Laravel 的 Session机制简介
前些天，为了解答一个问题，就去研究了Laravel的源码，讲讲我的收获：
这个是问题源：
http://segmentfault.com/q/1010000003776645?_ea=365137
本文讲的Laravel全部使用5.1版本。
我们首先看Laravel是如何创建Session组件的。
首先我们可以看见在Kernerl.php中注册了StartSession这个类（这里不去讨论Laravel的DI和IoC），看下这个类是如何使用的。
Session Start
我们在这个类中的handle方法看到如下代码
public function handle($request, Closure $next){ $this->sessionHandled = true; // If a session driver has been configured, we will need to start the session here // so that the data is ready for an application. Note that the Laravel sessions // do not make use of PHP "native" sessions in any way since they are crappy. if ($this->sessionConfigured()) { $session = $this->startSession($request); $request->setSession($session); } $response = $next($request); // Again, if the session has been configured we will need to close out the session // so that the attributes may be persisted to some storage medium. We will also // add the session identifier cookie to the application response headers now. if ($this->sessionConfigured()) { $this->storeCurrentUrl($request, $session); $this->collectGarbage($session); $this->addCookieToResponse($response, $session); } return $response;}OK，一个典型的过滤器，在这个StartSession中获取Session的方法是getSession。
Get Session
它使用了Laravel注入的SessionManager 这个类的完整限定名是：\Illuminate\Session\SessionManager
/** * Start the session for the given request. * * @param \Illuminate\Http\Request $request * @return \Illuminate\Session\SessionInterface */protected function startSession(Request $request){ with($session = $this->getSession($request))->setRequestOnHandler($request); $session->start(); return $session;}/** * Get the session implementation from the manager. * * @param \Illuminate\Http\Request $request * @return \Illuminate\Session\SessionInterface */public function getSession(Request $request){ $session = $this->manager->driver(); $session->setId($request->cookies->get($session->getName())); return $session;}我们在这个函数中看到，它调用了SessionManager的drive方法，drive方法是SessionManager的父类Manager中定义的，看到它的实现是这样的
/** * Get a driver instance. * * @param string $driver * @return mixed */public function driver($driver = null){ $driver = $driver ?: $this->getDefaultDriver(); // If the given driver has not been created before, we will create the instances // here and cache it so we can return it next time very quickly. If there is // already a driver created by this name, we'll just return that instance. if (! isset($this->drivers[$driver])) { $this->drivers[$driver] = $this->createDriver($driver); } return $this->drivers[$driver];}/** * Create a new driver instance. * * @param string $driver * @return mixed * * @throws \InvalidArgumentException */protected function createDriver($driver){ $method = 'create'.ucfirst($driver).'Driver'; // We'll check to see if a creator method exists for the given driver. If not we // will check for a custom driver creator, which allows developers to create // drivers using their own customized driver creator Closure to create it. if (isset($this->customCreators[$driver])) { return $this->callCustomCreator($driver); } elseif (method_exists($this, $method)) { return $this->$method(); } throw new InvalidArgumentException("Driver [$driver] not supported.");}也就是调用getDefaultDriver方法
/** * Get the default session driver name. * * @return string */public function getDefaultDriver(){ return $this->app['config']['session.driver'];}这里就是我们在app/config/session.php中定义的driver字段，获取这个字段后，我们可以从createDriver的源码看到，它实际上是调用createXXXXDriver的方式获取到driver的。
OK，我们看下最简单的File，也就是createFileDriver
/** * Create an instance of the file session driver. * * @return \Illuminate\Session\Store */protected function createFileDriver(){ return $this->createNativeDriver();}/** * Create an instance of the file session driver. * * @return \Illuminate\Session\Store */protected function createNativeDriver(){ $path = $this->app['config']['session.files']; return $this->buildSession(new FileSessionHandler($this->app['files'], $path));}/** * Build the session instance. * * @param \SessionHandlerInterface $handler * @return \Illuminate\Session\Store */protected function buildSession($handler){ if ($this->app['config']['session.encrypt']) { return new EncryptedStore($this->app['config']['session.cookie'], $handler, $this->app['encrypter'] ); } else { return new Store($this->app['config']['session.cookie'], $handler); }}这个Store就是我们Session的实体了，它的具体读写调用使用抽象的Handler进行，也就是说如果是读文件，就new FileHandler如果是Redis就是new RedisHandler实现read和write即可。
Session ID
回到StartSession 这个类，我们再看getSession这个方法
$session->setId($request->cookies->get($session->getName()));了解Web的人都应该知道，session是根据cookie中存的key来区分不同的会话的，所以sessionId尤为重要，这里我们先不关心Cookie是如何读取，我们知道在这里读取了cookie中存放的SessionId就够了。
接下去看怎么加载数据
Data
依然是StartSession这个类，
我们看startSession这个方法，下一句调用的就是$session->start();，这句话是干嘛呢？经过前面的分析，我们知道$session已经是Store对象了，那么它的start方法如下：
public function start(){ $this->loadSession(); if (! $this->has('_token')) { $this->regenerateToken(); } return $this->started = true;}/** * Load the session data from the handler. * * @return void */protected function loadSession(){ $this->attributes = array_merge($this->attributes, $this->readFromHandler()); foreach (array_merge($this->bags, [$this->metaBag]) as $bag) { $this->initializeLocalBag($bag); $bag->initialize($this->bagData[$bag->getStorageKey()]); }} /** * Read the session data from the handler. * * @return array */protected function readFromHandler(){ $data = $this->handler->read($this->getId()); if ($data) { $data = @unserialize($this->prepareForUnserialize($data)); if ($data !== false && $data !== null && is_array($data)) {return $data; } } return [];}依次调用了loadSession和readFromHandler 好，可以看到从handler中调用了read方法，这个方法就是从我们之前定义的各种handler中读取数据了，它是个抽象方法，在子类中具体实现，然后返回给Store（存入到Store中的attributes数组中去了，到此为止，Session的初始化方法都结束了，
public function get($name, $default = null){ return Arr::get($this->attributes, $name, $default);}这个函数去读取Session中的数据了。
Summary
OK，Session整个初始化的过程总结下：
StartSession#handle->StartSession#startSession->StartSession#getSession->SessionManager#getDefaultDriver->SessionManager#createXXXHandler->Store->Store#setId->Store#startSession
Laravel巧妙的使用了面向对象的接口方式，为我们提供了各种各样不同的存储方式，一旦我们了解了存储方式和加密规则，让不同的web容器进行Session共享的目的也可以达到~