code: public static String genBackupName(String fileName) {
  		if (isEmpty(fileName))
  			return "";
  		int i = fileName.lastIndexOf(POINT);
  		String leftFileName = fileName.substring(0, i);
  		String rightFileName = fileName.substring(i + 1);
  		String newFileName = leftFileName + "_backup" + POINT + rightFileName;
  		return newFileName;
  	}
docs: 将传入的文件名(xxx)改名 <br>
  结果为： xxx_backup.cer
  
  @param fileName
  @return