How to set a default controller if no other routes match

Question

I my project I would like to have a default route or controller that handles any request that does not match any available route. For example on requesting /abcd it now returns 404 as there is route defined for the path /abcd. I want any such request to be handle by a default controller that consider 'abcd' or any other string for that matter as a parameter. How can I implement that. If I define a route for path /{param} wouldn't that override other active routes

Answer 1

Go to /admin/config/system/site-information and set Default 404 (not found) page to the path of your custom controller.

In the controller place a typecasted $request in the argument list. Then get the path from the request and either build a custom 404 response or redirect to another page:

use Symfony\Component\HttpFoundation\Request;

  public function myController(Request $request) {
    $path = $request->getPathInfo();
    if ($path === '/abc') {
      return ['#markup' => 'Path "abc" does not exist!'];
    }
    else {
      return $this->redirect('<front>');
    }
  }

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To answer the deleted comment. The controller can't set a status code 200, because it responds to a sub request and the master request is set to 404 no matter which 2xx status you set in the controller. For this you need an event subscriber: Does not redirect in EventSubscriber.