ProgressDialog doesn't show up in AsyncTask

Issue

I am creating an android app that depends on data that the app gets from the database. To get this data I have the following class (this class gets data from the database in JSON, translates it and returns it):

public class Json {
    public String jsonResult;

    private Activity activity;
    private String url = "http://json.example.org/json.php";
    private String db, query;

    public Json(Activity activity) {
            this.activity = activity;
        }

    public String accessWebService(String db, String query) {
        JsonReadTask task = new JsonReadTask();

        this.db = db;
        this.query = query;

        task.execute(new String[] { url });

        try {
            task.get();
        } catch (InterruptedException e) {
            Toast.makeText(activity.getApplicationContext(), "FATAL ERROR: The thread got interrupted",
                    Toast.LENGTH_LONG).show();
        } catch (ExecutionException e) {
            Toast.makeText(activity.getApplicationContext(), "FATAL ERROR: The thread wasn't able to execute",
                    Toast.LENGTH_LONG).show();
        }
        return jsonResult;
    }

    // Async Task to access the web
    private class JsonReadTask extends AsyncTask<String, Void, String> {

        private final ProgressDialog dialog = new ProgressDialog(activity);

        protected String doInBackground(String... params) {
            HttpClient httpclient = new DefaultHttpClient();
            HttpPost httppost = new HttpPost(params[0]);
            try {
                // add post data
                List<NameValuePair> nameValuePairs = new ArrayList<NameValuePair>();
                nameValuePairs.add(new BasicNameValuePair("db", db));
                nameValuePairs.add(new BasicNameValuePair("query", query));
                httppost.setEntity(new UrlEncodedFormEntity(nameValuePairs));
                HttpResponse response = httpclient.execute(httppost);
                jsonResult = inputStreamToString(response.getEntity().getContent()).toString();
                if (jsonResult.isEmpty()) {
                    Toast.makeText(activity.getApplicationContext(),
                            "Error, connection is up but didn't receive data. That's strange...", Toast.LENGTH_LONG)
                            .show();
                    this.cancel(true);
                }

            } catch (ClientProtocolException e) {
                // Toast.makeText(activity.getApplicationContext(),
                // "Error, Client Protocol Exception in JSON task",
                // Toast.LENGTH_LONG).show();
                Log.i("Json", "Error, Client Protocol Exception in JSON task");
                this.cancel(true);
            } catch (IOException e) {
                // Toast.makeText(activity.getApplicationContext(),
                // "Error, Please check your internet connection",
                // Toast.LENGTH_LONG).show();
                Log.i("Json", "Error, Please check your internet connection");
                this.cancel(true);
            }
            return null;
        }

        private StringBuilder inputStreamToString(InputStream is) {
            String rLine = "";
            StringBuilder answer = new StringBuilder();
            BufferedReader rd = new BufferedReader(new InputStreamReader(is));

            try {
                while ((rLine = rd.readLine()) != null) {
                    answer.append(rLine);
                }
            } catch (IOException e) {
                Toast.makeText(activity.getApplicationContext(), "Error..." + e.toString(), Toast.LENGTH_LONG).show();
            }
            return answer;
        }
    }// end async task
}

I noticed that my app freezes while accessing the database. After some googling, I found out it was the .get() method in the accessWebService() method caused this. I tried to implement a progressDialog like so (I also deleted the .get() method):

private final ProgressDialog dialog = new ProgressDialog(activity);

protected void onPreExecute() {
    super.onPreExecute();
    this.dialog.setMessage("Loading...");
    this.dialog.setCancelable(false);
    this.dialog.show();
}

protected void onPostExecute(String result) {
    if (this.dialog.isShowing()) {
        this.dialog.dismiss();
    }
}

but the dialog didn't show up and I got NullPointerException because the app only works when there is data:

result = json.accessWebService(db, query);

(maybe an important thing to mention: I also use this method in for loops)

So now my question is: How can I change my app so that I get a ProgressDialog while accessing the database and without getting NullPointerException? I fear that I have to rearchitect my whole app and if I have to do this, how do I do this? I hope you guys understand my question and have a fix for this because I really need help. Thanks in advance.

P.S. Sorry if my English is not that good, I'm not a native speaker.

Solution

... I found out it was the .get() method in the accessWebService() method caused this. I tried to implement a progressDialog...

That is right. get() is a blocking call and simply adding a ProgressDialog won't fix it. You need to remove .get() and that will probably fix the issue of your ProgressDialog not showing.

An AsyncTask must be executed on the main Thread so make sure you are doing that.

Another problem you have is Toast.LENGTH_LONG).show(); runs on the UI and you have it in doInBackground() which cannot happen. You need to send the result to onPostExecute() and you can display your Toast there if need. This could also be done in onProgressUpdate().

Answered By - codeMagic