How to override a controller class in Drupal?

In Drupal, you can override a controller class by implementing a custom route subscriber. Here are the steps to do so:

1. Create a custom module: Create a custom Drupal module if you haven't already. You can do this by creating a new folder with the module name in the modules/custom directory of your Drupal installation, and adding a *.info.yml file to define the module.

2. Create a route subscriber class: In your custom module, create a PHP class that implements the RouteSubscriberInterface interface. This class will be responsible for overriding the controller class. For example, you can create a MyCustomRouteSubscriber class with the following content:
    

   namespace Drupal\my_module\Routing;
   
   use Drupal\Core\Routing\RouteSubscriberBase;
   use Symfony\Component\Routing\RouteCollection;
   
   /**
    * Overrides the SomeController class.
    */
   class MyCustomRouteSubscriber extends RouteSubscriberBase {
   
     /**
      * {@inheritdoc}
      */
     public function alterRoutes(RouteCollection $collection) {
       // Override the original controller class with your custom controller class.
       if ($route = $collection->get('some_module.some_route_name')) {
         $route->setDefault('_controller', '\Drupal\my_module\Controller\MyCustomController::someMethod');
       }
     }
   }
   

   Make sure to replace some_module.some_route_name with the route name of the route that has the controller you want to override, and \Drupal\my_module\Controller\MyCustomController::someMethod with the class name and method name of your custom controller that will override the original controller.

3. Declare the route subscriber in your module's services.yml file: In your module's .services.yml file, declare the route subscriber class you created in the services section. For example:

   services:
     my_module.my_custom_route_subscriber:
       class: Drupal\my_module\Routing\MyCustomRouteSubscriber
       tags:
         - { name: event_subscriber }
   

   Make sure to adjust the class path and route subscriber ID to match your custom module's namespace and class name.

4. Clear cache

5. Test the overridden controller: You can now test the overridden controller by accessing the route that you specified in the alterRoutes() method of your route subscriber, and Drupal will automatically use your custom controller with your custom logic.