public static String GetSignKey() throws Exception {
        String nonce_str = WXPayUtil.generateNonceStr();//生成随机字符
        Map<String, String> param = new HashMap<String, String>();
        param.put("mch_id", "自己的商户号");//需要真实商户号
        param.put("nonce_str", nonce_str);//随机字符
        String sign = WXPayUtil.generateSignature(param,API_KEY,WXPayConstants.SignType.MD5);//通过SDK生成签名其中API_KEY为商户对应的真实密钥
        param.put("sign", sign);
        String xml = WXPayUtil.mapToXml(param);//将map转换为xml格式
        String url = "https://api.mch.weixin.qq.com/sandboxnew/pay/getsignkey";//沙箱密钥获取api
        String SignKey = HttpUtil.postData(url, xml);//
        System.out.println("signkey+"+SignKey);
        Map<String, String> param1 = new HashMap<String, String>();
        param1 = WXPayUtil.xmlToMap(SignKey);
        String key = param1.get("sandbox_signkey");
        return key;
    }