生成驗證碼算法

/**
	 * 獲得一個隨機的字符串
	 * 
	 * @param baseString 隨機字符選取的樣本
	 * @param length 字符串的長度
	 * @return 隨機字符串
	 */
	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();
	}