multithreading - Android AsyncTask threads limits? -

i developing application need update info every time user logs in system, use database in phone. operations (updates, retrieving data db , etc.) use async tasks. till didn't see why shouldn't use them, experienced if operations of async tasks stop on pre-execute , don't jump doinbackground. strange leave that, developed simple application check whats wrong. , strange enough, same behavior when count of total async tasks reach 5, 6th 1 stops on pre-execute.

does android have limit of asynctasks on activity/app? or bug , should reported? did experience same problem , maybe found workaround it?

here code:

simply create 5 of threads work in background:

private class longasync extends asynctask<string, void, string> {     @override     protected void onpreexecute()     {         log.d("testbug","onpreexecute");         isrunning = true;     }      @override     protected string doinbackground(string... params)     {         log.d("testbug","doinbackground");         while (isrunning)         {          }         return null;     }      @override     protected void onpostexecute(string result)     {         log.d("testbug","onpostexecute");     } } 

and create thread. enter preexecute , hang (it not go doinbackground).

private class testbug extends asynctask<string, void, string> {     @override     protected void onpreexecute()     {         log.d("testbug","onpreexecute");          waiting = new progressdialog(testactivity.this);         waiting.setmessage("loading data");         waiting.setindeterminate(true);         waiting.setcancelable(true);         waiting.show();     }      @override     protected string doinbackground(string... params)     {         log.d("testbug","doinbackground");         return null;     }      @override     protected void onpostexecute(string result)     {         waiting.cancel();         log.d("testbug","onpostexecute");     } } 

all asynctasks controlled internally shared (static) threadpoolexecutor , linkedblockingqueue. when call execute on asynctask, threadpoolexecutor execute when ready time in future.

the 'when ready?' behavior of threadpoolexecutor controlled 2 parameters, core pool size , maximum pool size. if there less core pool size threads active , new job comes in, executor create new thread , execute immediately. if there @ least core pool size threads running, try queue job , wait until there idle thread available (i.e. until job completed). if not possible queue job (the queue can have max capacity), create new thread (up-to maximum pool size threads) jobs run in. non-core idle threads can decommissioned according keep-alive timeout parameter.

before android 1.6, core pool size 1 , maximum pool size 10. since android 1.6, core pool size 5, , maximum pool size 128. size of queue 10 in both cases. keep-alive timeout 10 seconds before 2.3, , 1 second since then.

with of in mind, becomes clear why asynctask appear execute 5/6 of tasks. 6th task being queued until 1 of other tasks complete. reason why should not use asynctasks long-running operations - prevent other asynctasks ever running.

for completeness, if repeated exercise more 6 tasks (e.g. 30), see more 6 enter doinbackground queue become full , executor pushed create more worker threads. if kept long-running task, should see 20/30 become active, 10 still in queue.