Re: weird CopyOnWriteArraySet error

Patricia Shanahan <>
Sun, 29 Oct 2006 00:14:12 GMT
Daisy wrote:

I'm getting a java.util.NoSuchElementException at high loads using
java.util.concurrent.CopyOnWriteArraySet. I have one guess why and
would like to hear if it makes sense. Could this error occur because
two threads call the same instance?

For example, thread A executes i.hasNext() which returns true. Then
thread B runs and happens to start executing at When thread
A runs again, it will execute the but B has already advanced
the iterator to the end of the list.

I'm using CopyOnWriteArraySet to avoid synchronizing. Do I have to
sync to avoid this issue?

Thanks for the opinions!

public class DistributionSet extends CopyOnWriteArraySet {

   private Iterator i;

  public void enqueue( Object message ) {

           for ( i = this.iterator( ) ; i.hasNext( ) ; ) {

            // .next() call is throwing an error - why? either:
            // copy hasn't completed or .hasNext() has a different
count, or some listener was removed
            EventListener listener = ( EventListener ) );
             listener.eventObserved( message ) ;


   public boolean add( EventListener consumer ) {
                  super.add( consumer );
         return true;


If enqueue can be called in multiple threads, then there is a risk of
NoSuchElementException. For example:

Suppose thread A calls enqueue, and enqueue assigns to i the Iterator
reference iterator() returned, and i.hasNext() returns true. Now A gets
interrupted, thread B starts running, and thread B calls enqueue.

Thread B assigns to i the Iterator reference that its iterator() call
returned. At that point, the thread A Iterator becomes unreachable.
Thread B completes its enqueue call, leaving i referencing an iterator
that returned false from i.hasNext().

Now thread A gets control back, but i references the Iterator that
thread B was using. The thread A next call fails.

If the uses of the shared Iterator are finely interleaved, as they could
be on a dual processor, you could get other problems such as not passing
a particular message to some listeners.

However, it is a problem you created, by choosing to make i an instance
field forcing multiple threads to share the iterator reference. Why is
that necessary?

If i were local, each enqueue activation would be able to remember its
own Iterator reference.


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