Execution order in AsyncTask inside loop


I have a problem concerning execution order.I get some data from server ( cities names , LatLng) and on onPostExecute i'm trying to calculate distances between cities. During last onPostExecute there are some distance calculations. The problem is that in

for(j=0;j<3;j++) { or = markerPoints.get(j); dest = markerPoints.get(j+1); Log.d("or", or.toString()); Log.d("dest", dest.toString()); // Getting URL to the Google Directions API String url = getDirectionsUrl(or, dest); DownloadTask downloadTask = new DownloadTask(); // Start downloading json data from Google Directions API downloadTask.execute(url); }

downloadTask is executed for the first time after loop "for" is completed, that means when j equals 2. Why not 3 times; Isn't that weird;

My whole code

public class Check_Itineraries1 extends FragmentActivity { private ProgressDialog pDialog; String username , origin_lat ,origin_lng ,destination_lat ,destination_lng , waypoint1_lat ,waypoint1_lng ,waypoint2_lat ,waypoint2_lng ,waypoints , origin_lat_pro ,origin_lng_pro ,destination_lat_pro ,destination_lng_pro , start_pro , final_pro; int i, j ,b ; double x,z ,t ,s; TextView v1 , v2; ArrayList<Double> Array = new ArrayList<Double>(); ArrayList<Double> Array_add = new ArrayList<Double>(); ArrayList<Double> Array_dif = new ArrayList<Double>(); ListView list; ArrayList<LatLng> markerPoints; // URL to get contacts JSON private static String LOGIN_URL = "http://**************************"; public static final String PREFS_NAME = "MyPreferencesFile"; // JSON Node names JSONParser jsonParser = new JSONParser(); // contacts JSONArray JSONArray contacts = null; // Hashmap for ListView ArrayList<HashMap<String, String>> itinList , final_itinList ; @Override public void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); setContentView(R.layout.checkitineraries); itinList = new ArrayList<HashMap<String, String>>(); new GetData().execute(); } private class GetData extends AsyncTask<Void, Void, Void> { public void onPreExecute() { super.onPreExecute(); Log.d("meg", "meg"); pDialog = new ProgressDialog(Check_Itineraries1.this); pDialog.setMessage("Επεξεργάζομαι τα δεδομένα..."); pDialog.setIndeterminate(false); pDialog.setCancelable(true); pDialog.show(); SharedPreferences settings = getSharedPreferences(PREFS_NAME, 0); username = settings.getString("username", "nikos"); } protected Void doInBackground(Void... args) { // TODO Auto-generated method stub try { // Building Parameters List<NameValuePair> params = new ArrayList<NameValuePair>(); params.add(new BasicNameValuePair("username", username)); JSONObject json = jsonParser.makeHttpRequest(LOGIN_URL, "POST", params); // JSONObject jObj = new JSONObject(); JSONArray itin_results = json.getJSONArray("itin_results"); for (int i = 0; i < itin_results.length(); i++) { JSONObject c = itin_results.getJSONObject(i); String username = c.getString("username"); String startPoliPro = c.getString("startPoliPro"); String start_lat_pro = c.getString("start_lat_pro"); String start_lng_pro = c.getString("start_lng_pro"); String finalPoliPro = c.getString("finalPoliPro"); String final_lat_pro = c.getString("final_lat_pro"); String final_lng_pro = c.getString("final_lng_pro"); LinkedHashMap<String, String> pinakas = new LinkedHashMap<String, String>(); // adding each child node to HashMap key => value pinakas.put("username", username); pinakas.put("startPoliPro", startPoliPro); pinakas.put("start_lat_pro", start_lat_pro); pinakas.put("start_lng_pro", start_lng_pro); pinakas.put("finalPoliPro", finalPoliPro); pinakas.put("final_lat_pro", final_lat_pro); pinakas.put("final_lng_pro", final_lng_pro); // Διαλέγω αφετηρία και προορισμό του δρομολογίου του επαγγελματία Log.d("1", username); Log.d("2", startPoliPro); Log.d("3", start_lat_pro); Log.d("4", start_lng_pro); Log.d("5", finalPoliPro); Log.d("6", final_lat_pro); Log.d("7", final_lng_pro); itinList.add(pinakas); b = itin_results.length(); // Είναι τα δρομολόγια των πελατων που ταιριάζουν ΣΥΝ το δρομολόγιο του επαγγελματία Log.d("b", String.valueOf(b)); } } catch (JSONException e) { // TODO Auto-generated catch block e.printStackTrace(); } return null; } @Override protected void onPostExecute(Void result) { super.onPostExecute(result); // Dismiss the progress dialog if (pDialog.isShowing()) pDialog.dismiss(); origin_lat_pro = itinList.get(0).get("start_lat_pro").toString(); origin_lng_pro = itinList.get(0).get("start_lng_pro").toString(); destination_lat_pro = itinList.get(0).get("final_lat_pro").toString(); destination_lng_pro = itinList.get(0).get("final_lng_pro").toString(); start_pro = itinList.get(0).get("startPoliPro").toString(); final_pro = itinList.get(0).get("finalPoliPro").toString(); LatLng origin_pro = new LatLng(Double.parseDouble(origin_lat_pro),Double.parseDouble(origin_lng_pro)); LatLng destination_pro = new LatLng(Double.parseDouble(destination_lat_pro),Double.parseDouble(destination_lng_pro)); for (i = 0; i <b; i++) { markerPoints = new ArrayList<LatLng>(); origin_lat = itinList.get(i).get("start_lat_pro").toString(); origin_lng = itinList.get(i).get("start_lng_pro").toString(); destination_lat = itinList.get(i).get("final_lat_pro").toString(); destination_lng = itinList.get(i).get("final_lng_pro").toString(); LatLng or = new LatLng(Double.parseDouble(origin_lat),Double.parseDouble(origin_lng)); LatLng dest = new LatLng(Double.parseDouble(destination_lat),Double.parseDouble(destination_lng)); markerPoints.add(origin_pro); markerPoints.add(or); markerPoints.add(dest); markerPoints.add(destination_pro); Log.d("a", markerPoints.get(0).toString()); Log.d("b", markerPoints.get(1).toString()); Log.d("c", markerPoints.get(2).toString()); Log.d("d", markerPoints.get(3).toString()); z = 0; for(j=0;j<3;j++) { or = markerPoints.get(j); dest = markerPoints.get(j+1); Log.d("or", or.toString()); Log.d("dest", dest.toString()); // Getting URL to the Google Directions API String url = getDirectionsUrl(or, dest); DownloadTask downloadTask = new DownloadTask(); // Start downloading json data from Google Directions API downloadTask.execute(url); } } } } //Αυτοτελές private String getDirectionsUrl(LatLng or, LatLng dest) { // Origin of route String str_origin = "origin=" + or.latitude + ","+ or.longitude; // Destination of route String str_dest = "destination=" + dest.latitude + "," + dest.longitude; // Sensor enabled String sensor = "sensor=false"; Log.d("sos1", "sos1"); // Building the parameters to the web service String parameters = str_origin + "&" + str_dest + "&" + sensor; // Output format String output = "json"; // Building the url to the web service String url = "https://maps.googleapis.com/maps/api/directions/" + output + "?" + parameters; return url; } //Αυτοτελές// //Αυτοτελές private String downloadUrl(String strUrl) throws IOException { String data = ""; InputStream iStream = null; HttpURLConnection urlConnection = null; try { URL url = new URL(strUrl); // Creating an http connection to communicate with url urlConnection = (HttpURLConnection) url.openConnection(); // Connecting to url urlConnection.connect(); Log.d("sos2", "sos2"); // Reading data from url iStream = urlConnection.getInputStream(); BufferedReader br = new BufferedReader(new InputStreamReader( iStream)); StringBuffer sb = new StringBuffer(); String line = ""; while ((line = br.readLine()) != null) { sb.append(line); } data = sb.toString(); br.close(); } catch (Exception e) { Log.d("Exception while downloading url", e.toString()); } finally { iStream.close(); urlConnection.disconnect(); } return data; } //Αυτοτελές// // Fetches data from url passed //Αυτοτελές private class DownloadTask extends AsyncTask<String, Void, String> { // Downloading data in non-ui thread @Override protected String doInBackground(String... url) { // For storing data from web service String data = ""; try { // Fetching the data from web service data = downloadUrl(url[0]); Log.d("sos3", "sos3"); } catch (Exception e) { Log.d("Background Task", e.toString()); } return data; } // Executes in UI thread, after the execution of // doInBackground() @Override protected void onPostExecute(String result) { super.onPostExecute(result); ParserTask parserTask = new ParserTask(); Log.d("sos4", "sos4"); // Invokes the thread for parsing the JSON data parserTask.execute(result); } } //Αυτοτελές// public class ParserTask extends AsyncTask<String, Integer, List<List<HashMap<String, String>>>> { // Parsing the data in non-ui thread @Override protected List<List<HashMap<String, String>>> doInBackground(String... jsonData) { JSONObject jObject; List<List<HashMap<String, String>>> routes = null; try { jObject = new JSONObject(jsonData[0]); DirectionsJSONParser parser = new DirectionsJSONParser(); Log.d("sos5", "sos5"); // Starts parsing data routes = parser.parse(jObject); Log.d("json", jObject.toString()); } catch (Exception e) { e.printStackTrace(); } return routes; } // Executes in UI thread, after the parsing process @Override protected void onPostExecute(List<List<HashMap<String, String>>> result) { ArrayList<LatLng> points = null; String distance = ""; String duration = ""; Log.d("sos6", "sos6"); Log.d("result", String.valueOf(result.size())); // Traversing through all the routes for (int i = 0; i < result.size(); i++) { points = new ArrayList<LatLng>(); // Fetching i-th route List<HashMap<String, String>> path = result.get(i); // Fetching all the points in i-th route for(int j=0;j<path.size();j++){ HashMap<String,String> point = path.get(j); if(j==0){ // Get distance from the list distance = point.get("distance"); continue; }else if(j==1){ // Get duration from the list duration = point.get("duration"); continue; } double lat = Double.parseDouble(point.get("lat")); double lng = Double.parseDouble(point.get("lng")); LatLng position = new LatLng(lat, lng); points.add(position); // // } } Log.d("finito", distance); String asd[] = distance.split(" "); Log.d("ass", asd[0]); Log.d("ass", asd[1]); double x = Double.parseDouble(asd[0]); if(asd[1].equals("m")){ x = x/1000; } Log.d("intValue", String.valueOf(x)); x = Math.round(x); Array.add(x); Log.d("Array", Array.toString()); if(Array.size()==3){ z = Array.get(0) + Array.get(1) + Array.get(2); Array_add.add(z); Array.clear(); } Log.d("size", String.valueOf(Array_add.size())); Log.d("Αθροισμα", String.valueOf(z)); } } //Αυτοτελές }


<a href="http://developer.android.com/reference/android/os/AsyncTask.html" rel="nofollow">Android Developers</a> says:


When first introduced, AsyncTasks were executed serially on a single background thread. Starting with {android.os.Build.VERSION_CODES#DONUT}, this was changed to a pool of threads allowing multiple tasks to operate in parallel. Starting with { android.os.Build.VERSION_CODES#HONEYCOMB}, tasks are executed on a single thread to avoid common application errors caused by parallel execution.


Also since executor of AsyncTask is static so you get single executor of your application.

Now it already running GetData asyncTask and it runs asynsTask serially so it cannot execute your DownloadTask. It can run only after GetDataTask is finished.

If you want to run multiple asynctasks in parallel, you can look at Asynctask.executeOnExecutor().


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