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AsyncTask stops when app is killed

My code runs fine, but if I go back to the home screen and return the code stops running on the text view shows nothing. Shouldn't it restart when I return to the page? It throws no errors, but if I eliminate the task and initialize the app it all over again it works but I must stay on that activity because If I go to Home and return it stops.
public class IsstatusActivity extends AppCompatActivity {
JSONParser jsonparser = new JSONParser();
TextView latitude;
TextView longitude;
String lat;
String longit;
JSONObject jobj = null;
private final ReentrantLock lock = new ReentrantLock();
private final Condition tryAgain = lock.newCondition();
#Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.isstatus);
latitude = (TextView) findViewById(R.id.Coordinates);
longitude = (TextView) findViewById(R.id.longitude);
new Retrievedata().execute();
}
class Retrievedata extends AsyncTask<Void, String, Void> {
#Override
protected Void doInBackground(Void... params) {
try {
while (!isCancelled()) {
jobj = jsonparser.makeHttpRequest("http://api.wheretheiss.at/v1/satellites/25544");
String lat = "latitude : " + jobj.getString("latitude");
String longit ="longitude : " + jobj.getString("longitude");
// this will cause onProgressUpdate to be called with lat & long
publishProgress(lat,longit);
// it's okay to sleep within the background thread
Thread.sleep(1500);
}
} catch (InterruptedException e) {
Log.w("RetrieveData", "thread was interrupted", e);
} catch (JSONException e) {
Log.e("RetrieveData", "parse error", e);
}
return null;
}
#Override
protected void onProgressUpdate(String... values) {
latitude.setText(values[0]);
longitude.setText(values[1]);
}
}
}

Shouldn't it restart when I return to the page?
Not necessarily. From the AsyncTask documentation:
When first introduced, AsyncTasks were executed serially on a single background thread. Starting with DONUT, this was changed to a pool of threads allowing multiple tasks to operate in parallel. Starting with HONEYCOMB, tasks are executed on a single thread to avoid common application errors caused by parallel execution.
So by default the execution of AsyncTasks on versions of Android from Honeycomb onwards is serialised, meaning that only one can execute at any given time.
Looking at Retrievedata, it is set up to loop until it is cancelled (or it is interrupted or encounters a JSON related exception). However, the task isn't ever cancelled in IsstatusActivity (say, in onDestroy()), so barring interruptions or errors it will run forever (or at least until your app is killed).
Part of the reason for this is that AsyncTasks are not tied to the activity lifecycle. That means that they don't get automatically stopped when the activity they were created in finishes - rather, they keep on running until they finish whatever they're tasked with doing. Again, because Retrievedata loops until it's cancelled and this never happens, it will happily block the execution of new instances of Retrievedata created when the IsstatusActivity is subsequently started again (not to mention any other AsyncTask instances you may be using in other activities).
For more background I recommend checking out:
https://stackoverflow.com/a/13147992/2259854
http://blog.danlew.net/2014/06/21/the-hidden-pitfalls-of-asynctask/
You may be able to solve the issue by cancelling the task when you finish the activity, but as other posters have stated it may be better to avoid using AsyncTask at all for this kind of task. My understanding of AsyncTask is that it is best used for short running operations - indeed, this is what the documentation says:
AsyncTask is designed to be a helper class around Thread and Handler and does not constitute a generic threading framework. AsyncTasks should ideally be used for short operations (a few seconds at the most.) If you need to keep threads running for long periods of time, it is highly recommended you use the various APIs provided by the java.util.concurrent package such as Executor, ThreadPoolExecutor and FutureTask.
As an aside, there's another issue with the code posted. As Retrievedata is an inner class (as opposed to being static) of IsstatusActivity, each instance of it will keep an implicit reference to the instance of IsstatusActivity it was created in. This means that even after you finish any given instance of IsstatusActivity, it will remain in memory until the Retrievedata instance created within it has finished and can be garbage collected. As per the above, this will never happen because the task runs forever.
This is called activity/context leaking. Every time IsstatusActivity is started and finished, the instance will hang around consuming memory until the app is killed. Activities are heavy objects so this is definitely something to be avoided.
Hope that helps!

Hold calling thread until multiple asynctask finishes

I have a background thread which calls 3 asynctasks to perform tasks simultaneously. The calling thread acts as a Queue for 3 sets of these tasks.
So basically I need to call 3 asynctasks simultaneously and once they are completed I want to call the next three tasks on the queue and repeat.
However I am having trouble pausing the caller thread until the three asynctask finishes. As a result the next three tasks in the queue start running before the previous three tasks are completed.
So is there anyway to hold the caller thread until the asynctasks are completed. I know that you can user .get() in asynctask but it will not enable the three asynctasks to run simultaneously.

Following code is rather a pseudocode of the idea. Basically, you'll declare an interface which will check for firing next three AsyncTasks. You'll also need to maintain a counter to see if the number of received response from AsyncTask is multiplied by 3. If it is then you can trigger next three AsyncTasks.
public interface OnRunNextThree{
void runNextThreeTasks();
}
public class MainClass extends Activity implements OnRunNextThree {
private int asyncTasksCounter = 0;
public void onCreate() {
//Initiate and run first three of your DownloadFilesTask AsyncTasks
// ...
}
public void runNextThreeTasks() {
if (asyncTasksCounter % 3 == 0) {
// you can execute next three of your DownloadFilesTask AsyncTasks now
// ...
} else {
// Otherwise, since we have got response from one of our previously
// initiated AsyncTasks so let's update the counter value by one.
asyncTasksCounter++;
}
}
private class DownloadFilesTask extends AsyncTask<Void, Void, Void> {
private OnRunNextThree onRunNextThree;
public DownloadFilesTask(OnRunNextThree onRunNextThree) {
this.onRunNextThree = onRunNextThree;
}
protected Void doInBackground(Void... voids) {
// Do whatever you need to do in background
return null;
}
protected void onPostExecute(Void aVoid) {
super.onPostExecute(aVoid);
//Got the result. Great! Now trigger the interface.
this.onRunNextThree.runNextThreeTasks();
}
}
}

Async tasks are meant to do things asynchronously.... so this can't be done in a straight forward way...
Even if you manage to do this, it basically defeats the whole point of asynchronous operation.
You should look for a Synchronous network operation.
Check out Volley... It is a google library specially made for network operations and it supports Synchronous operations
http://www.truiton.com/2015/02/android-volley-making-synchronous-request/
There are many other libraries available ... Retrofit is one other good library..

Android AsyncTask waiting/queued

I have a class extending an AsyncTask that sends messages to a WCF web service. Simple messages one by one will work fine, but if I send a message on a new thread that takes 30 seconds to complete, then midway through that I send a quick request it won't execute the AsyncTask until the long one has returned.
I thought the whole idea of AsyncTask was these two messages would run on different threads and therefore wouldn't stack?
Here is my code:
private class RunnableTask extends AsyncTask<RunnableObj, RunnableObj, RunnableObj> {
#Override
protected RunnableObj doInBackground(RunnableObj... params) {
try {
if (params[0].requestBody != (null)) {
params[0].request.body(new JSONObject(params[0].requestBody));
}
params[0].request.asVoid();
return params[0];
}
catch (Throwable e) {
params[0].handler.onFailure(e);
e.printStackTrace();
}
return null;
}
#Override
protected void onPostExecute(RunnableObj runnableObj) {
super.onPostExecute(runnableObj);
runnableObj.handler.onSuccess();
}
}
This is my AsyncTask above.
public void put(final String urlStr, final String requestBody, final HttpResponseHandler httpResponseHandler) {
RunnableObj obj = new RunnableObj();
obj.handler = httpResponseHandler;
obj.request = webb.put(urlStr)
.header(ServiceConstants.SessionTokenHeader, MyApplication.getSessionToken())
.ensureSuccess();
obj.requestBody = requestBody;
new RunnableTask().execute(obj);
}
This is the method I use to call the Async.
As you can see in the method I use to call the service, I initialise a new instance of RunnableTask each time.
How it performs:
The long request will go to the web service and start it's 30 seconds of doing things.
10 seconds later my quick little PUT creates it's object, then the last thing the debugger shows is the break point on the "new RunnableTask().execute(obj);" line and then it just disappears.
20 seconds later the first line of my RunnableTasks doInBackground method will hit and it will perform the PUT.
Please can someone help? Or at least tell me I'm doing something very stupid..

You can execute multiple AsyncTask by using executeOnExecutor
if( Build.VERSION.SDK_INT >= Build.VERSION_CODES.HONEYCOMB ) {
new MyAsyncTask().executeOnExecutor(AsyncTask.THREAD_POOL_EXECUTOR);
} else {
new MyAsyncTask().execute();
}
For more check the AsyncTask documentation

According to the AsyncTask Docs:
When first introduced, AsyncTasks were executed serially on a single
background thread. Starting with DONUT, this was changed to a pool of
threads allowing multiple tasks to operate in parallel. Starting with
HONEYCOMB, tasks are executed on a single thread to avoid common
application errors caused by parallel execution.
If you truly want parallel execution, you can invoke
executeOnExecutor(java.util.concurrent.Executor, Object[]) with
THREAD_POOL_EXECUTOR.
As you can see, AsyncTasks currently only operate on a single background thread, meaning multiple queued tasks will have to fire one after another. If you would like to execute concurrent tasks, you'll want to follow the instructions above.

How to implement a more flexible AsyncTask?

while it is very convenient to use, from my understanding, AsyncTask has two important limitations:
doInBackground of any instances will share the same worker
thread, i.e. one long running AsyncTasks can block all others.
execute, onPostExecute and other "synchronizing" methods must/will always be executed on the UI-thread, i.e. not on the Thread, which wants to start the task.
I ran into trouble, when I tried to reuse some existing AsyncTasks in a background IntentService that are responsible for the client-server communication of my app. The tasks of the service would fight over time in the worker thread with those of the UI activities. Also they would force the service to fall back onto the UI-thread, although that service should perform its work quietly in the background.
How would I go about removing/circumventing these limitations? I basically want to achieve:
A framework that closely resembles AsyncTask (because I need to migrate a lot of critical code there).
Each instance of such a task should run its doInBackground on its own thread instead of a single worker thread for all instances.
Edit: Thx to VinceFR for pointing out this can be achieved by simply calling executeOnExecutor instead of execute.
The callbacks like onPostExecute should be called on the same thread that started the task by calling execute, which should not need to be the UI-thread.
I figure, I'm not the first person to require something like this. Therefore I wonder: Is there already some third-party library that can be recommended to accomplish this? If not, what would be a way to implement this?
Thanks in advance!

The solution looks like this:
All classes that spawn AsyncTasks that might interfere with each other get their own Executor like this one (make that elaborate as you like using thread pools etc.):
private Executor serviceExecutor = new Executor() {
public void execute(Runnable command) {
new Thread(command).start();
}
};
As pointed out by VinceFR you can run an AsyncTask on a given Executor by calling it like this (where payload are the parameters that you would regularly pass to a task):
task.executeOnExecutor(serviceExecutor, payload);
However, this breaks backwards-compatibility to Gingerbread and earlier. Also, if you want to support Honeycomb, you need to make sure, this call happens on the UI thread. Jelly Bean will take care of this automatically.
Now the trickier part: Keeping the service running on its own thread. As many things in Android this seems harder than it needs to be (or maybe I'm lacking some information here). You can't use an IntentService, because that will shut down automatically the first time an AsyncTask takes over and let's the onHandleIntent callback complete.
You need to setup your own thread and event loop on the service:
public class AsyncService extends Service {
private static final String TAG = AsyncService.class.getSimpleName();
private class LooperThread extends Thread {
public Handler threadHandler = null;
public void run() {
Looper.prepare();
this.threadHandler = new Handler();
Looper.loop();
}
}
private LooperThread serviceThread = null;
private Handler serviceThreadHandler = null;
#Override
// This happens on the UI thread
public void onCreate() {
super.onCreate();
}
#Override
// This happens on the UI thread
public int onStartCommand(Intent intent, int flags, int startId) {
this.serviceThread = new LooperThread();
this.serviceThread.start();
while(this.serviceThread.threadHandler == null) {
Log.d(TAG, "Waiting for service thread to start...");
}
this.serviceThreadHandler = this.serviceThread.threadHandler;
this.serviceThreadHandler.post(new Runnable() {
#Override
public void run() {
doTheFirstThingOnTheServiceThread();
}
});
return Service.START_STICKY;
}
// doTheFirstThingOnTheServiceThread
}
No you need to make sure that each time an AsyncTask returns to the UI thread, you end up in your service thread instead:
// This happens on the serviceThread
private void doTheFirstThingOnTheServiceThread() {
// do some stuff
// here we can reuse a class that performs some work on an AsyncTask
ExistingClassWithAsyncOperation someUsefullObject = new ExistingClassWithAsyncOperation();
// the existing class performs some work on an AsyncTask and reports back via an observer interface
someUsefullObject.setOnOperationCompleteListener(new OnOperationCompleteListener() {
#Override
// This happens on the UI thread (due to an ``AsyncTask`` in someUsefullObject ending)
public void onOperationComplete() {
serviceThreadHandler.post(new Runnable() {
#Override
public void run() {
doTheSecondThingOnTheServiceThread();
}
});
}
}
someUsefulObject.performOperation();
}
// This happens on the serviceThread
private void doTheSecondThingOnTheServiceThread() {
// continue working on the serviceThread
}
So, this works for me. I'd be delighted to see a simpler solution for this. Note that the solution requires the service to know that is will be called back by the ExistingClassWithAsyncOperation on the UI thread. I don't particularly like this dependency, but don't know how to do better right now. However, I don't have to rewrite a lot of existing classes that perform asynchronous operations using AsyncTask.

Android AsyncTask - avoid multiple instances running

I have AsyncTask that processes some background HTTP stuff. AsyncTask runs on schedule (Alarms/service) and sometime user executes it manually.
I process records from SQLite and I noticed double-posts on server which tells me that sometime scheduled task runs and at the same time user runs it manually causing same record to be read and processed from DB twice. I remove records after they processed but still get this.
How should I handle it ? Maybe organize some kind of queing?

You can execute your AsyncTask's on an Executor using executeOnExecutor()
To make sure that the threads are running in a serial fashion please use: SERIAL_EXECUTOR.
Misc: How to use an Executor
If several activities are accessing your DB why don't create a sort of gateway database helper and use the synchronized block to ensure only one thread has access to it at an instant

Or, you can try this to see if the Task is currently running or not:
if (katitsAsyncTask.getStatus().equals(AsyncTask.Status.FINISHED))
katitsAsyncTask.execute();
else
// wait until it's done.

Initialize the AsyncTask to null. Only create a new one if it is null. In onPostExecute, set it to null again, at the end. Do the same in onCancelled, in case the user cancels this. Here's some untested code to illustrate the basic idea.
import android.app.Activity;
import android.os.AsyncTask;
import android.os.Bundle;
import android.view.View;
import android.widget.Button;
public class FooActivity extends Activity {
private class MyAsyncTask extends AsyncTask<Foo, Foo, Foo> {
#Override
protected void onPostExecute(Foo foo) {
// do stuff
mMyAsyncTask = null;
}
#Override
protected void onCancelled() {
// TODO Auto-generated method stub
mMyAsyncTask = null;
}
#Override
protected Foo doInBackground(Foo... params) {
try {
// dangerous stuff
} catch (Exception e) {
// handle. Now we know we'll hit onPostExecute()
}
return null;
}
}
private MyAsyncTask mMyAsyncTask = null;
#Override
public void onCreate(Bundle bundle) {
Button button = (Button) findViewById(R.id.b2);
button.setOnClickListener(new View.OnClickListener() {
public void onClick(View v) {
if (mMyAsyncTask == null) {
mMyAsyncTask = new MyAsyncTask();
mMyAsyncTask.execute(null);
}
}
});
}
}

I know this was a while ago now, and you have solved your problem. but I just had a similar problem. Reno's suggestion put me on the right track, but for those who have been finding it difficult to fill in the gaps. Here is how I overcame a similar issue to that of katit's.
I wanted a particular AsyncTask to only run if it was not currently running. And as a forward from Reno's suggestion, the AsyncTask interface has been created to handle all the nitty gritty processes in properly dealing with threads for Android. Which means, the Executor is built in. As this blog suggests:
"When execute(Object.. params) is invoked on an AsyncTask the task is executed in a background thread. Depending on the platform AsyncTasks may be executed serially (pre 1.6 and potentially again in 4+), or concurrently (1.6-3.2).
To be sure of running serially or concurrently as you require, from API Level 11 onwards you can use the executeOnExecutor(Executor executor, Object.. params) method instead, and supply an executor. The platform provides two executors for convenience, accessable as AsyncTask.SERIAL_EXECUTOR and AsyncTask.THREAD_POOL_EXECUTOR respectively. "
So with this in mind, you can do thread blocking via the AsyncTask interface, it also implies you can simply use the AsyncTasks.getStatus() to handle thread blocking, as DeeV suggests on this post.
In my code, I managed this by:
Creating a global variable defined as:
private static AsyncTask<String, Integer, String> mTask = null;
And in onCreate, initialising it as an instance of my AsyncTask called CalculateSpecAndDraw:
mTask = new CalculateAndDrawSpec();
Now when ever I wish to call this AsyncTask I surround the execute with the following:
if(mTask.getStatus() == AsyncTask.Status.FINISHED){
// My AsyncTask is done and onPostExecute was called
mTask = new CalculateAndDrawSpec().execute(Integer.toString(progress));
}else if(mTask.getStatus() == AsyncTask.Status.PENDING){
mTask.execute(Integer.toString(progress));
}else{
Toast.makeText(PlaySpecActivity.this, "Please Wait..", 1000).show();
}
This spawns a new thread if it is finished, or if the thread state is PENDING, the thread is defined but has not been started we start it. But otherwise if the thread is running we don't re-run it, we simply inform the user that it is not finished, or perform what ever action we wish. Then if you wanted to schedule the next event rather than just block it from re running, take a look at this documentation on using executors.

How about just wrapping your check-what-to-send and send-it logic in a synchronized method? This approach seems to work for us.

try having some instance boolean value that gets set to "true" on the asynctask's preexecute then "false" on postexecute. Then maybe in doinbackground check if that boolean is true. if so, then call cancel on that particular duplicate task.

You could keep the state of the task in shared preferences. Check the value (Boolean perhaps) before starting the task. Set the state to finished(true?) in onPostExecute and false in onPreExecute or in the constructor