String val = "";
       Random random = new Random();
       for (int i = 0; i < length; i++) {
           String charOrNum = random.nextInt(2) % 2 == 0 ? "char" : "num"; // 输出字母还是数字

           if ("char".equalsIgnoreCase(charOrNum)) // 字符串
           {
               int choice = random.nextInt(2) % 2 == 0 ? 65 : 97; // 取得大写字母还是小写字母
               val += (char) (choice + random.nextInt(26));
               // int choice = 97; // 指定字符串为小写字母
               val += (char) (choice + random.nextInt(26));
           } else if ("num".equalsIgnoreCase(charOrNum)) // 数字
           {
               val += String.valueOf(random.nextInt(10));
           }
       }
       return val;
   }