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

You could use sockets - a ServerSocket can only listen on a port that's not already in use. The first launch successfully creates a ServerSocket instance on the port - while that program is running, no other ServerSocket can successfully be created on that port.

import java.io.IOException;
import java.net.ServerSocket;

public class OneInstance {

    private static ServerSocket SERVER_SOCKET;

    public static void main(String[] args) {
        try {
            SERVER_SOCKET = new ServerSocket(1334);
            System.out.println("OK to continue running.");
            System.out.println("Press any key to exit.");
            System.in.read();
        } catch (IOException x) {
            System.out.println("Another instance already running... exit.");
        }
    }
}