ConcurrentModificationException in iterator.next()

Question

I have next code in background thread

private List<IStartAction> mActions = Collections.synchronizedList(new ArrayList<IStartAction>()); 

protected void removeNonApplicableActions() {
        Iterator<IStartAction> iterator = mActions.iterator();
        while (iterator.hasNext()) {
            IStartAction action = iterator.next();
            if (!action.isApplicable()) {
                iterator.remove();
            }
        }
    }

When i run this in main thread got ConcurrentModificationException into iterator.next(). Why is this happening? I use thread-safe collection and remove items through iterator. Collection used in only this thread.

synchronized means that you can modify list in more than 1 thread under specific conditions (please see javadoc for synchronizedList() method). Here is a different issue, you are iterating and removing at the same time. Please use listIterator instead of plain iterator. Use listIterator() method. — Tomek
– Tomek, Commented May 5, 2015 at 20:23
Could you provide a minimal working example of your code so that we can reproduce the problem? — Mick Mnemonic
– Mick Mnemonic, Commented May 5, 2015 at 20:25
ConcurrentModificationException isn't necessarily indicating you have multiple threads modifying your list, it's that you are concurrently iterating through your list and modifying it at the same time. — Ryan J
– Ryan J, Commented May 5, 2015 at 20:28
What does isApplicable() do? Does it modify mActions in any way? — Paul Boddington
– Paul Boddington, Commented May 5, 2015 at 20:34

Peter Lawrey · Accepted Answer · 2015-05-05 21:20:48Z

2

Thread safety for a synchronized collection only applies to one method call. Between method calls the lock is released and another thread can lock the collection. If you perform two operations, anything could happen in the meantime, unless you lock it your self. e.g.

// to add two elements in a row, you must hold the lock.
synchronized(mAction) {
    mAction.add(x);
    // without holding the lock, anything could happen in between
    mAction.add(y);
}

Similarly to iterate over a synchronized collection, you have to hold the lock, otherwise anything could happen between method calls to the iterator.

synchronized (mAction) {
    for(Iterator<IStartAction> iter = mActions.iterator(); iter.hashNext();) {
        IStartAction action = iter.next();
        if (!action.isApplicable()) {
            iter.remove();
        }
    }
}

answered May 5, 2015 at 21:20

Peter Lawrey

535k83 gold badges770 silver badges1.2k bronze badges

Sign up to request clarification or add additional context in comments.

Collectives™ on Stack Overflow

ConcurrentModificationException in iterator.next()

1 Answer 1

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

Comments

Your Answer

Sign up or log in

Post as a guest

Related