class MaintenanceModeSubscriber

class MaintenanceModeSubscriber implements EventSubscriberInterface {
    
    /**
     * The maintenance mode.
     *
     * @var \Drupal\Core\Site\MaintenanceMode
     */
    protected $maintenanceMode;
    
    /**
     * The current account.
     *
     * @var \Drupal\Core\Session\AccountInterface
     */
    protected $account;
    
    /**
     * Constructs a new MaintenanceModeSubscriber.
     *
     * @param \Drupal\Core\Site\MaintenanceModeInterface $maintenance_mode
     *   The maintenance mode.
     * @param \Drupal\Core\Session\AccountInterface $account
     *   The current user.
     */
    public function __construct(MaintenanceModeInterface $maintenance_mode, AccountInterface $account) {
        $this->maintenanceMode = $maintenance_mode;
        $this->account = $account;
    }
    
    /**
     * Logout users if site is in maintenance mode.
     *
     * @param \Symfony\Component\HttpKernel\Event\GetResponseEvent $event
     *   The event to process.
     */
    public function onKernelRequestMaintenance(GetResponseEvent $event) {
        $request = $event->getRequest();
        $route_match = RouteMatch::createFromRequest($request);
        if ($this->maintenanceMode
            ->applies($route_match)) {
            // If the site is offline, log out unprivileged users.
            if ($this->account
                ->isAuthenticated() && !$this->maintenanceMode
                ->exempt($this->account)) {
                user_logout();
                // Redirect to homepage.
                $event->setResponse(new RedirectResponse(Url::fromRoute('<front>')->toString()));
            }
        }
    }
    
    /**
     * {@inheritdoc}
     */
    public static function getSubscribedEvents() {
        $events[KernelEvents::REQUEST][] = [
            'onKernelRequestMaintenance',
            31,
        ];
        return $events;
    }

}