Drupal - Redirect Anonymous user to login page

You can test a user's status very early with event subscriber in a custom module that subscribes to KernelEvents::REQUEST.

First, you register the event subscriber in mymodule.services.yml in your module folder:

services:
  mymodule.event_subscriber:
    class: Drupal\mymodule\EventSubscriber\RedirectAnonymousSubscriber
    arguments: []
    tags:
      - {name: event_subscriber}

Then add RedirectAnonymousSubscriber.php for your custom event subscriber in your module in the /src/EventSubscriber/ folder.

namespace Drupal\mymodule\EventSubscriber;

use Symfony\Component\HttpFoundation\RedirectResponse;
use Symfony\Component\HttpKernel\KernelEvents;
use Symfony\Component\HttpKernel\Event\GetResponseEvent;
use Symfony\Component\EventDispatcher\EventSubscriberInterface;

/**
 * Event subscriber subscribing to KernelEvents::REQUEST.
 */
class RedirectAnonymousSubscriber implements EventSubscriberInterface {

  public function __construct() {
    $this->account = \Drupal::currentUser();
  }

  public function checkAuthStatus(GetResponseEvent $event) {

    if ($this->account->isAnonymous() && \Drupal::routeMatch()->getRouteName() != 'user.login') {

      // add logic to check other routes you want available to anonymous users,
      // otherwise, redirect to login page.
      $route_name = \Drupal::routeMatch()->getRouteName();
      if (strpos($route_name, 'view') === 0 && strpos($route_name, 'rest_') !== FALSE) {
        return;
      }

      $response = new RedirectResponse('/user/login', 301);
      $event->setResponse($response);
      $event->stopPropagation();
    }
  }

  public static function getSubscribedEvents() {
    $events[KernelEvents::REQUEST][] = array('checkAuthStatus');
    return $events;
  }

}

First, create a service for your event subscriber in module-name.services.yml

Code -

services:
    [MODULE-NAME]_event_subscriber:
        class: Drupal\MODULE-NAME\EventSubscriber\[Event-Subscriber-class]
        tags:
        - {name: event_subscriber}

Create your own eventsubscriber class inside modules/module-name/src/EventSubscriber directory:

use Symfony\Component\EventDispatcher\EventSubscriberInterface;
use Symfony\Component\HttpFoundation\RedirectResponse;
use Symfony\Component\HttpKernel\Event\GetResponseEvent;
use Symfony\Component\HttpKernel\KernelEvents;

class Event-Subscriber-class implements EventSubscriberInterface {

  private $redirectCode = 301;

  public function checkForRedirection2(GetResponseEvent $event) {
    $account = \Drupal::currentUser(); 
    if (empty($account->id()) {
      $response = new RedirectResponse('/', $this->redirectCode);
      $response->send();
      exit(0);
    }
  }

  public static function getSubscribedEvents() {
    $events[KernelEvents::REQUEST][] = array('checkForRedirection2');
    return $events;
  }
}

On Drupal 8.3.3, this code causes an endless redirection. I've fixed that by adding that instead.

..
$response = new RedirectResponse('/user/login', 301);
$response->send();
..