code: public ExcelWriter flush(File destFile) throws IORuntimeException {
  		Assert.notNull(destFile, "[destFile] is null, and you must call setDestFile(File) first or call flush(OutputStream).");
  		return flush(FileUtil.getOutputStream(destFile), true);
  	}
docs: 将Excel Workbook刷出到文件<br>
  如果用户未自定义输出的文件，将抛出{@link NullPointerException}
  
  @param destFile 写出到的文件
  @return this
  @throws IORuntimeException IO异常
  @since 4.0.6