/**
 * 針對微信支付生成商戶訂單號，為了避免微信商戶訂單號重複（下單單位支付），
*
 * @return
*/
public static String generateOrderSN() {
    StringBuffer orderSNBuffer = new StringBuffer();
    orderSNBuffer.append(System.currentTimeMillis());
    orderSNBuffer.append(getRandomString(7));
    return orderSNBuffer.toString();
}

/**
 * 獲取隨機字串
 
* @param len
* @return
*/
public static String getRandomString(int len) {
    StringBuilder sb = new StringBuilder(len);
    for (int i = 0; i < len; i++)
        sb.append(AB.charAt(rnd.nextInt(AB.length())));
    return sb.toString();
}