How to redirect an anonymous user to the login form after a 403 error in Drupal 8 & 9

How to redirect an anonymous user to the login form after a 403 error in Drupal 8 & 9

Code snippet that can be used to redirect an anonymous user to the login form after a 403 error in Drupal 8 & 9.

First step you need to add custom service, in MODULENAME.services.yml:

services:
  mymodule.exception403.subscriber:
    class: Drupal\mymodule\EventSubscriber\RedirectOn403Subscriber
    tags:
      - { name: event_subscriber }
    arguments: ['@current_user']

Then add RedirectOn403Subscriber.php for your custom event subscriber in your module in the /src/EventSubscriber/ folder.

<?php

namespace Drupal\mymodule\EventSubscriber;

use Drupal\Core\EventSubscriber\HttpExceptionSubscriberBase;
use Symfony\Component\HttpKernel\Event\GetResponseForExceptionEvent;
use Symfony\Component\HttpFoundation\RedirectResponse;
use Drupal\Core\Session\AccountInterface;
use Drupal\Core\Url;

class RedirectOn403Subscriber extends HttpExceptionSubscriberBase {

  protected $currentUser;

  public function __construct(AccountInterface $current_user) {
    $this->currentUser = $current_user;
  }

  protected function getHandledFormats() {
    return ['html'];
  }

  public function on403(GetResponseForExceptionEvent $event) {
    $request = $event->getRequest();
    $is_anonymous = $this->currentUser->isAnonymous();
    $route_name = $request->attributes->get('_route');
    $is_not_login = $route_name != 'user.login';
    if ($is_anonymous && $is_not_login) {
      $query = $request->query->all();
      $query['destination'] = Url::fromRoute('<current>')->toString();
      $login_uri = Url::fromRoute('user.login', [], ['query' => $query])->toString();
      $returnResponse = new RedirectResponse($login_uri);
      $event->setResponse($returnResponse);
    }
  }

}

if you want to redirect user for specific page you can use check your path page with this code:

\Drupal::service('path.current')->getPath() == '/node/path'

Example 2: 

<?php

namespace Drupal\mymodule\EventSubscriber;

use Drupal\Core\EventSubscriber\HttpExceptionSubscriberBase;
use Symfony\Component\HttpKernel\Event\GetResponseForExceptionEvent;
use Symfony\Component\HttpFoundation\RedirectResponse;
use Drupal\Core\Session\AccountInterface;
use Drupal\Core\Url;

class RedirectOn403Subscriber extends HttpExceptionSubscriberBase {

  protected $currentUser;

  public function __construct(AccountInterface $current_user) {
    $this->currentUser = $current_user;
  }

  protected function getHandledFormats() {
    return ['html'];
  }

  public function on403(GetResponseForExceptionEvent $event) {
    $request = $event->getRequest();
    $is_anonymous = $this->currentUser->isAnonymous();
    if ($is_anonymous && \Drupal::service('path.current')->getPath() == '/node/add/article') {
      $query = $request->query->all();
      $query['destination'] = Url::fromRoute('<current>')->toString();
      $login_uri = Url::fromRoute('user.login', [], ['query' => $query])->toString();
      $returnResponse = new RedirectResponse($login_uri);
      $event->setResponse($returnResponse);
    }
  }

}