apollo-portal/src/main/java/com/ctrip/framework/apollo/portal/controller/PermissionController.java/PermissionController/checkUserExists
class PermissionController:  private void checkUserExists(String userId) {
    if (userService.findByUserId(userId) == null) {
      throw new BadRequestException(String.format("User %s does not exist!", userId));
    }
  }