How do I make sure only one instance of my program can be executed?

Question

I want my program, a Java executable .jar, to be run just once. I made a program but now I want users not to be able to open more than one instance ....thanks for your time.

Answer 1

it also depends what "more than one instance" means.

1. once per machine -> tcp / mailslot / atom / fixed path
2. once per user -> lock file in user homedir
3. once user session -> a little more complicated and more dependent on platform