You can use sockets - ServerSocket can only listen on a port that is not yet in use. The first start successfully creates an instance of ServerSocket on the port - while this program is running, no other ServerSocket server can be successfully created on this 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.");
}
}
}