How to implement a single instance Java application?

Question

Sometime I see many application such as msn, windows media player etc that are single instance applications (when user executes while application is running a new applicatio

Answer 1

You can use JUnique library. It provides support for running single-instance java application and is open-source.

http://www.sauronsoftware.it/projects/junique/

The JUnique library can be used to prevent a user to run at the same time more instances of the same Java application.

JUnique implements locks and communication channels shared between all the JVM instances launched by the same user.

public static void main(String[] args) {
    String appId = "myapplicationid";
    boolean alreadyRunning;
    try {
        JUnique.acquireLock(appId, new MessageHandler() {
            public String handle(String message) {
                // A brand new argument received! Handle it!
                return null;
            }
        });
        alreadyRunning = false;
    } catch (AlreadyLockedException e) {
        alreadyRunning = true;
    }
    if (!alreadyRunning) {
        // Start sequence here
    } else {
        for (int i = 0; i < args.length; i++) {
            JUnique.sendMessage(appId, args[0]));
        }
    }
}

Under the hood, it creates file locks in %USER_DATA%/.junique folder and creates a server socket at random port for each unique appId that allows sending/receiving messages between java applications.