Running multiple AsyncTasks at the same time -- not possible

Question

I m trying to run two AsyncTasks at the same time   Platform is Android 1 5  HTC Hero   However  only the first gets executed  Here s a simple snippet to describe my problem   public class AndroidJunk extends Activity    class PrinterTask extends AsyncTask lt String  Void  Void gt         protected Void doInBackground String     x          while  true           System out println x 0           try           Thread sleep 1000            catch  InterruptedException ie            ie printStackTrace                                            Override     public void onCreate Bundle savedInstanceState            super onCreate savedInstanceState           setContentView R layout main            new PrinterTask   execute  bar bar bar            new PrinterTask   execute  foo foo foo             System out println  onCreate   is done               The output I expect is   onCreate   is done  bar bar bar foo foo foo bar bar bar foo foo foo   And so on  However  what I get is   onCreate   is done  bar bar bar bar bar bar bar bar bar   The second AsyncTask never gets executed  If I change the order of the execute   statements  only the foo task will produce output   Am I missing something obvious here and or doing something stupid  Is it not possible to run two AsyncTasks at the same time   Edit  I realized the phone in question runs Android 1 5  I updated the problem descr  accordingly  I don t have this problem with an HTC Hero running Android 2 1  Hmmm

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It is posible  My android device version is 4 0 4 and android os Build VERSION SDK INT is 15  I have 3 spinners  Spinner c fruit  Spinner  findViewById R id fruits   Spinner c vegetable  Spinner  findViewById R id vegetables   Spinner c beverage  Spinner  findViewById R id beverages     And also I have a Async-Tack class   Here is my spinner loading code  RequestSend reqs fruit   new RequestSend this   reqs fruit where  Get fruit List   reqs fruit title  Loading fruit   reqs fruit execute     RequestSend reqs vegetable   new RequestSend this   reqs vegetable where  Get vegetable List   reqs vegetable title  Loading vegetable   reqs vegetable execute     RequestSend reqs beverage   new RequestSend this   reqs beverage where  Get beverage List   reqs beverage title  Loading beverage   reqs beverage execute      This is working perfectly  One by one my spinners loaded  I didn t user executeOnExecutor   Here is my Async-task class  public class RequestSend  extends AsyncTask lt String  String  String  gt         private ProgressDialog dialog   null      public Spinner spin      public String where      public String title      Context con      Activity activity            String   items       public RequestSend Context activityContext            con   activityContext          dialog   new ProgressDialog activityContext           this activity   activityContext              Override     protected void onPostExecute String result            try               ArrayAdapter lt String gt  adapter   new ArrayAdapter lt String gt   activity  android R layout simple spinner item  items                      adapter setDropDownViewResource android R layout simple spinner dropdown item               spin setAdapter adapter             catch  NullPointerException e                Toast makeText activity   Can not load list  Check your connection   Toast LENGTH LONG  show                e printStackTrace              catch  Exception e                 Toast makeText activity   Can not load list  Check your connection   Toast LENGTH LONG  show                e printStackTrace                      super onPostExecute result            if  dialog    null              dialog dismiss                  protected void onPreExecute             super onPreExecute            dialog setTitle title           dialog setMessage  Wait               dialog setCancelable false            dialog show                Override     protected String doInBackground String    Strings            try               Send Request                  catch  NullPointerException e                    e printStackTrace                  catch  Exception e                    e printStackTrace                          return null             public void Send Request   throws JSONException            try               String DataSendingTo    http   www example com AppRequest     where                HttpClient             HttpClient httpClient   new DefaultHttpClient                  Post header             HttpPost httpPost   new HttpPost DataSendingTo                 Adding data             List lt NameValuePair gt  nameValuePairs   new ArrayList lt NameValuePair gt  2                nameValuePairs add new BasicNameValuePair  authorized   001                  httpPost setEntity new UrlEncodedFormEntity nameValuePairs                   execute HTTP post request             HttpResponse response   httpClient execute httpPost                BufferedReader reader              try                   reader   new BufferedReader new InputStreamReader response getEntity   getContent                      StringBuilder builder   new StringBuilder                    String line   null                  while   line   reader readLine       null                        builder append line                                       JSONTokener tokener   new JSONTokener builder toString                     JSONArray finalResult   new JSONArray tokener                   items   new String finalResult length                         looping through All details and store in public String array                 for int i   0  i  lt  finalResult length    i                          JSONObject c   finalResult getJSONObject i                       items i  c getString  data name                                     catch  ClientProtocolException e                    e printStackTrace                  catch  IOException e                    e printStackTrace                             catch  ClientProtocolException e                e printStackTrace              catch  IOException e                e printStackTrace

User · Answer

AsyncTask uses a thread pool pattern for running the stuff from doInBackground    The issue is initially  in early Android OS versions  the pool size was just 1  meaning no parallel computations for a bunch of AsyncTasks  But later they fixed that and now the size is 5  so at most 5 AsyncTasks can run simultaneously  Unfortunately I don t remember in what version exactly they changed that   UPDATE   Here is what current  2012-01-27  API says on this      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  After HONEYCOMB  it is planned to change this back to a single thread to avoid common application errors caused by parallel execution  If you truly want parallel execution  you can use the executeOnExecutor Executor  Params     version of this method with THREAD POOL EXECUTOR  however  see commentary there for warnings on its use    DONUT is Android 1 6  HONEYCOMB is Android 3 0   UPDATE  2  See the comment by kabuko from Mar 7 2012 at 1 27   It turns out that for APIs where  a pool of threads allowing multiple tasks to operate in parallel  is used  starting from 1 6 and ending on 3 0  the number of simultaneously running AsyncTasks depends on how many tasks have been passed for execution already  but have not finished their doInBackground   yet   This is tested confirmed by me on 2 2  Suppose you have a custom AsyncTask that just sleeps a second in doInBackground    AsyncTasks use a fixed size queue internally for storing delayed tasks  Queue size is 10 by default  If you start 15 your custom tasks in a row  then first 5 will enter their doInBackground    but the rest will wait in a queue for a free worker thread  As soon as any of the first 5 finishes  and thus releases a worker thread  a task from the queue will start execution  So in this case at most 5 tasks will run simultaneously  However if you start 16 your custom tasks in a row  then first 5 will enter their doInBackground    the rest 10 will get into the queue  but for the 16th a new worker thread will be created so it ll start execution immediately  So in this case at most 6 tasks will run simultaneously   There is a limit of how many tasks can be run simultaneously  Since AsyncTask uses a thread pool executor with limited max number of worker threads  128  and the delayed tasks queue has fixed size 10  if you try to execute more than 138 your custom tasks the app will crash with java util concurrent RejectedExecutionException   Starting from 3 0 the API allows to use your custom thread pool executor via AsyncTask executeOnExecutor Executor exec  Params    params  method  This allows  for instance  to configure the size of the delayed tasks queue if default 10 is not what you need   As  Knossos mentions  there is an option to use AsyncTaskCompat executeParallel task  params   from support v 4 library to run tasks in parallel without bothering with API level  This method became deprecated in API level 26 0 0   UPDATE  3   Here is a simple test app to play with number of tasks  serial vs  parallel execution  https   github com vitkhudenko test asynctask  UPDATE  4  thanks  penkzhou for pointing this out   Starting from Android 4 4 AsyncTask behaves differently from what was described in UPDATE  2 section  There is a fix to prevent AsyncTask from creating too many threads   Before Android 4 4  API 19  AsyncTask had the following fields   private static final int CORE POOL SIZE   5  private static final int MAXIMUM POOL SIZE   128  private static final BlockingQueue lt Runnable gt  sPoolWorkQueue           new LinkedBlockingQueue lt Runnable gt  10     In Android 4 4  API 19  the above fields are changed to this   private static final int CPU COUNT   Runtime getRuntime   availableProcessors    private static final int CORE POOL SIZE   CPU COUNT   1  private static final int MAXIMUM POOL SIZE   CPU COUNT   2   1  private static final BlockingQueue lt Runnable gt  sPoolWorkQueue           new LinkedBlockingQueue lt Runnable gt  128     This change increases the size of the queue to 128 items and reduces the maximum number of threads to the number of CPU cores   2   1  Apps can still submit the same number of tasks

User · Answer

The android developers example of loading bitmaps efficiently uses a custom asynctask  copied from jellybean  so you can use the executeOnExecutor in apis lower than  lt  11  http   developer android com training displaying-bitmaps index html  Download the code and go to util package

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if you want to execute tasks parallel you need call the method executeOnExecutor AsyncTask THREAD POOL EXECUTOR   your task name   after Android version of 3 0  but this method is not exist before Android 3 0 and after 1 6 because it execute parallel by itself  So I suggest you customize your own AsyncTask class in your project to avoid throw exception in different Android version

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This allows for parallel execution on all android versions with API 4   Android 1 6      TargetApi Build VERSION CODES HONEYCOMB     API 11 void startMyTask AsyncTask asyncTask        if Build VERSION SDK INT  gt   Build VERSION CODES HONEYCOMB          asyncTask executeOnExecutor AsyncTask THREAD POOL EXECUTOR  params       else         asyncTask execute params       This is a summary of Arhimed s excellent answer   Please make sure you use API level 11 or higher as your project build target  In Eclipse  that is Project  gt  Properties  gt  Android  gt  Project Build Target  This will not break backward compatibility to lower API levels  Don t worry  you will get Lint errors if your accidentally use features introduced later than minSdkVersion  If you really want to use features introduced later than minSdkVersion  you can suppress those errors using annotations  but in that case  you need take care about compatibility yourself  This is exactly what happened in the code snippet above

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Just to include the latest update  UPDATE 4  in  Arhimed  s immaculate answer in the very good summary of  sulai   void doTheTask AsyncTask task        if  Build VERSION SDK INT  gt   Build VERSION CODES KITKAT       Android 4 4  API 19  and above            Parallel AsyncTasks are possible  with the thread-pool size dependent on device            hardware         task execute params         else if  Build VERSION SDK INT  gt   Build VERSION CODES HONEYCOMB       Android 3 0 to            Android 4 3            Parallel AsyncTasks are not possible unless using executeOnExecutor         task executeOnExecutor AsyncTask THREAD POOL EXECUTOR  params         else      Below Android 3 0            Parallel AsyncTasks are possible  with fixed thread-pool size         task execute params

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Making  sulai suggestion more generic     TargetApi Build VERSION CODES HONEYCOMB     API 11 public static  lt T gt  void executeAsyncTask AsyncTask lt T       gt  asyncTask  T    params        if Build VERSION SDK INT  gt   Build VERSION CODES HONEYCOMB          asyncTask executeOnExecutor AsyncTask THREAD POOL EXECUTOR  params       else         asyncTask execute params

[android] Running multiple AsyncTasks at the same time -- not possible?

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