class DefaultExceptionSubscriber

class DefaultExceptionSubscriber extends HttpExceptionSubscriberBase {
    
    /**
     * The serializer.
     *
     * @var \Symfony\Component\Serializer\Serializer
     */
    protected $serializer;
    
    /**
     * The available serialization formats.
     *
     * @var array
     */
    protected $serializerFormats = [];
    
    /**
     * DefaultExceptionSubscriber constructor.
     *
     * @param \Symfony\Component\Serializer\SerializerInterface $serializer
     *   The serializer service.
     * @param array $serializer_formats
     *   The available serialization formats.
     */
    public function __construct(SerializerInterface $serializer, array $serializer_formats) {
        $this->serializer = $serializer;
        $this->serializerFormats = $serializer_formats;
    }
    
    /**
     * {@inheritdoc}
     */
    protected function getHandledFormats() {
        return $this->serializerFormats;
    }
    
    /**
     * {@inheritdoc}
     */
    protected static function getPriority() {
        // This will fire after the most common HTML handler, since HTML requests
        // are still more common than HTTP requests. But it has a lower priority
        // than \Drupal\Core\EventSubscriber\ExceptionJsonSubscriber::on4xx(), so
        // that this also handles the 'json' format. Then all serialization formats
        // (::getHandledFormats()) are handled by this exception subscriber, which
        // results in better consistency.
        return -70;
    }
    
    /**
     * Handles all 4xx errors for all serialization failures.
     *
     * @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();
        $request = $event->getRequest();
        $format = $request->getRequestFormat();
        $content = [
            'message' => $exception->getMessage(),
        ];
        $encoded_content = $this->serializer
            ->serialize($content, $format);
        $headers = $exception->getHeaders();
        // Add the MIME type from the request to send back in the header.
        $headers['Content-Type'] = $request->getMimeType($format);
        // If the exception is cacheable, generate a cacheable response.
        if ($exception instanceof CacheableDependencyInterface) {
            $response = new CacheableResponse($encoded_content, $exception->getStatusCode(), $headers);
            $response->addCacheableDependency($exception);
        }
        else {
            $response = new Response($encoded_content, $exception->getStatusCode(), $headers);
        }
        $event->setResponse($response);
    }

}