Executing AsyncTasks on the Executor doesn't fetch all the data - multithreading

Question

I'm using asyncTask to download some files over the internet. This is the code I've written which works
downloadUrl task = new downloadUrl(url1,"jsonData1","/sdcard/appData/LocalJson/jsonData1",context);
task.execute();
downloadUrl task1 = new downloadUrl(url2,"jsonData2","/sdcard/appData/LocalJson/jsonData2",context);
task1.execute();
downloadUrl task2 = new downloadUrl(url3,"jsonData3","/sdcard/appData/LocalJson/jsonData3",context);
task2.execute();
downloadUrl task3 = new downloadUrl(url4,"jsonData4","/sdcard/appData/LocalJson/jsonData4",context);
task3.execute();
Now, the tasks run in parallel considering the UI-Thread but they run serialized between one another, which is time consuming. So instead I've tried to execute them on the executor But the thing is that this way I'm missing some files, meaning that when they run serialized I end up with 38 files downloaded while the run on the Executor I end up with 20. I'm pretty sure that is, because I messed up something in the multi-threading code So I'll post it that to:
#Override
protected String doInBackground(String... args) {
downloadAndStoreJson(url,targetFolder);
JSONObject jsonObj = loadJSONObject(pathForLoad);
try {
processJsonData(jsonObj);
} catch (IOException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
return "done";
}
#Override
protected void onPostExecute(String result) {
s(targetFolder+" Finished!");
++mutex;
progressBar.setProgress(25*mutex);
if(mutex==4){
mutex=0;
progressBar.setProgress(100);
progressBar.dismiss();
s(monuments.size());
Intent intent = new Intent (getApplicationContext(),NextClass.class);
intent.putExtra("monuments", monuments);
startActivity(intent);
}
}
private void downloadAndStoreJson(String url,String tag){
JSONParser jParser = new JSONParser();
JSONObject json = jParser.getJSONFromUrl(url);
String jsonString = json.toString();
byte[] jsonArray = jsonString.getBytes();
File fileToSaveJson = new File("/sdcard/appData/LocalJson/",tag);
BufferedOutputStream bos;
try {
bos = new BufferedOutputStream(new FileOutputStream(fileToSaveJson));
bos.write(jsonArray);
bos.flush();
bos.close();
} catch (FileNotFoundException e4) {
// TODO Auto-generated catch block
e4.printStackTrace();
} catch (IOException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
finally {
jsonArray=null;
jParser=null;
System.gc();
}
}
private JSONObject loadJSONObject(String path){
JSONObject jsonObj = null;
File readFromJson = new File(path);
byte[] lala;
try {
lala= org.apache.commons.io.FileUtils.readFileToByteArray(readFromJson);
s("---------------"+lala.length);
String decoded = new String(lala, "UTF-8");
jsonObj = new JSONObject(decoded);
} catch (IOException e5) {
// TODO Auto-generated catch block
e5.printStackTrace();
} catch (JSONException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
return jsonObj;
}
and processJsonData is a long method which parses the json files, creates objects and then stores them in an ArrayList, that's where a problem might exist.

Answer 1

You need to make sure your code is Reentrant, meaning it must be possible to run it by several threads at the same time. Or if some code is used to syncronize the execution between your threads you need to make sure it is synchronized.
Looking at your code I see that the mutex is a static variable, which you use to keep track of your threads. Make sure that the operation on the mutex is synchronized, just to keep it clean. But that will not cause you problem...
I dont see your error in this code-snippet, either I fail to see the problem or it might be located in some other methods? Can you please share "downloadAndStoreJson"?