code: public int refreshNodes() throws IOException {
      int exitCode = -1;
  
      DistributedFileSystem dfs = getDFS();
      if (dfs == null) {
        System.err.println("FileSystem is " + getFS().getUri());
        return exitCode;
      }
  
      dfs.refreshNodes();
      exitCode = 0;
     
      return exitCode;
    }
docs: Command to ask the namenode to reread the hosts and excluded hosts
  file.
  Usage: java DFSAdmin -refreshNodes
  @exception IOException