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

The Unique4j library can be used for running a single instance of a Java application and pass messages. You can see it at https://github.com/prat-man/unique4j. It supports Java 1.6+.

It uses a combination of file locks and dynamic port locks to detect and communicate between instances with the primary goal of allowing only one instance to run.

Following is a simple example of the same:

import tk.pratanumandal.unique4j.Unique4j;
import tk.pratanumandal.unique4j.exception.Unique4jException;

public class Unique4jDemo {

    // unique application ID
    public static String APP_ID = "tk.pratanumandal.unique4j-mlsdvo-20191511-#j.6";

    public static void main(String[] args) throws Unique4jException, InterruptedException {

        // create unique instance
        Unique4j unique = new Unique4j(APP_ID) {
            @Override
            public void receiveMessage(String message) {
                // display received message from subsequent instance
                System.out.println(message);
            }

            @Override
            public String sendMessage() {
                // send message to first instance
                return "Hello World!";
            }
        };

        // try to obtain lock
        boolean lockFlag = unique.acquireLock();

        // sleep the main thread for 30 seconds to simulate long running tasks
        Thread.sleep(30000);

        // try to free the lock before exiting program
        boolean lockFreeFlag = unique.freeLock();

    }

}

Disclaimer: I created and maintain Unique4j library.