Android AsyncTask Incorrectly Built

In the application that I'm building, I have to load some datas from a database.
I load the datas when the user select an item from an alert dialog.
I use an AsyncTask class to connect to the database. Here's the code:

public class GetTask extends AsyncTask<Void,Void,Void>{

        @Override
        protected Void doInBackground(Void... arg0) {
            getProdotti();
            return null;
        }

        @Override
          protected void onPostExecute(Void result) {
            pg.dismiss();
            for(int w=0;w<all_id.length;w++){
                _id.add(all_id[w]);
                nomi.add(all_names[w]);
                foto.add(all_images[w]);
                prezzi.add(all_prices[w]);
                descr.add(all_desc[w]);
            }
            lista = (ListView)findViewById(R.id.lista_prodotti);
            ListViewAdapter lva = new ListViewAdapter(nomi , foto , prezzi , _id , descr , context);
            lista.setAdapter(lva);
          }

          protected void onPreExecute(Void result) {
              pg = ProgressDialog.show(context, "", "Caricamento in corso...");
          }

          @Override
          protected void onProgressUpdate(Void... values) {
          }

     }/**/

And i call it so

GetTask cat = new GetTask();
cat.execute();

The progress dialog is shown, but it doesn't disappear and the ListView is not populated.
What i'm doing wrong?

Here's getProdotti():

private void getProdotti(){
    try{
        httpclient = new DefaultHttpClient();
        httppost = new HttpPost(host_url);
        datas = new ArrayList<NameValuePair>(1);
        datas.add(new BasicNameValuePair("categoria",selected));

        Log.d("INVIO",selected);

        httppost.setEntity(new UrlEncodedFormEntity(datas));
        response = httpclient.execute(httppost);
        HttpEntity entity = response.getEntity();
        inputStream = entity.getContent();

    }catch (Exception e){
        Toast.makeText(Catalogo.this, "error"+e.toString(), Toast.LENGTH_LONG).show();
    }

    if(inputStream != null){

        try{
            BufferedReader reader = new BufferedReader(new InputStreamReader(inputStream,"iso-8859-1"),8);
            StringBuilder sb = new StringBuilder();
            String line = null;
            while ((line = reader.readLine()) != null) {
                    sb.append(line + "\n");
            }
            inputStream.close();
            result = sb.toString();
            Log.d("RESULT",result);

        }catch(Exception e){
            Log.e("TEST", "Errore nel convertire il risultato "+e.toString());
            }
        try{
            JSONArray jArray = new JSONArray(result);
            all_id = new String[jArray.length()];
            all_names = new String[jArray.length()];
            all_prices = new String[jArray.length()];
            all_images = new String[jArray.length()];
            all_desc = new String[jArray.length()];
            for(int i=0;i<jArray.length();i++){
                JSONObject json_prod = jArray.getJSONObject(i);
                Log.d("Debug",json_prod.getString("id_prodotto")+"--"+json_prod.getString("nome_prodotto"));
                all_id[i]=json_prod.getString("id_prodotto");
                all_names[i]=json_prod.getString("nome_prodotto");
                all_prices[i]=json_prod.getString("prezzo");
                all_images[i]=json_prod.getString("foto_prodotto");
                all_desc[i]=json_prod.getString("descrizione");
            }
        }catch(JSONException e){
            Log.e("log_tag", "Error parsing data "+e.toString());
                }
    }

}

Try adding lva.notifyDataSetChanged() inside onPostExecute like this :

        lista = (ListView)findViewById(R.id.lista_prodotti);
        ListViewAdapter lva = new ListViewAdapter(nomi , foto , prezzi , _id , descr , context);
        lista.setAdapter(lva);
        lva.notifyDataSetChanged();

Hope this helps, good luck ^^