--- java/org/apache/catalina/tribes/transport/nio/NioReceiver.java	(revision 1496923)
+++ java/org/apache/catalina/tribes/transport/nio/NioReceiver.java	(working copy)
@@ -27,6 +27,7 @@ 
 import java.nio.channels.Selector;
 import java.nio.channels.ServerSocketChannel;
 import java.nio.channels.SocketChannel;
+import java.util.ConcurrentModificationException;
 import java.util.Iterator;
 import java.util.LinkedList;
 import java.util.Set;
@@ -378,19 +379,49 @@ 
         Selector selector = this.selector.getAndSet(null);
         if (selector==null) return;
         try {
-            Iterator<SelectionKey> it = selector.keys().iterator();
-            // look at each key in the selected set
-            while (it.hasNext()) {
-                SelectionKey key = it.next();
-                key.channel().close();
-                key.attach(null);
-                key.cancel();
-            }
-        }catch ( IOException ignore ){
-            if (log.isWarnEnabled()) {
-                log.warn("Unable to cleanup on selector close.",ignore);
-            }
-        }catch ( ClosedSelectorException ignore){}
+            int closeAttempt = 1;
+            int maxCloseAttempts = 3;
+            boolean goodClose = false;
+            do {
+                try {
+                    // attempt to close known keys/channels
+                    Iterator<SelectionKey> it = selector.keys().iterator();
+
+                    // in order to see a ConcurrentModificationException thrown
+                    // it is helpful to sleep after obtaining the key set iterator
+                    //try {
+                    //    System.err.println("sleeping...");
+                    //    Thread.sleep(2000);
+                    //} catch (InterruptedException ignore) {}
+
+                    while (it.hasNext()) {
+                        SelectionKey key = it.next();
+                        try {
+                            key.channel().close();
+                        } catch (IOException ignore) {
+                            if (log.isWarnEnabled()) {
+                                log.warn("Unable to cleanup on selector close.",ignore);
+                            }
+                        }
+                        key.attach(null);
+                        key.cancel();
+                    }
+                    goodClose = true;
+                } catch (ConcurrentModificationException concurrentException) {
+                    // This will occur when another thread is processing closed
+                    // connections because when connections are closed, the
+                    // associated channel is closed and the key is canceled.
+                    // See concurrency notes for Selector.
+                    if (log.isDebugEnabled()) {
+                        log.debug("Caught exception during close attempt " + closeAttempt + " of " +
+                                maxCloseAttempts + ". Retrying.", concurrentException);
+                    }
+                    try { Thread.sleep(100 * closeAttempt); } catch(InterruptedException ignore) {}
+                }
+            } while (!goodClose && closeAttempt++ < maxCloseAttempts);
+        }catch ( ClosedSelectorException ignore){
+            ignore.printStackTrace();
+        }
         selector.close();
     }