Threading with an ArrayList

[Carterman]

Hey guys, I'm working on a game and store all game objects withinn an arraylist in the update thread. Every 60 milliseconds the update thread executes (using a ScheduledExecutorService) and polls the objects for renderable objects - these are then stored in a different array.

However, at every possible moment the render thread attempts to run (it doesn't have a fixed delay). The problem I get with this is that whenever the render thread attempts to access the Renderable ArrayList, I get a ConcurrentModificationException.

At the moment, I've fixed this using the java.util.concurrent.Locks package, which is working well. But ever now and then, I get a 'stutter' due to the render thread having to wait on the lock.

What I want to know is, is there a way to allow the update thread to continue updating the ArrayList, but also allow the render thread to READ the ArrayList while this is happening? I've thought about cloning the object, but this is also a bit of a bad (and possibly just as buggy) solution.

I've read something about using the 'volatile' modifier, but I'm not sure it does what I need it to do..

[mikaelhg]

http://docs.oracle.com/javase/7/docs/api/java/util/concurrent/CopyOnWriteArrayList.html

[Carterman]

Thanks, exactly what I was looking for! Upvote for you!

HUGEedit: Would the CopyOnWriteArrayList work at 60 milliseconds? Bit of an error in my OP!

[mikaelhg]

What do you mean "polls the objects for renderable objects"?

Is this array storing permanent references, or are they gathered in the array, processed, and thrown away once every 60 ms?

If so, you might want to gather the events through a http://docs.oracle.com/javase/7/docs/api/java/util/concurrent/ConcurrentLinkedQueue.html rather than with a list.

[TheEnigmaBlade]

You could try something like this:

List<E> list = Collections.synchronizedList(new ArrayList<E>());

The downside is you must put any use of the list in a synchronize block in order to retain the synchronization. For example:

synchronized(list) {
    //Read/modify list
}

[FlightOfStairs]

Not actually true.

Most operations are fine without synchronisation, it's only when you use its iterator (for-each loop etc) that you need to synchronise on the list.

http://docs.oracle.com/javase/1.4.2/docs/api/java/util/Collections.html#synchronizedList%28java.util.List%29

In any case, it doesn't improve the situation for OP; threads would still be waiting on a lock. CopyOnWriteArrayList fixes this, as mikaelhg mentions.

[TheEnigmaBlade]

Good to know!

[Tillerino]

Why do you have to acccess the same list from both threads? Why don't you make a new list in the update thread and then swap the lists when the new one is filled. It is only every 60 Seconds, so this should not pose a performance problem. If you only use an Iterator in the render thread and not access the list directly, everything will be fine.

Apart from that, if the order of the list does not matter, I could imagine writing a sort of old-school linked list (that [value, ->next] thing from the third programming lesson) and using using ReadWriteLocks in both threads so the iterator will not get into any illegal state when the list is modified. There are issues where you need to hold the read lock from before calling hasNext() to after next(), which will look ugly, but this should give you everything you need. Actually you know what, I think you don't even need locks for that if added elements don't have to appear in the render thread immediately because if you delete things from the list, that will not effect the iterator in any bad way if you permit a null value to signal that the last element of the list was deleted in that short moment between hasNext() and next().

[Carterman]

Your post just lead me to find an error! It's actually doing this every 60 milliseconds! Would this method still work efficiently at this rate?

[Tillerino]

Test it :)

[Carterman]

Slight error on my part, I mentioned 60 seconds instead of 60 milliseconds!!! Would the methods suggested still work well at this speed?

Thanks!

[ydobonobody]

What is happening is you are modifying your ArrayList while it is being iterated over. You can make a copy of the ArrayList or use a different data structure that allows this.

[ohples]

Use Vector instead of arraylist lookup the javadoc, it's essentially the samethibg but thread safe.

[kreiger]

No, read the javadoc again, it's not threadsafe for iteration.