code: protected IndexReader openIndex(Path path, Configuration conf) throws IOException {
      return DirectoryReader.open(new LuceneHdfsDirectory(path, path.getFileSystem(conf)));
    }
docs: Open an index stored in path
  Override if you want more control over how indexes are opened
  
  @param path path to the index
  @param conf job conf
  @return an IndexReader of the index in path
  @throws IOException