Rather than return null when we request a channel that doesn't exist, throw an exception containing a vaguely informative message and the relevant channel number. This means that at least when this bug occurs you'll be able to get a better idea of where it's occurring.

Arguably this is an API change, which might be problematic for a 1.7.1 release, but it's an API change to code which is really internal, so I don't think it's an issue.

Author: David R. MacIver

src/com/rabbitmq/client/impl/AMQConnection.java

@@ -428,9 +428,9 @@ private class MainLoop extends Thread {
                                     // for non-zero channels (and any inbound commands on
                                     // channel zero that aren't Connection.CloseOk) must
                                     // be discarded.
-                                    ChannelN channel = _channelManager.getChannel(frame.channel);
-                                    // FIXME: catch NullPointerException and throw more informative one?
-                                    channel.handleFrame(frame);
+                                    _channelManager
+                                        .getChannel(frame.channel)
+                                        .handleFrame(frame);
                                 }
                             }
                         }

src/com/rabbitmq/client/impl/ChannelManager.java

@@ -62,9 +62,13 @@ public synchronized void setChannelMax(int value) {
      * Public API - Looks up an existing channel associated with this connection.
      * @param channelNumber the number of the required channel
      * @return the relevant channel descriptor
+     * @throws UnknownChannelException if there is no Channel associated with the 
+     *         required channel number.
      */
     public ChannelN getChannel(int channelNumber) {
-        return _channelMap.get(channelNumber);
+        ChannelN result = _channelMap.get(channelNumber);
+        if(result == null) throw new UnknownChannelException(channelNumber);
+        return result;
     }
 
     public void handleSignal(ShutdownSignalException signal) {

src/com/rabbitmq/client/impl/UnknownChannelException.java

@@ -0,0 +1,45 @@
+//   The contents of this file are subject to the Mozilla Public License
+//   Version 1.1 (the "License"); you may not use this file except in
+//   compliance with the License. You may obtain a copy of the License at
+//   http://www.mozilla.org/MPL/
+//
+//   Software distributed under the License is distributed on an "AS IS"
+//   basis, WITHOUT WARRANTY OF ANY KIND, either express or implied. See the
+//   License for the specific language governing rights and limitations
+//   under the License.
+//
+//   The Original Code is RabbitMQ.
+//
+//   The Initial Developers of the Original Code are LShift Ltd,
+//   Cohesive Financial Technologies LLC, and Rabbit Technologies Ltd.
+//
+//   Portions created before 22-Nov-2008 00:00:00 GMT by LShift Ltd,
+//   Cohesive Financial Technologies LLC, or Rabbit Technologies Ltd
+//   are Copyright (C) 2007-2008 LShift Ltd, Cohesive Financial
+//   Technologies LLC, and Rabbit Technologies Ltd.
+//
+//   Portions created by LShift Ltd are Copyright (C) 2007-2009 LShift
+//   Ltd. Portions created by Cohesive Financial Technologies LLC are
+//   Copyright (C) 2007-2009 Cohesive Financial Technologies
+//   LLC. Portions created by Rabbit Technologies Ltd are Copyright
+//   (C) 2007-2009 Rabbit Technologies Ltd.
+//
+//   All Rights Reserved.
+//
+//   Contributor(s): ______________________________________.
+//
+
+package com.rabbitmq.client.impl;
+
+class UnknownChannelException extends RuntimeException{
+    private final int channelNumber;
+    
+    public UnknownChannelException(int channelNumber){
+        super("Unknown channel number " + channelNumber + " for this this connection");
+        this.channelNumber = channelNumber;
+    }
+
+    public int getChannelNumber(){
+        return channelNumber;
+    }
+}