How to get the logged-in user's ID in Symfony

I am developing an application using Symfony and Doctrine ORM. I would like to know how to get the currently logged-in user's ID.

Current Symfony versions (Symfony 4, Symfony >=3.2)

Since Symfony >=3.2 you can simply expect a UserInterface implementation to be injected to your controller action directly. You can then call getId() to retrieve user's identifier:

class DefaultController extends Controller
{
    // when the user is mandatory (e.g. behind a firewall)
    public function fooAction(UserInterface $user)
    {
        $userId = $user->getId(); 
    }

    // when the user is optional (e.g. can be anonymous)
    public function barAction(UserInterface $user = null) 
    {
        $userId = null !== $user ? $user->getId() : null;
    }
}

You can still use the security token storage as in all Symfony versions since 2.6. For example, in your controller:

$user = $this->get('security.token_storage')->getToken()->getUser();

Note that the Controller::getUser() shortcut mentioned in the next part of this answer is no longer encouraged.

Legacy Symfony versions

The easiest way to access the user used to be to extend the base controller, and use the shortcut getUser() method:

$user = $this->getUser();

Since Symfony 2.6 you can retrieve a user from the security token storage:

$user = $this->get('security.token_storage')->getToken()->getUser();

Before Symfony 2.6, the token was accessible from the security context service instead:

$user = $this->get('security.context')->getToken()->getUser();

Note that the security context service is deprecated in Symfony 2 and was removed in Symfony 3.0.