public static FileReaderManager getInstance() {
    return FileReaderManagerHelper.INSTANCE;
  }
the key of unclosedFileReaderMap is the file path and the value of unclosedFileReaderMap is the
file's reference count.
iotdb-core/datanode/src/main/java/org/apache/iotdb/db/storageengine/dataregion/read/control/FileReaderManager.java