本以為申請退款應該就沒那么多坑了，沒想到...

申請退款這個環節需要商戶證書，那么好吧，我們按官方文檔給的地址下載一下：微信商戶平臺(pay.weixin.qq.com)-->賬戶中心-->賬戶設置-->API安全-->證書下載

解壓之后有四個文件：

F4AA095C-4C45-4720-B5E3-60664348E70B.png

經過一番百度，原來java web開發只需要使用apiclient_cert.p12這個文件。

我們需要把這個文件放到自己的項目中，我放到了這里：

BD85CFBE-AD83-4B42-BC8E-3AE760AAD107.png

ok,廢話說完了，接下來上代碼：

public static String payHttps(String data,String id) throws Exception {
        //指定讀取證書格式為PKCS12
        KeyStore keyStore = KeyStore.getInstance("PKCS12");
        //讀取本機存放的PKCS12證書文件
        FileInputStream instream = new FileInputStream(new File("doc/apiclient_cert.p12"));
        try {
            //指定PKCS12的密碼, 商戶ID
            keyStore.load(instream, id.toCharArray());
        } finally {
            instream.close();
        }
        SSLContext sslcontext = SSLContexts.custom().loadKeyMaterial(keyStore, id.toCharArray()).build();
        //指定TLS版本
        SSLConnectionSocketFactory sslsf = new SSLConnectionSocketFactory(
                sslcontext,new String[] { "TLSv1" },null,
                SSLConnectionSocketFactory.BROWSER_COMPATIBLE_HOSTNAME_VERIFIER);
        //設置httpclient的SSLSocketFactory
        CloseableHttpClient httpclient = HttpClients.custom().setSSLSocketFactory(sslsf).build();
        try {
            /**
             * wx 頭信息設置
             */
            //因為牽扯到證書, 所以不使用HttpURLConnection, 改用HttpClient
            HttpPost httpost = new HttpPost("https://api.mch.weixin.qq.com/secapi/pay/refund"); // 設置響應頭信息
            httpost.addHeader("Connection", "keep-alive");
            httpost.addHeader("Accept", "*/*");
            httpost.addHeader("Content-Type", "application/x-www-form-urlencoded; charset=UTF-8");
            httpost.addHeader("Host", "api.mch.weixin.qq.com");
            httpost.addHeader("X-Requested-With", "XMLHttpRequest");
            httpost.addHeader("Cache-Control", "max-age=0");
            httpost.addHeader("User-Agent", "Mozilla/4.0 (compatible; MSIE 8.0; Windows NT 6.0) ");
            httpost.setEntity(new StringEntity(data, "UTF-8"));
            CloseableHttpResponse response = httpclient.execute(httpost);
            try {
                HttpEntity entity = response.getEntity();
                String jsonStr = EntityUtils.toString(response.getEntity(), "UTF-8");
                EntityUtils.consume(entity);
                System.out.println("jsonStr" + jsonStr);
                return jsonStr;
            } finally {
                response.close();
            }
        } finally {
            httpclient.close();
        }
    }

筆者能力有限，不足之處歡迎指出。