Symfony 5 custom 404 page - symfony

Question

I'm trying to create a custom 404 page for a Symfony 5 project that must:
Output a simple JSON-encoded string, like "Not found".
Said string must be read from a translation resource.
Have an additional Content-Type: application/json header.
There is a section in the Symfony docs, that attempts to explain how this can be achieved, but the information seems incomplete/incorrect, apparently being written for the 4.X version, even pointing to non-existent source files on GitHub.
I have managed to create an error controller, but it swallows all errors:
# config/packages/framework.yaml
framework:
error_controller: App\Controller\ErrorController::errorHandler
// src/Controller/ErrorController.php
class ErrorController extends AbstractController
{
public function errorHandler(TranslatorInterface $translator) : JsonResponse
{
return new JsonResponse($translator->trans('not_found'));
}
}
The problem is that this results in any error (including internal ones) returning a 404 page.
How can I make this controller/method handle only 404 errors and leave everything else to be handled as before by the framework itself?

Answer 1

For anyone else that is looking for a solution to a JSON 404 page for a Symfony application:
I was looking for a way to use a controller to handle specific error cases as it seemed the easiest option on the surface, but this does not seem to be possible, or at least I have not figured out how.
In the end, I reached a solution using events and event listeners:
Configuration:
# config/services.yaml
services:
...
# This listener handles only 404 errors in PROD mode
App\EventListener\ExceptionListener:
tags:
- { name: kernel.event_listener, event: kernel.exception }
Event listener:
// src/EventListener/ExceptionListener.php
class ExceptionListener {
public function onKernelException(ExceptionEvent $event) : void
{
if (
$_ENV['APP_ENV'] != 'prod'
|| !$event->isMasterRequest()
|| !$event->getThrowable() instanceof NotFoundHttpException
) {
return;
}
// Send a not found in JSON format
$event->setResponse(new JsonResponse($this->translator->trans('not_found')));
}
}