Creating another AsyncTask inside of doInBackground

Issue

I recently looked through SO to find the answer to the same question here, but there was not an answer directed at the question in regards to the risk of doing so. But basically I want to run another AsyncTask inside the doInBackground() method of another AsyncTask. Is this a bad approach and/or does it leave any potential side effects?

I know that running it in the onPostExecute() works and so far from past experiences I have not had any issues due to the fact that onPostExecute() runs back on the main thread which started a AsyncTask to begin with.

Solution

From the API docs:

•The task instance must be created on the UI thread.

doInBackground() runs on the background thread. So you cannot create and run another asynctask from doInBackground().

http://developer.android.com/reference/android/os/AsyncTask. Have a look at the topic under threading rules.

When an asynchronous task is executed, the task goes through 4 steps: (Straight from the doc)

1.onPreExecute(), invoked on the UI thread before the task is executed. This step is normally used to setup the task, for instance by showing a progress bar in the user interface.

2.doInBackground(Params...), invoked on the background thread immediately after onPreExecute() finishes executing. This step is used to perform background computation that can take a long time. The parameters of the asynchronous task are passed to this step. The result of the computation must be returned by this step and will be passed back to the last step. This step can also use publishProgress(Progress...) to publish one or more units of progress. These values are published on the UI thread, in the onProgressUpdate(Progress...) step.

3.onProgressUpdate(Progress...), invoked on the UI thread after a call to publishProgress(Progress...). The timing of the execution is undefined. This method is used to display any form of progress in the user interface while the background computation is still executing. For instance, it can be used to animate a progress bar or show logs in a text field.

4.onPostExecute(Result), invoked on the UI thread after the background computation finishes. The result of the background computation is passed to this step as a parameter.

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.

Update 05-04-2023

AsyncTask is no more used in the android world for async stuff. Consider using Coroutines. Suggest to switch to coroutines ASAP

https://developer.android.com/kotlin/coroutines?gclid=Cj0KCQjwla-hBhD7ARIsAM9tQKuy9BV88avgYYrrwkcCHJ1zktoctbOrdrWavXT0NTDktIfPdfb8Q-4aAqXIEALw_wcB&gclsrc=aw.ds

The codelab sample is availabe at https://developer.android.com/codelabs/kotlin-coroutines#0

Answered By - Raghunandan