public static String getRandomString(int length) { //length表示生成字符串的长度
        String base = "ABCDEFHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789 ";   
        Random random = new Random();   
        StringBuffer sb = new StringBuffer();   
        for (int i = 0; i < length; i++) {   
            int number = random.nextInt(base.length());   //为啥这里要用int类型去接收随机字符串啊？
            sb.append(base.charAt(number));   
        }   
        return sb.toString();   
     }