how can an AsyncTask wait for another without deadLock

Question

I'm trying to implement 2 Async Tasks where one should wait for the other to finish.

I make a test case for it, that looks like this:

public class MainActivity extends Activity{

    public static boolean SYNCING = false;
    public static Object LOCK = new Object();

    public static void setSync(){
        Log.i(TAG, "setting Sync=true");
        synchronized (LOCK) {
            SYNCING = true;
        }
    }

    public static void waitForSync(){
        Log.i(TAG, "waiting for Sync");
        synchronized (LOCK) {
            try{
                while (SYNCING){
                    LOCK.wait();
                }
            } catch (InterruptedException ignore){}
        }
    }

    public static void setSyncFinish(){
        Log.i(TAG, "setSyncFinish to notify all");
        synchronized (LOCK) {
            SYNCING = false;
            LOCK.notifyAll();
        }
    }

    public class WaitingOnLoaderTask extends AsyncTask<Void, Void, Void>{

        @Override
        protected Void doInBackground(Void... params) {
            Log.i(TAG, "doInBackground of WaitingForLoaderTask");

            try {
                Thread.sleep(500);
            } catch (InterruptedException e) {

            }

            waitForSync();
            Log.i(TAG, "finishedWaiting!");
            return null;
        }

    }

    public class LoaderTask extends AsyncTask<Void, Void, Void>{

        long mRuntime;
        int mTimes;

        public LoaderTask(long runtime, int times){
            mRuntime = runtime;
            mTimes = times;
        }

        @Override
        protected Void doInBackground(Void... params) {
            while(mTimes >= 0){
                Log.i(TAG, "sleeping for " + mTimes + " times for " + mRuntime + " milliseconds");
                mTimes--;
                try {
                    Thread.sleep(mRuntime);
                } catch (InterruptedException e) {
                    Log.e("MainActivity", "Interrupted", e);
                }
            }

            setSyncFinish();

            return null;
        }

    }
}

But I can't get the WaitingOnLoaderTask to wait correctly. if I run this in onCreate

new WaitingOnLoaderTask().execute((Void[])null);
new LoaderTask(1000, 5).execute((Void[])null);

I get a logOutput like this (a DeadLock happens):

03-08 18:17:23.309: I/myUniqueLogTag(4432): setting Sync=true
03-08 18:17:23.317: I/myUniqueLogTag(4432): doInBackground of WaitingForLoaderTask
03-08 18:17:23.817: I/myUniqueLogTag(4432): waiting for Sync

and on

new LoaderTask(1000, 5).execute((Void[])null);
new WaitingOnLoaderTask().execute((Void[])null);

i get a Log like this:

03-08 18:18:53.082: I/myUniqueLogTag(4621): sleeping for 5 times for 1000 milliseconds
03-08 18:18:54.082: I/myUniqueLogTag(4621): sleeping for 4 times for 1000 milliseconds
03-08 18:18:55.090: I/myUniqueLogTag(4621): sleeping for 3 times for 1000 milliseconds
03-08 18:18:56.082: I/myUniqueLogTag(4621): sleeping for 2 times for 1000 milliseconds
03-08 18:18:57.090: I/myUniqueLogTag(4621): sleeping for 1 times for 1000 milliseconds
03-08 18:18:58.106: I/myUniqueLogTag(4621): sleeping for 0 times for 1000 milliseconds
03-08 18:18:59.113: I/myUniqueLogTag(4621): doInBackground of WaitingForLoaderTask
03-08 18:18:59.113: I/myUniqueLogTag(4621): setSyncFinish to notify all
03-08 18:18:59.621: I/myUniqueLogTag(4621): waiting for Sync
03-08 18:18:59.621: I/myUniqueLogTag(4621): finishedWaiting!

so I guess one is preventing the other to run. Did they compute their doInBackground actually in the same Thread, and LOCK.wait() blocks this Thread from executing further?

Is there a way to fix it (and stay compatible to Android 2.2)?

Note: the second Time is basically what I want, but in other Task implementations I cannot be shure that WaitingOnLoaderTask is only executed when the other one has finished.

Answer 1

Your best bet would be to use Geobits suggestion.

You can use the onPostExecute() function to call the next Async task so it will perform it:

class WaitingOnLoaderTask extends AsyncTask<Void,Void,Void>
{
    protected void onPreExecute(Void)
    {
        super.onPreExecute();
    }

    protected void doInBackground(Void... params)
    {
        //Commands for first AsyncTask
    }

    protected void onPostExecute(Void)
    {
        //Call AsyncTask 2
    }
}

That way it will wait until Task 1 completes.

Answer 2

Your code works as expected on Android 3.2 and lower. On higher Android versions AsyncTasks are not executed parallel try this to make it work on Android 4+:

LoaderTask loaderTask = new LoaderTask(1000, 5);
WaitingOnLoaderTask waitingTask = new WaitingOnLoaderTask();
if (Build.VERSION.SDK_INT >= 14) {
    loaderTask.executeOnExecutor(AsyncTask.THREAD_POOL_EXECUTOR);
    waitingTask.executeOnExecutor(AsyncTask.THREAD_POOL_EXECUTOR);
} else {
    loaderTask.execute();
    waitingTask.execute();
}

Note: You should mark your AsyncTasks as static to avoid leaking Activity on configuration change (eg rotation).