Question

In Java, I need to grab a port number for communication between multiple instances of the same program.
Now, I could simply pick some fixed number and go with it. But I’m wondering if there’s a way to dynamically choose the port number, so that I don’t have to bother my users with setting the port number.

Here’s one idea I had, which works like this:

• There’s a fixed initial port number A.
• Program ‘MyApp’ starts, tries to grab port A.
• If it succeeds, then it’s the first instance of ‘MyApp’. Done.
• If it fails, it asks over port A whether the program on A is an instance of ‘MyApp’. If yes, communicate with that instance. Done. If not, try to grab port A+1. And if there’s another program using that port (not an instance of ‘MyApp’ either), then grab A+2, then A+3, and so on.

Does this strategy make sense? Or is there a better way to dynamically choose a port number?

Answer 1

If you bind to port 0, Java will use a system-generated port. 🙂 So, that’s probably the easiest way to fall back if your desired port is already used.

ServerSocket s = new ServerSocket(0);
int port = s.getLocalPort();    // returns the port the system selected