Develop Reference

AsyncTask run on one single thread in Android 2.3.3 (VERSION.SDK_INT = 10)

Following the Android's document
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.
Is there any way to run AsyncTask on one single thread in Android 2.3.3 ? I intended to use executeOnExecutor but it only support for version sdk > 11

Following the guide in this link How do I cancel an AsyncTask running BitmapFactory.decodeFile() and clean-up . I put the synchronized to the doInBackground method and see it worked.

Prevent multiple parallell AsyncTask's on API10?

According to the documentation for AsyncTask, it uses a pool of parallell threads on certain older API's:
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.
What if I truly want non-parallel execution even on pre-Honeycomb?
On newer API's, they end up running one after the other. Is there any way to get the same behaviour on pre-Honeycomb as well?

In short, no, not with the API by itself. It really only affects Donut, Eclair, Froyo and Gingerbread. Instead of using AsyncTask, you could use your own thread pool executor with a single thread so it serializes things. But, the API itself is different within the framework in versions prior to Honyecomb.

Multiple AsyncTask vs multiple requests within one AsyncTask

I'm following this tutorial to create an XML reader with options to download multiple feeds at once. Are there any downsides to executing multiple AsyncTasks (max 3) simultaneously? I'm going to add a timer to check if they finished.

Running multiple AsyncTasks at the same time -- not possible? It is possible. However it could be, that due to the cpu depending on each device, one is faster than the other. But as #Alex already answered, you wont get "real" multitasking. Haven't it tried yet I would assume, that doing it all in one AsyncTask is faster. And you could reuse the connection you established to the server.

For better architecture I'll choose 1 AsyncTask per request. It is easier to manage actual request. Also it would be easier to modify (add/remove request).
Downsides of executing multiple AsyncTasks depend on your knowledge of AsyncTask lifecycle. You need to execute it correctly (depends on android version).
Good article about the dark side of Async tasks http://bon-app-etit.blogspot.com.by/2013/04/the-dark-side-of-asynctask.html

They won't be run simultaneously, but serially. See here:
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

Starting AsyncTasks from Handlers in Android

How does the handler message queue work? I know for a fact that the message queue is tied to the thread it is initialized in. If i have 2 tasks(each download from the web), and I initiate an async task from the handler,one for each, will the 2 tasks be executed simultaneously?
I just need to understand, how the queue works..
could anyone please help! :)

First of all, AsyncTask can be executed only on the UI thread. So, even if you have two separate handlers (one for each AsyncTask) they should be both associated with the UI thread.
Secondary, several AsyncTask instances may run either simultaneously or one by one. It depends on the API version. It is better to read documentation about this:
public final AsyncTask execute (Params... params)
Executes the task with the specified parameters.
The task returns itself (this) so that the caller can keep a reference
to it.
Note: this function schedules the task on a queue for a single
background thread or pool of threads depending on the platform
version. 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 HONEYCOMB, tasks are back to being executed on 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.

ASyncTask does not execute when task.execute() is called

I have an Android application which has the main UI thread, and several other worker threads which I have spawned using ASyncTask.
However, there is a scenario that I can produce every time in which I attempt to spawn a new thread with ASyncTask using task.execute(), and it does not call doInBackground(). The thread never seems to start. Then my application seems to spin for a little while, and then begins to hang.
Here are the threads I am using:
And here is the memory usage on the device:
It does not look as though it is failing to spawn due to memory issues.
Is there some other underlying reason that I do not know of? Maximum number of threads? Is there any way for me to find out why it is not executing?

AsyncTask uses a ThreadPoolExecutor used internally with a core pool size of 5 and a LinkedBlockingQueue. In simpler terms: you can have atmost 5 AsyncTasks active at the same time. Additional tasks will be queued till one of the other AsyncTasks does not return from doInBackground().
You may want to review your code to free up some AsyncTasks. If thats not possible, you can try to create a CustomAsyncTask class in your project based on the original AsyncTask code which can be found here. Try setting the CORE_POOL_SIZE variable to a higher value or using a SynchronousQueue.

Did you read the docs?
Note: this function schedules the task on a queue for a single
background thread or pool of threads depending on the platform
version. 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.
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