Android Show Loading dialog into AsyncTask

i want to show loading dialog before Json data get loaded into JsonObject using AsyncTask.

The problem im using AsyncTask and calling it from another class (a fragment) ..

so i have created : public static ProgressDialog dialog ; in the fragment activity (using it as a global var)

then calling the Async task :

    JSONParser jsonParser = new JSONParser();

    jsonParser.execute(url,this);

    jObj = jsonParser.get();

Here is the JSONParser (Asynch task)

   public class JSONParser extends AsyncTask<Object, Integer, JSONObject>  {

static InputStream is = null;
static JSONObject jObj = null;
static String json = "";
private MovieListBoxOffice act; // The fragment Activity 
// constructor
public JSONParser() {
     super();
}
ProgressDialog dialog;

@Override
protected void onPreExecute() {
    super.onPreExecute();
    dialog = act.dialog;
    dialog.setMessage("Loading...");

     dialog.setCancelable(true);
     dialog.show();
}


@Override
protected JSONObject doInBackground(Object... params) {


       // Making HTTP request
    try {
        // defaultHttpClient
        DefaultHttpClient httpClient = new DefaultHttpClient();

        HttpGet httpGet = new HttpGet((String)params[0]);

        HttpResponse httpResponse = httpClient.execute(httpGet);
        HttpEntity httpEntity = httpResponse.getEntity();
        is = httpEntity.getContent();

    } catch (UnsupportedEncodingException e) {
        e.printStackTrace();
    } catch (ClientProtocolException e) {
        e.printStackTrace();
    } catch (IOException e) {
        e.printStackTrace();
    }

    try {
        BufferedReader reader = new BufferedReader(new InputStreamReader(
                is, "iso-8859-1"), 8);
        StringBuilder sb = new StringBuilder();
        String line = null;
        while ((line = reader.readLine()) != null) {
            sb.append(line + "\n");
            System.out.println(line);
        }
        is.close();
        json = sb.toString();

    } catch (Exception e) {
        Log.e("Buffer Error", "Error converting result " + e.toString());
    }

    // try parse the string to a JSON object
    try {
        jObj = new JSONObject(json);
    } catch (JSONException e) {
        Log.e("JSON Parser", "Error parsing data " + e.toString());
        System.out.println("error on parse data in jsonparser.java");
    }


    return jObj;
    }


protected void onPostExecute(String result) {

    dialog.dismiss();
}

The result is the loading dialogue don't show until the data are loaded (after the async task finish) !!!

i have doubt that jsonParser.get(); is causing the problem and how can i manage that without changing my code from The fragment class to the postexecute ,