How do I close a port in a case of program termination?

It sounds like your program is listening on a socket. Normally, when your program exits the OS closes all sockets that might be open (including listening sockets). However, for listening sockets the OS normally reserves the port for some time (several minutes) after your program exits so it can handle any outstanding connection attempts. You may notice that if you shut down your program abnormally, then come back some time later it will start up just fine.

If you want to avoid this delay time, you can use setsockopt() to configure the socket with the SO_REUSEADDR option. This tells the OS that you know it's OK to reuse the same address, and you won't run into this problem.

You can set this option in Java by using the ServerSocket.setReuseAddress(true) method.

You want to set the SO_REUSEADDR flag on the socket.

See setReuseAddress().

The operating system should handle things such as that automatically, when the JVM process has ended. There might be a short delay before the port is closed, though.