docs: public void downLoadFile(String fullFilename, OutputStream outputStream) {
          TrackerServer trackerServer = null;
          StorageClient1 storageClient1 = null;
          try {
              trackerServer = fastDfsClientPool.borrowObject();
              String suffx = fullFilename.substring(fullFilename.lastIndexOf(EXT_SEPERATOR));
              File file = filePool.createFile(suffx);
              storageClient1 = new StorageClient1(trackerServer, null);
              storageClient1.downloadFile1(fullFilename, file.getAbsolutePath());
              Streams.writeAndClose(outputStream, new FileInputStream(file));
          } catch (Exception e) {
              throw Lang.makeThrow("[FastdfsService] download file error : %s", e.getMessage());
          } finally {
              if (trackerServer != null)
                  fastDfsClientPool.returnObject(trackerServer);
              storageClient1 = null;
          }
      }