@Override
  public void println(int logLevel, String tag, String msg) {
    long timeMillis = System.currentTimeMillis();
    if (USE_WORKER) {
      if (!worker.isStarted()) {
        worker.start();
      }
      worker.enqueue(new LogItem(timeMillis, logLevel, tag, msg));
    } else {
      doPrintln(timeMillis, logLevel, tag, msg);
    }
  }
Make sure the folder of log file exists.
xlog/src/main/java/com/elvishew/xlog/printer/file/FilePrinter.java