler::class,
App\Exceptions\Handler::class
);
/*
|--------------------------------------------------------------------------
| Return The Application
|--------------------------------------------------------------------------
|
| This script returns the application instance. The instance is given to
| the calling script so we can separate the building of the instances
| from the actual running of the application and sending responses.
|
*/
return $app;
Application是Container的子类，所以$app不仅是Application类的对象，还是Container的对象，所以，新出现的singleton函数我们就可以到Container类的源代码文件里查。bind函数和singleton的区别见这篇博文。
singleton这个函数，前一个参数是实际类名，后一个参数是类的“别名”。
$app对象声明了3个单例模型对象，分别是HttpKernel，ConsoleKernel，ExceptionHandler。请注意，这里并没有创建对象，只是声明，也只是起了一个“别名”。
大家有没有发现，index.php中也有一个$kernel变量，但是只保存了make出来的HttpKernel变量，因此本文不再讨论，ConsoleKernel，ExceptionHandler。。。
继续在文件夹下找到App\Http\Kernel.php，既然我们把实际的HttpKernel做的事情都写在这个php文件里，就从这份代码里看看究竟做了哪些事？
<?php
namespace App\Http;
use Illuminate\Foundation\Http\Kernel as HttpKernel;
class Kernel extends HttpKernel
{
/**
* The application‘s global HTTP middleware stack.
*
* These middleware are run during every request to your application.
*
* @var array
*/
protected $middleware = [
\Illuminate\Foundation\Http\Middleware\CheckForMaintenanceMode::class,
//\App\Http\Middleware\MyMiddleware::class,
];
/**
* The application‘s route middleware groups.
*
* @var array
*/
protected $middlewareGroups = [
‘web‘ => [
\App\Http\Middleware\EncryptCookies::class,
\Illuminate\Cookie\Middleware\AddQueuedCookiesToResponse::class,
\Illuminate\Session\Middleware\StartSession::class,
\Illuminate\View\Middleware\ShareErrorsFromSession::class,
\App\Http\Middleware\VerifyCsrfToken::class,
],
‘api‘ => [
‘throttle:60,1‘,
],
];
/**
* The application‘s route middleware.
*
* These middleware may be assigned to groups or used individually.
*
* @var array
*/
protected $routeMiddleware = [
‘auth‘ => \App\Http\Middleware\Authenticate::class,
‘auth.basic‘ => \Illuminate\Auth\Middleware\AuthenticateWithBasicAuth::class,
‘guest‘ => \App\Http\Middleware\RedirectIfAuthenticated::class,
‘throttle‘ => \Illuminate\Routing\Middleware\ThrottleRequests::class,
‘mymiddleware‘=>\App\Http\Middleware\MyMiddleware::class,
];
}
一目了然，HttpKernel里定义了中间件数组。
该做的做完了，就开始了请求到响应的过程，见index.php
$response = $kernel->handle(
$request = Illuminate\Http\Request::capture()
);
$response->send();
最后在中止，释放所有资源。
/**
* Call the terminate method on any terminable middleware.
*
* @param \Illuminate\Http\Request $request
* @param \Illuminate\Http\Response $response
* @return void
*/
public function terminate($request, $response)
{
$this->terminateMiddleware($request, $response);
$this->app->terminate();
}
总结一下，简单归纳整个过程就是：
1.index.php加载\bootstrap\app.php，创建$app这个容器，然后创建1个单例模式的对象HttpKernel。
2.其中HttpKernel在请求来访之前，开始做初始化工作，比如中间件的声明。
原文：http://www.cnblogs.com/sweng/p/6391542.html