ExceptionJsonSubscriber.php

<?php

namespace Drupal\Core\EventSubscriber;

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

/**
 * Default handling for JSON errors.
 */
class ExceptionJsonSubscriber extends HttpExceptionSubscriberBase {
    
    /**
     * {@inheritdoc}
     */
    protected function getHandledFormats() {
        return [
            'json',
            'drupal_modal',
            'drupal_dialog',
            'drupal_ajax',
        ];
    }
    
    /**
     * {@inheritdoc}
     */
    protected static function getPriority() {
        // This will fire after the most common HTML handler, since HTML requests
        // are still more common than JSON requests.
        return -75;
    }
    
    /**
     * Handles all 4xx errors for JSON.
     *
     * @param \Symfony\Component\HttpKernel\Event\ExceptionEvent $event
     *   The event to process.
     */
    public function on4xx(ExceptionEvent $event) {
        
        /** @var \Symfony\Component\HttpKernel\Exception\HttpExceptionInterface $exception */
        $exception = $event->getThrowable();
        // If the exception is cacheable, generate a cacheable response.
        if ($exception instanceof CacheableDependencyInterface) {
            $response = new CacheableJsonResponse([
                'message' => $event->getThrowable()
                    ->getMessage(),
            ], $exception->getStatusCode(), $exception->getHeaders());
            $response->addCacheableDependency($exception);
        }
        else {
            $response = new JsonResponse([
                'message' => $event->getThrowable()
                    ->getMessage(),
            ], $exception->getStatusCode(), $exception->getHeaders());
        }
        $event->setResponse($response);
    }

}