code: public static String getClientUser(AlluxioConfiguration conf) throws AccessControlException {
      try {
        User user = get(conf);
        if (user == null) {
          throw new AccessControlException(
              ExceptionMessage.AUTHORIZED_CLIENT_USER_IS_NULL.getMessage());
        }
        return user.getName();
      } catch (IOException e) {
        throw new AccessControlException(ExceptionMessage.AUTHENTICATION_IS_NOT_ENABLED.getMessage());
      }
    }
docs: Gets the user name from the {@link ThreadLocal} variable.
  
  @param conf Alluxio configuration
  @return the client user in string
  @throws AccessControlException there is no authenticated user for this thread or
  the authentication is not enabled