Re: SocketChannels, Selectors, Iterators and ConcurrentModificationExceptions, oh my.

[Note: Reposted my question from comp.lang.java. This group is more
appropriate, not to mention more active.]

Hi there,

I'm trying to make an instant messaging connection library that's rather
sparse with Threads, due to it being meant for a transport for XMPP. Most
libraries out there create a Thread for damn near everything that's going
on, so I'm writing my own.

I'm running into a bit of trouble with my Selector, which I periodically
poll from the lone Thread I have so far. If I have just one SocketChannel
registered, I have no problems, and the thing connects fine and receives
all the input it should. If I have two connections open at the same time,
then when I get my SelectionKey iterator and hit 'next()', it blows up
with a java.util.ConcurrentModificationException.

While the iterator can throw the Exception if there is even the chance of
the iterator's underlying data being modified, I am confused by this not
being a problem if I only have the single channel open. I have checked and
verified that I am not accidentally starting two Threads (and even so,
wouldn't the synchronization on the iterator be sufficient to satify it?)

I'm not experienced at all with socket programming in general, and
java.nio.* in particular, so I wonder if anyone here could shed some
light?

Some code from my manager Thread's run method:

public void run()
{
  [...]
  while(!stopped)
  {
    this.selector.selectNow();
    Iterator<SelectionKey> selectionKeyIterator =
                                 selector.selectedKeys().iterator();
    synchronized(selectionKeyIterator)
    {
      SelectionKey selectionKey = null;
      while(selectionKeyIterator.hasNext())
      {
        selectionKey = selectionKeyIterator.next(); // This goes BOOM.
        selectionKeyIterator.remove();
        /**
         * Verify that the key is valid, get the socketchannel from the
         * key, get my connection object, have it read the data, send it
         * for processing by a 'listener' of sorts, etc...
      }
    }

    Thread.sleep(50); // Complete with try/catch, natch.
  }
}

--
Frank