微信 JS SDK 中的添加卡券问题

function addCard() {
    wx.addCard({
        cardList: [{
            cardId: '100001',
            cardExt:'{"code":"","openid":"","nonceStr":"${api_nonceStr}", "timestamp": ${timestamp}, "signature":"${signature}"}'
        }],
        success: function (res) {
            alert('已添加卡券：' + JSON.stringify(res.cardList));
        }
    });
}

public static String makeApiSignature(String api_ticket, String timestamp, String nonce_str) {
    String card_id = "100001";
    String[] values = {api_ticket, timestamp, card_id, nonce_str};
    Arrays.sort(values);
    String sourceStr = "";
    for (int i = 0; i < values.length; i++) {
        sourceStr += values[i];
    }

    LOGGER.info("source:" + sourceStr);
    String signature = "";
    try {
        MessageDigest md = MessageDigest.getInstance("SHA-1");
        md.reset();
        md.update(sourceStr.getBytes("UTF-8"));
        signature = byteToHex(md.digest());
    } catch (Exception e) {
        e.printStackTrace();
    }
    return signature.toUpperCase();
}

private static String byteToHex(final byte[] hash) {
    Formatter formatter = new Formatter();
    for (byte b : hash) {
        formatter.format("%02x", b);
    }
    String result = formatter.toString();
    formatter.close();
    return result;
}