HttpURLConnection is subclass of URLConnection with support for HTTP-specific features, for example: open a connection, retrieve message and post data to server.

First of all, we need declare a global variable cookie:

public static String cookie = null;

Secondly, create a method GetCookieFromURL, open a URL connection, read the cookie from opened connection and assign to global variable cookie:
Please note the connection will be closed when method is finished(it’s one-off connection).

    public static String GetCookieFromURL() {

        String result = null;
        HttpURLConnection con = null;
        try {
            String urlName = "http://xxx.com/trade/index.jsp";
            URL url = new URL(urlName);
            con = (HttpURLConnection) (url.openConnection());
            con.setConnectTimeout(40000);
            con.setReadTimeout(40000);
            con.setRequestMethod("POST");
            con.setRequestProperty("User-Agent", "Mozilla/4.0 (compatible; MSIE 6.0; Windows 2000)");
            con.setFollowRedirects(true);
            con.setDoOutput(true);
            con.setDoInput(true);
            con.setUseCaches(false);
            con.setAllowUserInteraction(false);
            con.setRequestProperty("Content-Type", "application/x-www-form-urlencoded");
            con.setRequestProperty("Content-Language", "en-US");
            result = con.getHeaderField("Set-Cookie");
       } catch (Exception e) {
            e.printStackTrace();
        } finally {
            con.disconnect();
            con = null;
        }
        return result;
    }

Third, we will open a different HTTP connection with cookies we got previously, prior to opening collection, use urlConn.setRequestProperty set the cookie value, by this way, we are able to pass-by session across multiple requests, it’s very useful, for example, after login, we need use login session for a while to read and  modify data.

    public static byte[] getByteFromFile(String urlName) throws AvException {
        InputStream input = null;
        HttpURLConnection urlConn = null;
        try {
            urlConn = (HttpURLConnection) (new URL(urlName).openConnection());
            urlConn.setRequestMethod("POST");
            urlConn.setRequestProperty("User-Agent", "Mozilla/4.0 (compatible; MSIE 6.0; Windows 2000)");
            urlConn.setFollowRedirects(true);
            urlConn.setDoOutput(true);
            urlConn.setDoInput(true);
            urlConn.setUseCaches(false);
            urlConn.setAllowUserInteraction(false);
            urlConn.setRequestProperty("Content-Type", "application/x-www-form-urlencoded");
            urlConn.setRequestProperty("Content-Language", "en-US");
            urlConn.setRequestProperty("Cookie", cookie);
            input = urlConn.getInputStream();
            ByteArrayOutputStream baos = new ByteArrayOutputStream();
            int bytesRead = 0;
            byte[] buffer = new byte[8192];

            while ((bytesRead = input.read(buffer, 0, buffer.length)) != -1) {
                baos.write(buffer, 0, bytesRead);
            }

            return baos.toByteArray();
        } catch (Exception e) {
            throw new AvException("maybe the internet is disable", AvException.Type.INTERNET_DENIED);
        } finally {
            urlConn.disconnect();
            urlConn = null;
        }
    }