I just found out the hard way that there are two bugs when using a non-blocking DatagramChannel in Java with empty (zero payload size) UDP packets.
The first one is not so much a bug but more an API limitation: When sending an empty UDP packet, you cannot tell whether it has been actually sent. The method returns the number of bytes sent and returns zero when the packet has not been sent, but if you send an empty packet, the number of bytes sent is zero even if the send operation suceeded. So there is no way to tell whether the send operation was successful.
The second bug is more serious and this one clearly is a bug in the implementation. When using a DatagramChannel with a Selector, the selector does not return from its select operation when an empty packet has been received. This means that the select call might block forever and you will only see the empty packets received once a non-empty packet arrives.
I describe the two problems and a possible workaround in more detail in my wiki. In the project that I am working on, I can live with not knowing for sure whether the packet was sent (I just try again later if there is no reaction) and for the case where I have to receive packets, I am now using blocking I/O. However, I still think that this is a nasty bug that should be fixed in a future release of Java.