func (w *AsyncFileWriter) rotateFile() {
	select {
	case <-w.timeTicker.C:
		if err := w.flushAndClose(); err != nil {
			fmt.Fprintf(os.Stderr, "flush and close file error. err=%s", err)
		}
		if err := w.initLogFile(); err != nil {
			fmt.Fprintf(os.Stderr, "init log file error. err=%s", err)
		}
                // start from here
                if maxBackups > 0 {
		   if err := w.removeExpiredFile(); err != nil {
			fmt.Fprintf(os.Stderr, "remove expired file error. err=%s", err)
		   }
                }
                // end
	default:
	}
}