How to call launch() more than once in java

try this, I tried this and found successful

@Override
public void start() {
    super.start();
    try {
                    // Because we need to init the JavaFX toolkit - which usually Application.launch does
                    // I'm not sure if this way of launching has any effect on anything
        new JFXPanel();

        Platform.runLater(new Runnable() {
            @Override
            public void run() {
                // Your class that extends Application
                new ArtisanArmourerInterface().start(new Stage());
            }
        });
    } catch (Exception e) {
        e.printStackTrace();
    }
}

You can't call launch() on a JavaFX application more than once, it's not allowed.

From the javadoc:

It must not be called more than once or an exception will be thrown.

Suggestion for showing a window periodically

1. Just call Application.launch() once.
2. Keep the JavaFX runtime running in the background using Platform.setImplicitExit(false), so that JavaFX does not shutdown automatically when you hide the last application window.
3. The next time you need another window, wrap the window show() call in Platform.runLater(), so that the call gets executed on the JavaFX application thread.

For a short summary implementation of this approach:

• See the answer by sergioFC

If you are mixing Swing you can use a JFXPanel instead of an Application, but the usage pattern will be similar to that outlined above.

• For an example of the JFXPanel apprach, see Irshad Babar s answer.

Wumpus Sample

This example is bit more complicated than it needs to be because it also involves timer tasks. However it does provide a complete stand-alone example, which might help sometimes.

import javafx.animation.PauseTransition;
import javafx.application.*;
import javafx.geometry.Insets;
import javafx.scene.Scene;
import javafx.scene.control.Label;
import javafx.stage.Stage;
import javafx.util.Duration;

import java.util.*;

// hunt the Wumpus....
public class Wumpus extends Application {
    private static final Insets SAFETY_ZONE = new Insets(10);
    private Label cowerInFear = new Label();
    private Stage mainStage;

    @Override
    public void start(final Stage stage) {
        // wumpus rulez
        mainStage = stage;
        mainStage.setAlwaysOnTop(true);

        // the wumpus doesn't leave when the last stage is hidden.
        Platform.setImplicitExit(false);

        // the savage Wumpus will attack
        // in the background when we least expect
        // (at regular intervals ;-).
        Timer timer = new Timer();
        timer.schedule(new WumpusAttack(), 0, 5_000);

        // every time we cower in fear
        // from the last savage attack
        // the wumpus will hide two seconds later.
        cowerInFear.setPadding(SAFETY_ZONE);
        cowerInFear.textProperty().addListener((observable, oldValue, newValue) -> {
            PauseTransition pause = new PauseTransition(
                    Duration.seconds(2)
            );
            pause.setOnFinished(event -> stage.hide());
            pause.play();
        });

        // when we just can't take it  anymore,
        // a simple click will quiet the Wumpus,
        // but you have to be quick...
        cowerInFear.setOnMouseClicked(event -> {
            timer.cancel();
            Platform.exit();
        });

        stage.setScene(new Scene(cowerInFear));
    }

    // it's so scary...
    public class WumpusAttack extends TimerTask {
        private String[] attacks = {
                "hugs you",
                "reads you a bedtime story",
                "sings you a lullaby",
                "puts you to sleep"
        };

        // the restaurant at the end of the universe.
        private Random random = new Random(42);

        @Override
        public void run() {
            // use runlater when we mess with the scene graph,
            // so we don't cross the streams, as that would be bad.
            Platform.runLater(() -> {
                cowerInFear.setText("The Wumpus " + nextAttack() + "!");
                mainStage.sizeToScene();
                mainStage.show();
            });
        }

        private String nextAttack() {
            return attacks[random.nextInt(attacks.length)];
        }
    }

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

Update, Jan 2020

Java 9 added a new feature called Platform.startup(), which you can use to trigger startup of the JavaFX runtime without defining a class derived from Application and calling launch() on it. Platform.startup() has similar restrictions to the launch() method (you cannot call Platform.startup() more than once), so the elements of how it can be applied is similar to the launch() discussion and Wumpus example in this answer.

For a demonstration on how Platform.startup() can be used, see Fabian's answer to How to achieve JavaFX and non-JavaFX interaction?

I use something like this, similar to other answers.

private static volatile boolean javaFxLaunched = false;

public static void myLaunch(Class<? extends Application> applicationClass) {
    if (!javaFxLaunched) { // First time
        Platform.setImplicitExit(false);
        new Thread(()->Application.launch(applicationClass)).start();
        javaFxLaunched = true;
    } else { // Next times
        Platform.runLater(()->{
            try {
                Application application = applicationClass.newInstance();
                Stage primaryStage = new Stage();
                application.start(primaryStage);
            } catch (Exception e) {
                e.printStackTrace();
            }
        });
    }
}