code: public static List<String> getGroups(String userName, AlluxioConfiguration conf)
        throws IOException {
      GroupMappingService groupMappingService = GroupMappingService.Factory.get(conf);
      return groupMappingService.getGroups(userName);
    }
docs: Using {@link CachedGroupMapping} to get the group list of a user.
  
  @param userName Alluxio user name
  @param conf Alluxio configuration
  @return the group list of the user