code: public static String getConnectionUser(AlluxioConfiguration conf) throws AccessControlException {
      if (!SecurityUtils.isAuthenticationEnabled(conf)) {
        throw new AccessControlException(ExceptionMessage.AUTHENTICATION_IS_NOT_ENABLED.getMessage());
      }
      User user = sConnectionUserThreadLocal.get();
      if (user == null) {
        return null;
      }
      return user.getName();
    }
docs: Gets the connection user name from the {@link ThreadLocal} variable.
  
  @param conf Alluxio configuration
  @return the client user in string, null if the user is not present
  @throws AccessControlException if the authentication is not enabled