aSyncTask死锁第二个aSyncTask

Question

I encountered a problem today involving aSyncTasks in my Android project and after some research have found the answer and, as no-one I had spoken to had ever realised, so I thought I'd share with the SO community in case anyone finds it to be of any use.

A Brief overview of my problem:

I have a UI activity class to download packages of multiple files when the install button is clicked. The file "bundles" are split into up to four separate downloads and the "Group" information is stored in a custom class along with other information for the files. When the download button is clicked, (up to) four jobs are queued with the android DownloadManager via a utility, DownloadUtilities.

When a "group" download is enqueued by DownloadUtilities, the DownloadManager download references are stored inside the "group" custom class for later use.

The Download Utilities class features a BroadcastReceiver to acknowledge the files and update the download references to 0 for each element of the group as they finish downloading to show that they are finished.

Once all files in a group are finished downloading, they can be processed. This is done by an aSyncTask triggered by the DownloadReceiver.

This in turn, triggers a broadcast to notify the UI activity that the requested download has been finished so the UI can be updated accordingly.

Everything worked great until this point, however, I found an issue when I tried to add a progress bar to show the progress of the "group"

To update the ProgressBar, I created a new aSyncTask which could query the download manager to calculate the total size of the downloading files, and then poll once a second to update the ProgressBar to show the current download progress.

The doInBackground() thread consists of the following pseudocode:

Check if file1 is being downloaded, if it is, query the download manager for the total file size and add it to total
Same for files2-4

While the download references are not all 0:
If file 1 is still downloading, get file1 download total
Same for file2-4
Update progress to sum of downloaded bytes/total

The problem I found was that the system would deadlock. Logcat would should the download receiver being told a file had finished downloading by the DownloadManager, and that the ProcessDownload aSyncTask onPreExecute() was running, but the doInBackGround was never running.

In the meantime, the download monitor was constantly looping in the doInBackground() because the condition of the while loop was never being deasserted, because the deassertion is done in the ProcessDownload doInBackground() thread.

It became apparent that the aSyncTask doInBackground() methods were mutually exclusive and so causing a deadlock but I had no idea why. I had never knowingly structured my code in such a way in the past that this posed an issue...

My understanding was that aSyncTasks offered a developer friendly way to multithread your code but it seems that this is not the case...

Answer 1

So I spent several hours trying to work out what may have been causing the issue and came to the conclusion that based on my understanding of what aSyncTasks were, there was no reason my code shouldn't work... So I took to the internet to trawl documentation and eventually found the answer.

"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[]) withTHREAD_POOL_EXECUTOR."

It seems that my thinking did USED to be the way, however, as of Honeycomb, it was no longer. aSyncTasks DO offload code from the UI thread, however, it offloads it onto a single thread with multiple aSyncTask doInBackground() methods being executed sequentially.

So I went back and changed my code to use good 'ol Java Threads and lo and behold, my code works!

Anyone thinking of using dependent code, it's worth thinking carefully and looking into aSyncTasks to be sure you understand its behaviour. They're a great tool I've used a lot in the past, but with more 'advanced?' stuff, it may be better to go back to basics.

Hopefully this will be of use to someone, I have wasted a lot of time on this because of a simple misunderstanding of the behaviour of an aSyncTask -.-

Source that gave me the answer

EDIT:

Props to Krylez for pointing me to http://commonsware.com/blog/2012/04/20/asynctask-threading-regression-confirmed.html which also features a really eloquent way of maintaining compatibility with both pre and post Honeycomb devices. All that is necessary is to modify the aSyncTask call

if (Build.VERSION.SDK_INT>=Build.VERSION_CODES.HONEYCOMB) {
  myTask.executeOnExecutor(AsyncTask.THREAD_POOL_EXECUTOR);
}
else {
  myTask.execute();
}

which reverts Honeycomb+ behavior to how it used to be