Laravel 版本：5.4.0

Laravel 在引入 composer 自动加载 后，进行了极为重要的一步，Application 实例化。该类的全称是 Illuminate\Foundation\Application，查看它的 __construct 方法，可以看到它进行了如下的四步。

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

        $this->registerBaseBindings();

        $this->registerBaseServiceProviders();

        $this->registerCoreContainerAliases();
    }

第一步：setBasePath

第一步依赖于初始化时传入的路径参数，路径为项目的根目录。在 setBasePath 方法中，bindPathsInContainer 方法被调用。这里又用到了 instance 方法，把路径填入了类的 $instances 数组中。

    protected function bindPathsInContainer()
    {
        $this->instance('path', $this->path());
        $this->instance('path.base', $this->basePath());
        $this->instance('path.lang', $this->langPath());
        $this->instance('path.config', $this->configPath());
        $this->instance('path.public', $this->publicPath());
        $this->instance('path.storage', $this->storagePath());
        $this->instance('path.database', $this->databasePath());
        $this->instance('path.resources', $this->resourcePath());
        $this->instance('path.bootstrap', $this->bootstrapPath());
    }

第二步：registerBaseBindings

    protected function registerBaseBindings()
    {
        static::setInstance($this);

        $this->instance('app', $this);

        $this->instance(Container::class, $this);
    }

这里同样调用了 instance 方法，但与之前不同的是，这里传入的第二个参数是一个类，而之前的是一个字符串。在第二步中，第一行它为 Application 类注册了一个名为 $instance 的静态实例，二三两行则为 Application 实例中的 $instances 数组属性填入了两个 key 为 'app' 和 Container::class，value 为 $this 的值。

第三步：registerBaseServiceProviders

    protected function registerBaseServiceProviders()
    {
        $this->register(new EventServiceProvider($this));

        $this->register(new LogServiceProvider($this));

        $this->register(new RoutingServiceProvider($this));
    }

第三步注册了三个 ServiceProvider，在注册过程中，ServiceProvider 的 register 方法会被执行，每一个 ServiceProvider 一般都会有 register 方法。

    public function register($provider, $options = [], $force = false)
    {
        if (($registered = $this->getProvider($provider)) && ! $force) {
            return $registered;
        }

        // If the given "provider" is a string, we will resolve it, passing in the
        // application instance automatically for the developer. This is simply
        // a more convenient way of specifying your service provider classes.
        if (is_string($provider)) {
            $provider = $this->resolveProvider($provider);
        }

        if (method_exists($provider, 'register')) {
            $provider->register();
        }

        $this->markAsRegistered($provider);

        // If the application has already booted, we will call this boot method on
        // the provider class so it has an opportunity to do its boot logic and
        // will be ready for any usage by this developer's application logic.
        if ($this->booted) {
            $this->bootProvider($provider);
        }

        return $provider;
    }

第四步：registerCoreContainerAliases

    public function registerCoreContainerAliases()
    {
        $aliases = [
            'app'                  => [\Illuminate\Foundation\Application::class, \Illuminate\Contracts\Container\Container::class, \Illuminate\Contracts\Foundation\Application::class],
            'auth'                 => [\Illuminate\Auth\AuthManager::class, \Illuminate\Contracts\Auth\Factory::class],
            'auth.driver'          => [\Illuminate\Contracts\Auth\Guard::class],
            'blade.compiler'       => [\Illuminate\View\Compilers\BladeCompiler::class],
            'cache'                => [\Illuminate\Cache\CacheManager::class, \Illuminate\Contracts\Cache\Factory::class],
            'cache.store'          => [\Illuminate\Cache\Repository::class, \Illuminate\Contracts\Cache\Repository::class],
            'config'               => [\Illuminate\Config\Repository::class, \Illuminate\Contracts\Config\Repository::class],
            'cookie'               => [\Illuminate\Cookie\CookieJar::class, \Illuminate\Contracts\Cookie\Factory::class, \Illuminate\Contracts\Cookie\QueueingFactory::class],
            'encrypter'            => [\Illuminate\Encryption\Encrypter::class, \Illuminate\Contracts\Encryption\Encrypter::class],
            'db'                   => [\Illuminate\Database\DatabaseManager::class],
            'db.connection'        => [\Illuminate\Database\Connection::class, \Illuminate\Database\ConnectionInterface::class],
            'events'               => [\Illuminate\Events\Dispatcher::class, \Illuminate\Contracts\Events\Dispatcher::class],
            'files'                => [\Illuminate\Filesystem\Filesystem::class],
            'filesystem'           => [\Illuminate\Filesystem\FilesystemManager::class, \Illuminate\Contracts\Filesystem\Factory::class],
            'filesystem.disk'      => [\Illuminate\Contracts\Filesystem\Filesystem::class],
            'filesystem.cloud'     => [\Illuminate\Contracts\Filesystem\Cloud::class],
            'hash'                 => [\Illuminate\Contracts\Hashing\Hasher::class],
            'translator'           => [\Illuminate\Translation\Translator::class, \Illuminate\Contracts\Translation\Translator::class],
            'log'                  => [\Illuminate\Log\Writer::class, \Illuminate\Contracts\Logging\Log::class, \Psr\Log\LoggerInterface::class],
            'mailer'               => [\Illuminate\Mail\Mailer::class, \Illuminate\Contracts\Mail\Mailer::class, \Illuminate\Contracts\Mail\MailQueue::class],
            'auth.password'        => [\Illuminate\Auth\Passwords\PasswordBrokerManager::class, \Illuminate\Contracts\Auth\PasswordBrokerFactory::class],
            'auth.password.broker' => [\Illuminate\Auth\Passwords\PasswordBroker::class, \Illuminate\Contracts\Auth\PasswordBroker::class],
            'queue'                => [\Illuminate\Queue\QueueManager::class, \Illuminate\Contracts\Queue\Factory::class, \Illuminate\Contracts\Queue\Monitor::class],
            'queue.connection'     => [\Illuminate\Contracts\Queue\Queue::class],
            'queue.failer'         => [\Illuminate\Queue\Failed\FailedJobProviderInterface::class],
            'redirect'             => [\Illuminate\Routing\Redirector::class],
            'redis'                => [\Illuminate\Redis\RedisManager::class, \Illuminate\Contracts\Redis\Factory::class],
            'request'              => [\Illuminate\Http\Request::class, \Symfony\Component\HttpFoundation\Request::class],
            'router'               => [\Illuminate\Routing\Router::class, \Illuminate\Contracts\Routing\Registrar::class, \Illuminate\Contracts\Routing\BindingRegistrar::class],
            'session'              => [\Illuminate\Session\SessionManager::class],
            'session.store'        => [\Illuminate\Session\Store::class, \Illuminate\Contracts\Session\Session::class],
            'url'                  => [\Illuminate\Routing\UrlGenerator::class, \Illuminate\Contracts\Routing\UrlGenerator::class],
            'validator'            => [\Illuminate\Validation\Factory::class, \Illuminate\Contracts\Validation\Factory::class],
            'view'                 => [\Illuminate\View\Factory::class, \Illuminate\Contracts\View\Factory::class],
        ];

        foreach ($aliases as $key => $aliases) {
            foreach ($aliases as $alias) {
                $this->alias($key, $alias);
            }
        }
    }

这一步是在为一些核心类注册别名。

四步走下来，Application 的实例化就算完成了。