generateFallbackAccessDeniedMessage example

  public function post(EntityInterface $entity = NULL) {
    if ($entity == NULL) {
      throw new BadRequestHttpException('No entity content received.');
    }

    $entity_access = $entity->access('create', NULL, TRUE);
    if (!$entity_access->isAllowed()) {
      throw new AccessDeniedHttpException($entity_access->getReason() ?: $this->generateFallbackAccessDeniedMessage($entity, 'create'));
    }
    $definition = $this->getPluginDefinition();
    // Verify that the deserialized entity is of the type that we expect to     // prevent security issues.     if ($entity->getEntityTypeId() != $definition['entity_type']) {
      throw new BadRequestHttpException('Invalid entity type');
    }
    // POSTed entities must not have an ID set, because we always want to create     // new entities here.     if (!$entity->isNew()) {
      throw new BadRequestHttpException('Only new entities can be created');
    }