java background application on windows

Question

I want to have a main application and a background application each in it's own JVM.
When launching the main it has to check if the background is running, and launch it if not.
I will have multiple mains running but I want only one background application.
It is fine for the background to keep running even after all mains are closed.

It's my first making a background application, I don't know:

• how to launch a dettached Application. (that won't terminate together)
• how to check if an other application is running
• how the two applications can communicate between them (should I use sockets)

I am still in the design phase, any comment or suggestion even remotely related is welcome.
I will be grateful if you can provide example code or a link.

Answer 1

From my understanding, you are looking for SingleInstance application, and you don't want it to be terminate to perform some background process?

I don't understand The 3rd question, but I'm gonna give you straight answer for the first and second.

public class Main extends Application {

    @Override
    public void start(Stage primaryStage) throws Exception{

        checkIfRunning(); //Check first if the Application is aready running.

        Parent root = FXMLLoader.load(getClass().getResource("fxml/main.fxml"));
        primaryStage.setTitle("Hello World");
        Scene scene = new Scene(root);
        primaryStage.setScene(scene);
        primaryStage.show();

        Platform.setImplicitExit(false); //Prevent the Application from Terminating when it's close

    }

    public static void main(String[] args) {
        launch(args);
    }


    /**The function for checking if the application is already running start here**/
    private static final int PORT = 9999;
    private static ServerSocket socket;
    private static void checkIfRunning() {
        try {
            //Bind to localhost adapter with a zero connection queue
            socket = new ServerSocket(PORT,0, InetAddress.getByAddress(new byte[] {127,0,0,1}));
        }
        catch (BindException e) {
            System.err.println("Application already running.");
            System.exit(1);
        }
        catch (IOException e) {
            System.err.println("Unexpected error when checking if application is already running.");
            e.printStackTrace();
            System.exit(2);
        }
    }
}

try it and you'll notice when you close the application it will still running and you won't be able to start more than one of the application.