CopyOnWriteArrayList throwing CurrentModificationException

Question

I'm occasionally getting a ConcurrentModificationException when I iterate over a list. A Google search informs me that it's probably because I'm altering that list in another thread while iterating over it and that to make this problem go away I should use java.util.concurrent.CopyOnWriteArrayList....

... except I already am.

Apparently, I'm doing something really stupid somewhere.

Does anybody have any insight into how one might induce CopyOnWriteArrayList to toss a ConcurrentModificationException? If it matters, I'm using Java 5.

Edit: Since the mutators I'm using may matter, I'm modifying this list in two ways:

Adding elements to the front. (list.add(0, newElement);)
Using subList to let older items fall off the back. (list = list.subList(0, MAX_LIST_SIZE);)

Do those raise red flags? If so, why? My understanding was that because these operations make a copy of the thing first, any existing iterators would be pointing at the unmodified original and would thus not care. Do I have a hole in my knowledge?

Edit 2: The precise code that's causing the problem is still a bit murky, but I can at least post the exception I'm seeing:


java.util.ConcurrentModificationException
    at java.util.concurrent.CopyOnWriteArrayList$COWSubList.checkForComodification(Unknown Source)
    at java.util.concurrent.CopyOnWriteArrayList$COWSubList.iterator(Unknown Source)
    at....

... where it points to a for-each loop instantiation in my code.

That COWSubList does seem to imply that my call to subList is the root of my problem; I'd still like to understand why.

Edit 3: *facepalm*

CopyOnWriteArrayList.subList() returns a List, not a CopyOnWriteArrayList. The list it returns is under no implied obligation to provide any of COWAL's protections. Which makes using subList() like this to remove elements a Very Bad Idea.

Don't know for certain if this is my culprit, but it's damned suspicious and needs to be corrected regardless.

It’s an old question, but it still should noted that list = list.subList(0, MAX_LIST_SIZE); is a horrible way to remove elements, as the sub list still references the original list which contains all elements, hence, this creates a memory leak. The canonical way of removing elements at the end, is list.subList(MAX_LIST_SIZE, list.size()).clear(); Generally, sub lists are a tool for applying arbitrary list operations to a part of a list, so they should be held temporarily throughout the operation only. — Holger

Sbodd Sbodd · Accepted Answer · 2009-10-06T19:21:55

CopyOnWriteArrayList.subLists throw ConcurrentModificationExceptions if the containing list changes out from underneath it:

public class ListTest {

  private static List<int[]> intList;

  public static void main (String[] args) {
    CopyOnWriteArrayList<Integer> cowal = new CopyOnWriteArrayList<Integer>();
    cowal.add(1);
    cowal.add(2);
    cowal.add(3);

    List<Integer> sub = cowal.subList(1, 2);
    cowal.add(4);
    sub.get(0); //throws ConcurrentModificationException
  }
}

CopyOnWriteArrayList throwing CurrentModificationException

2 Answers