public String httpPost(String url, String queryString) throws Exception {
String ret = null;
final HTTPRequest request = new HTTPRequest();
request.setURL(new URL(url));
request.setRequestMethod(HTTPRequestMethod.POST);
request.addHeader(new HTTPHeader("Content-Type",
"application/x-www-form-urlencoded"));
if (queryString != null && !queryString.equals("")) {
request.setPayload(queryString.getBytes("UTF-8"));
}