code: public static User get(AlluxioConfiguration conf) throws UnauthenticatedException {
      if (sLoginUser == null) {
        synchronized (LoginUser.class) {
          if (sLoginUser == null) {
            sLoginUser = login(conf);
          }
        }
      }
      return sLoginUser;
    }
docs: Gets current singleton login user. This method is called to identify the singleton user who
  runs Alluxio client. When Alluxio client gets a user by this method and connects to Alluxio
  service, this user represents the client and is maintained in service.
  
  Note that until if the authentication type or login user changes between the first
  invocation of this method that it is possible the cached user won't respect the updated
  configuration properties.
  
  @param conf Alluxio's current configuration
  @return the login user