code: public static String generateMixString(int length) {
  		StringBuffer sb = new StringBuffer();
  		Random random = new Random();
  		for (int i = 0; i < length; i++) {
  			sb.append(allChar.charAt(random.nextInt(letterChar.length())));
  		}
  		return sb.toString();
  	}
docs: 返回一个定长的随机纯字母字符串(只包含大小写字母)
  
  @param length 随机字符串长度
  @return 随机字符串