How to alter existing routes in Drupal 8

After building routes (e.g. when a module is enabled or when caches are cleared), the RoutingEvents::ALTER event triggers the route alter process. The \Drupal\Core\Routing\RouteSubscriberBase class contains an event listener that listens to this event. You can alter existing routes by implementing the alterRoutes(RouteCollection $collection) method of this class.

To alter existing routes in Drupal 8 we need to create a service that extends RouteSubscriberBase. Below is how to do it with examples of how to disable the user registration page and user password reset page.

Create the mymodule.info.yml

name: 'My Module'
description: 'Custom Module'
package: Custom
type: module
core: 8.x

Create the mymodule.services.yml file:

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

Create the src/RouteSubscriber.php Class :

<?php
namespace Drupal\mymodule;

use Drupal\Core\Routing\RouteSubscriberBase;
use Symfony\Component\Routing\RouteCollection;

/**
 * Class RouteSubscriber
 * @package Drupal\mymodule
 */
class RouteSubscriber extends RouteSubscriberBase {

  /**
   * @param RouteCollection $collection
   */
  public function alterRoutes(RouteCollection $collection) {

    // Disable access to the registration page.
    if ($route = $collection->get('user.register')) {
      $route->setRequirement('_access', 'FALSE');
    }

    // Remove user password reset page.
    if ($route = $collection->get('user.pass')) {
      $route->setRequirement('_access', 'FALSE');
    }

  }

}