UnboundedQueueBasedMessageQueue

Try to dequeue the next message from this queue, return null failing that.

Try to enqueue the message to this queue, or throw an exception.

Called when the mailbox this queue belongs to is disposed of. Normally it is expected to transfer all remaining messages into the dead letter queue which is passed in. The owner of this MessageQueue is passed in if available (e.g. for creating DeadLetters()), “/deadletters” otherwise.

Note that we implement the method in a recursive manner mainly for atomicity (not touching the queue twice).

Indicates whether this queue is non-empty.

Should return the current number of messages held in this queue; may always return 0 if no other value is available efficiently. Do not use this for testing for presence of messages, use hasMessages instead.