ConcurrentModificationException on Iterator.next()

Question

I am getting a ConcurrentModificationException on the following code:

private static List<Task> tasks = new LinkedList<Task>();
...
public void doTasks(){
    synchronized(tasks){
        Iterator<Task> it = tasks.iterator();

        while(it.hasNext()){
            Task t = it.next(); < Exception is always thrown on this line.

            if(t.isDone()){
                it.remove();
            } else {
                t.run();
            }
        }
    }
}
...
public void addTask(Task t){
    synchronized(tasks){
        tasks.add(t);
    }
}
...
public void clearTasks(){
    synchronized(tasks){
        tasks.clear();
    }
}

The Object "tasks" is not used anywhere else in the class. I'm not sure why I'm getting the exception. Any help would be greatly appreciated.

I would use an ExecutorService. You can submit tasks to it and it is built in. — Peter Lawrey
– Peter Lawrey, Commented Apr 26, 2013 at 21:38
I added that information regarding where the exception is thrown into the source. — idunnololz
– idunnololz, Commented Apr 26, 2013 at 21:42

Trinimon · Accepted Answer · 2013-04-26 21:52:33Z

3

This is your issue:

if(t.isDone()){
    ...
} else {
    t.run(); // probably changing the task, so consequently the list tasks
}

EDIT: you can't change the tasks list in the loop. Check out the ConcurrentModificationException documentation for more details.

Cheers!

edited Apr 26, 2013 at 21:52

answered Apr 26, 2013 at 21:39

Trinimon

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3 Comments

idunnololz Over a year ago

I read elsewhere that you can if you use Iterator.remove(). stackoverflow.com/questions/1655362/…

Boris the Spider Over a year ago

@idunnololz yup, you're absolutely right. remove is an optional operation so it can throw an UnsupportedOperationException, otherwise it's supported by the underlying Collection.

Trinimon Over a year ago

I'm inclined to say it's the t.run() cause the tasks t might be changed by run() ...

idunnololz · Accepted Answer · 2013-04-26 21:55:34Z

1

Found the bug! I forgot the scenario where the task ran in doTask() can actually call addTask(). However I am a bit confused why this can happen as I thought the "tasks" object would be locked by the doTask() function.

answered Apr 26, 2013 at 21:55

idunnololz

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3 Comments

Boris the Spider Over a year ago

The Thread already has a lock so it works fine. synchronized prevents other Threads from acquiring the monitor on tasks but the Thread in question already has the monitor.

Brian Over a year ago

Exactly what @BoristheSpider said. The locks are reentrant, so if the thread tried to obtain the lock a second time, it would be granted. Ergo, Trinimon's answer was correct.

yshavit Over a year ago

If they weren't reentrant, your thread would have deadlocked itself! Think about it: it would block on trying to add a task, and it wouldn't be able to unblock until it completed the doTask that's blocked by the attempt to add. It's like stopping your car at a stop sign and not starting it back up again until you're home.

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