@SuppressWarnings("squid:S2095")
  public synchronized TsFileSequenceReader get(String filePath, boolean isClosed)
      throws IOException {
    return get(filePath, isClosed, null);
  }
Get the reader of the file(tsfile or unseq tsfile) indicated by filePath. If the reader already
exists, just get it from closedFileReaderMap or unclosedFileReaderMap depending on isClosing .
Otherwise a new reader will be created and cached.

@param filePath the path of the file, of which the reader is desired.
@param isClosed whether the corresponding file still receives insertions or not.
@return the reader of the file specified by filePath.
@throws IOException when reader cannot be created.
iotdb-core/datanode/src/main/java/org/apache/iotdb/db/storageengine/dataregion/read/control/FileReaderManager.java