import org.apache.hadoop.hdfs.server.namenode.ImageServlet; //导入依赖的package包/类
@Override
public void doGet(final HttpServletRequest request,
    final HttpServletResponse response) throws ServletException, IOException {
  FileInputStream editFileIn = null;
  try {
    final ServletContext context = getServletContext();
    final Configuration conf = (Configuration) getServletContext()
        .getAttribute(JspHelper.CURRENT_CONF);
    final String journalId = request.getParameter(JOURNAL_ID_PARAM);
    QuorumJournalManager.checkJournalId(journalId);
    final JNStorage storage = JournalNodeHttpServer
        .getJournalFromContext(context, journalId).getStorage();

    // Check security
    if (!checkRequestorOrSendError(conf, request, response)) {
      return;
    }

    // Check that the namespace info is correct
    if (!checkStorageInfoOrSendError(storage, request, response)) {
      return;
    }
    
    long segmentTxId = ServletUtil.parseLongParam(request,
        SEGMENT_TXID_PARAM);

    FileJournalManager fjm = storage.getJournalManager();
    File editFile;

    synchronized (fjm) {
      // Synchronize on the FJM so that the file doesn't get finalized
      // out from underneath us while we're in the process of opening
      // it up.
      EditLogFile elf = fjm.getLogFile(
          segmentTxId);
      if (elf == null) {
        response.sendError(HttpServletResponse.SC_NOT_FOUND,
            "No edit log found starting at txid " + segmentTxId);
        return;
      }
      editFile = elf.getFile();
      ImageServlet.setVerificationHeadersForGet(response, editFile);
      ImageServlet.setFileNameHeaders(response, editFile);
      editFileIn = new FileInputStream(editFile);
    }
    
    DataTransferThrottler throttler = ImageServlet.getThrottler(conf);

    // send edits
    TransferFsImage.copyFileToStream(response.getOutputStream(), editFile,
        editFileIn, throttler);

  } catch (Throwable t) {
    String errMsg = "getedit failed. " + StringUtils.stringifyException(t);
    response.sendError(HttpServletResponse.SC_INTERNAL_SERVER_ERROR, errMsg);
    throw new IOException(errMsg);
  } finally {
    IOUtils.closeStream(editFileIn);
  }
}