Re: single instance

Knute Johnson <>
Fri, 18 Jan 2013 20:50:37 -0800
On 1/18/2013 1:47 AM, Roedy Green wrote:

I have been working on polishing Knute's code. Mostly I have been
adding informal comments and renaming to help myself understand how it

I have not run it yet, but it is getting close to a test.


I have added UUIDs to break the tie for equal start times.
I have added app ids so different apps can share the same port.

I have added the ability avoid several different apps.
I have added the ability to permit two apps to run, so long as they
run on different files.

Below is what I ended up with. I'm curious about the UUID. How do you
create a time based UUID? And does it have less granularity that

package com.knutejohnson.classes;

import java.awt.*;
import java.lang.reflect.*;
import java.nio.charset.*;
import java.util.*;
import javax.swing.*;

public class Exclusive implements Runnable {
     private static final int port = 35798;
     private static final String addr = "";

     private final long myTime;
     private final String name;
     private final InetAddress group;
     private final MulticastSocket socket;
     private final String token;

     private volatile boolean runFlag;
     private volatile Thread thread;

     public Exclusive(String name) throws IOException {
         myTime = System.currentTimeMillis();
 = name;
         if (name.indexOf(",") >= 0)
             throw new IllegalArgumentException(
              "Comma character not allowed in name");

         group = InetAddress.getByName(addr);
         socket = new MulticastSocket(port);

         token = String.format("%s,%d",name,myTime);

     public void start() throws IOException {
         if (thread == null || !thread.isAlive()) {
             runFlag = true;
             thread = new Thread(this);


     public void sendToken() throws IOException {
         byte[] buf = token.getBytes(StandardCharsets.US_ASCII);
         DatagramPacket dp = new DatagramPacket(buf,buf.length,group,port);

     @Override public void run() {
         while (runFlag) {
             try {
                 // receive their time
                 byte[] buf = new byte[64];
                 DatagramPacket dp = new DatagramPacket(buf,buf.length);
                 String recStr = new String(dp.getData(),dp.getOffset(),
                 String[] arr = recStr.split(",");
                 // if names don't match there is nothing to do
                 if (!name.equals(arr[0]))
                 long theirTime = Long.parseLong(arr[1]);
                 // if we are seeing our own packet, do nothing
                 if (theirTime == myTime) {
                 // if their time is before my time, we need to shut down
                 } else if (theirTime < myTime) {
                     // can't use invokeLater()
                     try {
                         EventQueue.invokeAndWait(new Runnable() {
                             public void run() {
                               "Another Copy of this Program is Already
                     } catch (InterruptedException|
                      InvocationTargetException ex) {
                 // if their time is after my time, send out my time
                 } else if (theirTime > myTime) {
             } catch (IOException|NumberFormatException ex) {

     private void stop() {
         if (thread != null && thread.isAlive()) {
             runFlag = false;
             if (socket != null)

         // signal the waitFor() method to stop waiting
         synchronized (this) {

     // wait for up to two seconds to see if any other copy responds
     // returns true if no other copy is running.
     public synchronized boolean waitFor() throws InterruptedException {

         return runFlag;

     public void shutdown() {

     public static void main(String[] args) {
         try {
             Exclusive e = new Exclusive("Test");
             if (e.waitFor())
                 System.out.println("no other copy running!");
                 System.out.println("another copy is running");
         } catch (IOException|InterruptedException ex) {
             // probably don't want to start if you get an exception either


Knute Johnson

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