code: public static String randomString(String baseString, int length) {
  		final StringBuilder sb = new StringBuilder();
  
  		if (length < 1) {
  			length = 1;
  		}
  		int baseLength = baseString.length();
  		for (int i = 0; i < length; i++) {
  			int number = getRandom().nextInt(baseLength);
  			sb.append(baseString.charAt(number));
  		}
  		return sb.toString();
  	}
docs: 获得一个随机的字符串
  
  @param baseString 随机字符选取的样本
  @param length 字符串的长度
  @return 随机字符串