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Kernel.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~
