How to get the root dir of the Symfony2 application?

UPDATE 2018-10-21:

As of this week, getRootDir() was deprecated. Please use getProjectDir() instead, as suggested in the comment section by Muzaraf Ali.

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Use this:

$this->get('kernel')->getRootDir();

And if you want the web root:

$this->get('kernel')->getRootDir() . '/../web' . $this->getRequest()->getBasePath();

this will work from controller action method…

EDIT:
As for the services, I think the way you did it is as clean as possible, although I would pass complete kernel service as an argument… but this will also do the trick…