Java NIO UDP client with multiple servers

Question

I'm building a UDP client that can communicate with a selection of different servers. Given that an NIO application involves using a single receive thread, how can I dispatch incoming datagrams to the correct part of my application? i.e. associate incoming packets with the outgoing packets.

In theory, when sending (or connecting?) to a server, it should be possible to get the source ip/port in the outgoing Datagram and then recognise incoming packets as their responses by inspecting the destination ip/port. (because: http://www.dcs.bbk.ac.uk/~ptw/teaching/IWT/transport-layer/source-destination.gif)

Most UDP client examples seem to assume a single server, so that identifying incoming datagrams as responses to outgoing datagrams is trivial, for example:

ByteBuffer textToEcho = ByteBuffer.wrap("blah");
ByteBuffer echoedText = ByteBuffer.allocateDirect(MAX_PACKET_SIZE);

DatagramChannel datagramChannel = DatagramChannel.open(StandardProtocolFamily.INET)
datagramChannel.connect(new InetSocketAddress(REMOTE_IP, REMOTE_PORT));

while(true)
{
     int sent = datagramChannel.write(textToEcho);
     datagramChannel.read(echoedText);
}

Perhaps I could use multiple DatagramChannels and iteratively call read() on each, dispatching data to the appropriate to wherever my application is expecting responses?

Glenn Lane Glenn Lane · Accepted Answer · 2014-01-15T14:18:45

If you're dead-set on using just one channel (and one bound local port), you need to avoid using the connect and write methods. Instead, use the send method.

Looping through your servers, use the send method for each server. You may need to rewind() your byte buffer after each send... I'm not sure if send clones the buffer or not.
When all servers have been sent: In a loop, as long as there are servers that haven't responded, use receive to get both the data returned (buffer argument), and the server that returned it (the method return value). Keep looping until the server list is exhausted, but you want to put a time-limit on the loop itself (for dead servers or lost packets)

Ideally in the receive loop, you want the receive method to block for a short period of time before timing out. If you can't find a way to configure blocking, you could use non-blocking, and put a Thread.sleep in your loop instead. Try to get timed blocking working though, that's the best way.

Java NIO UDP client with multiple servers

2 Answers