How to override REST error message

Question

After installing "RESTful Web Services" module, if I execute this API request:

curl --request GET \
  --url 'http://localhost/resourceNotFound?_format=json'

I got following error (which is expected):

{
    "message": "No route found for \"GET http:\/\/localhost\/resourceNotFound\""
}

Let's say I would like to change that error to:

{
  "code": 60,
  "message": "Resource not found",
  "description": "The requested URI or the requested resource does not exist."
}

How to do that?

Answer 1

You don't want to override the exception messages when they are thrown in routing or elsewhere. Better use an exception handler to catch all JSON 404 exceptions:

/src/EventSubscriber/MymoduleExceptionSubscriber.php

<?php

namespace Drupal\mymodule\EventSubscriber;

use Drupal\Core\Cache\CacheableDependencyInterface;
use Drupal\Core\Cache\CacheableJsonResponse;
use Drupal\Core\EventSubscriber\HttpExceptionSubscriberBase;
use Symfony\Component\HttpFoundation\JsonResponse;
use Symfony\Component\HttpKernel\Event\ExceptionEvent;

class MymoduleExceptionSubscriber extends HttpExceptionSubscriberBase {

  protected static function getPriority() {
    // handle the exceptions we want for JSON before
    // core subscribers do this with priority -70/-75
    return -69;
  }

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

  public function on404(ExceptionEvent $event) {
    /** @var \Symfony\Component\HttpKernel\Exception\HttpExceptionInterface $exception */
    $exception = $event->getThrowable();

    $content = [
      'code' => 60,
      'message' => 'Resource not found',
      'description' => 'The requested URI or the requested resource does not exist.',
    ];
  
    // If the exception is cacheable, generate a cacheable response.
    if ($exception instanceof CacheableDependencyInterface) {
      $response = new CacheableJsonResponse($content, 404, $exception->getHeaders());
      $response->addCacheableDependency($exception);
    }
    else {
      $response = new JsonResponse($content, 404, $exception->getHeaders());
    }

    $event->setResponse($response);
  }

}

mymodule.services.yml

services:
  mymodule.404_subscriber:
    class: Drupal\mymodule\EventSubscriber\MymoduleExceptionSubscriber
    tags:
      - { name: event_subscriber }