How to Avoid \"Application Launch Must Not Be Called More than Once\" Exception in JavaFX

How to Deal with the Exception "Application Launch Must Not Be Called More than Once" in Java

Calling launch() more than once in a JavaFX application is not permitted. This is explicitly stated in the JavaFX documentation:

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

Suggestion for Displaying a Window Periodically

Instead of multiple calls to launch(), follow these steps:

1. Call Application.launch() once.
2. Keep the JavaFX runtime running in the background using Platform.setImplicitExit(false).
3. When you need to display another window, execute the window show() call within Platform.runLater() to ensure it's handled by the JavaFX application thread.

Example Implementation:

<code class="java">public class MyApplication extends Application {
    private Stage primaryStage;

    @Override
    public void start(Stage primaryStage) {
        this.primaryStage = primaryStage;
        primaryStage.setScene(new Scene(new Label("Hello, World!")));
        primaryStage.show();

        // Keep the JavaFX runtime running in the background
        Platform.setImplicitExit(false);
    }

    public void showNewWindow() {
        Platform.runLater(() -> {
            Stage newWindow = new Stage();
            newWindow.setScene(new Scene(new Label("New Window")));
            newWindow.show();
        });
    }

    public static void main(String[] args) {
        launch(args);
    }
}</code>

Alternative Approaches:

• Using JFXPanel: Instead of an Application, you can use a JFXPanel. However, the usage pattern remains similar.
• Using Platform.startup(): Java 9 introduced Platform.startup() to trigger JavaFX runtime without an Application class and launch() call. Similar restrictions apply as with launch().

Conclusion (Optional):

By adhering to these guidelines, you can avoid the "Application Launch Must Not Be Called More than Once" exception and display windows periodically in your JavaFX application.

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