Android - Thread safety while updating arraylist

Question

My Activity consists of a BroadcastReceiver and an AsyncTask, both of them update an ArrayList (very often). I understand that an AsyncTask runs in the background, and there may be a possibility where the BroadcastReceiver and the AsyncTask threads may update the ArrayList at the same time. How can I make them thread-safe ?

EDIT: As alexander mentioned, a BroadcastReceiver is run on the main thread unless you explicitly implement it otherwise.

developer.android.com/reference/java/util/Vector.html <<< this — Budius
– Budius, Commented Sep 15, 2014 at 12:34
Unless you show us the code (in both threads) that access and update the list, it is not possible to give a reliable answer. (There are various methods that might work ...) — Stephen C
– Stephen C, Commented Sep 15, 2014 at 12:49
Try to make it accessible only once at any point. Store/Queue any other incoming changes. Of course, this makes only sense depending on the importance of an immediate change. A very naive approach would be make a static variable and change it everytime something is changed and this is also the condition under which your ArrayList can be changed. Maybe there is a better way, but this should work. This Problem also exist with e.g. a StringBuilder and a BufferedStringBuilder (BufferedStringBuilder is Threadsecure). Maybe you can find something helpful there... I hope this helps a bit. John — JRsz
– JRsz, Commented Sep 15, 2014 at 12:57

alexander zak · Accepted Answer · 2014-09-15 12:52:05Z

2

AsyncTask runs as a separate thread. A BroadcastReceiver usually runs on the main thread, unless you specify otherwise in 'registerReceiver' (see here: http://developer.android.com/reference/android/content/Context.html#registerReceiver(android.content.BroadcastReceiver,%20android.content.IntentFilter,%20java.lang.String,%20android.os.Handler) )

You can either use a Vector as Budius wrote, or wrap all access to the ArrayList with a synchronized block:

synchronized(myList){
    myList.add(itemToAdd);
}

answered Sep 15, 2014 at 12:52

alexander zak

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1 Comment

user1164937 Over a year ago

Thanks, I did not know that a BroadcastReceiver ran on the main thread. Nice catch! Also, I decided to go with Vector thanks to Budius.

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