如何从任何地方访问Symfony2会话？

I did my own URLGenerator (Symfony\Component\Routing\Generator\UrlGenerator) extending the original URLGenerator. I want to access the Symfony2 session from there, but I can't.

use Symfony\Component\Routing\Generator\UrlGenerator as BaseUrlGenerator;

class UrlGenerator extends BaseUrlGenerator
{
protected function doGenerate($variables, $defaults, $requirements, $tokens, $parameters, $name, $referenceType, $hostTokens)
{
    $url=parent::doGenerate($variables, $defaults, $requirements, $tokens, $parameters, $name, $referenceType, $hostTokens);

   **//Here I want to access to session** 

    return $url;
}

}

I read the documentation but I dont know how to acess. Only can access to session from a controller? I searched here, but I want to invoke a Symfony Object to the session.

\Resoruces\config\services.yml

 parameters:
    router.options.generator_base_class: Ex\Bundle\ExBundle\Routing\Generator\UrlGenerator

UPDATE 1: I added:

ex.service:
    class: Ex\Bundle\ExBundle\Routing\Generator\UrlGenerator
    arguments: [@session]

and:

private $session;

public function __construct($session, RouteCollection $routes, RequestContext $context, LoggerInterface $logger = null)
{
    $this->routes = $routes;
    $this->context = $context;
    $this->logger = $logger;
    $this->session = $session;
}

protected function doGenerate($variables, $defaults, $requirements, $tokens, $parameters, $name, $referenceType, $hostTokens)
{

 $url=parent::doGenerate($variables, $defaults, $requirements, $tokens, $parameters,   $name, $referenceType, $hostTokens);

 $this->session->set($url,$url);

 return $url;
}

** ERROR here on ->set (: Call to a member function set() on a non-object in )

As Maerlyn said maybe you should use a construct to implement this. Have a look on how to use arguments in service: http://symfony.com/doc/current/book/service_container.html

Update 1 : Check the second answer. It could be helpful How do you access a users session from a service in Symfony2?

Update 2 :

For me this code works but Idon't know if my override of Symfony\Component\Routing\Generator\UrlGenerator is a good practice.

app/config/config.yml

ex.service:
  class: Test\YourBundle\Service\UrlGenerator
  arguments: [@router]
  calls:
       - [setSession, ["@session"]]

src/Test/YourBundle/Service/UrlGenerator.php

namespace Test\YourBundle\Service;
use \Symfony\Component\Routing\Generator\UrlGenerator as BaseUrlGenerator;
use \Symfony\Component\Routing\RequestContext as RequestContext;
class UrlGenerator extends BaseUrlGenerator {

    private $session;

    public function __construct(\Symfony\Component\Routing\Router $routes,  \Psr\Log\LoggerInterface $logger = null) {
        parent::__construct($routes->getRouteCollection(), new RequestContext(), $logger);
    }

    public function setSession($session) {
        $this->session = $session;
    }

    public function getSession() {
        return $this->session;
    }


    protected function doGenerate($variables, $defaults, $requirements, $tokens, $parameters, $name, $referenceType, $hostTokens)
    {
     $url=parent::doGenerate($variables, $defaults, $requirements, $tokens, $parameters,   $name, $referenceType, $hostTokens);

     $this->session->set($url,$url);

     return $url;
    }
}

src/Test/YourBundle/Controller/DefaultController.php

class DefaultController {
    // ...
    public function indexAction()
    {
            // Retrieve my service
            $service = $this->get('ex.service');
            echo $service->generate('_demo');
    }
    // ...
}

try this solution

use Symfony\Component\HttpFoundation\Session\Session;`

$session = new Session();
$session->get('variable');