Question

I’m writing a UDP client-server pair for a networks class, and I have hit on a problem. This is a rather unorthodox networks assignment, so a little background first:

The goal is to create a server to implement push-based notifications. The key point here is that the server has to contact the client at whatever address it was last seen, as well listen for the client’s control packets. So therefore, I have a thread running on the client periodically sending out UDP packets to the server, which logs their origin for when it needs to send out a response. This technique also busts through NAT’s, as the send refreshes the address translation.

So then, here is my dilemma: Unless I’m mistaken, the NAT maps its own address and a generated port number onto it’s clients address port combination. Therefore, in order to successfully traverse the NAT, I need to move all my packets through one port on the client machine. The updater thread would simply have to listen for a time, push out an update packet, and go back to listening.

Then here is where it get hairy. If the original thread, which wants to perform some action, wants the port, it has to wake the announcer, which is blocking while waiting for the response.

How can I pull this off in Java?

P.S.: If it turns out that the NAT would allow a communication on a different port to go through, then things are awesome.

Answer 1

Note: I am not necessarily telling you this is the right way to solve your larger problem.

But the answer to your top-line question, “How do I signal a sleeping thread in Java” is: Call interrupt() on the thread. You’ll need a more elaborate mechanism in place to communicate why it has been interrupted, but that’s a start. interrupt() will wake a sleep()ing or wait()ing thread with an InterruptedException, but I don’t think that’s really what you’re asking.

This will not wake up a thread blocked on a read() call, say a socket. It sounds like you are using a DatagramSocket, in which case you have a couple of options:

• Use a non-blocking implementation. (aka, “Selector-based”, or New I/O (nio) in Java lingo) See e.g. DatagramChannel; also maybe this SO question and/or this one
• Use normal Java I/O, set a socket timeout of suitable length, and wrap your calls to read() in a loop, checking for the appropriate condition.