How do I launch JavaFX applications on Java 8 Ubuntu?

I tried to start the application with an error:

java.lang.ClassNotFoundException: com.sun.glass.ui.gtk.GtkPlatformFactory   at
  java.net.urlclassloader.findClass(urlclassloader.java:381)  at
  java.lang.ClassLoader.loadClass(ClassLoader.java:424)   at
  java.lang.ClassLoader.loadClass(ClassLoader.java:357)   at
  java.lang.Class.forName0(Native Method)   at
  java.lang.Class.forName(Class.java:264)   at
  com.sun.glass.ui.PlatformFactory.getPlatformFactory(PlatformFactory.java:42)
    at com.sun.glass.ui.Application.run(Application.java:146)   at
  com.sun.javafx.tk.quantum.QuantumToolkit.startup(QuantumToolkit.java:257)
    at
  com.sun.javafx.application.PlatformImpl.startup(PlatformImpl.java:211)
    at
  com.sun.javafx.application.LauncherImpl.startToolkit(LauncherImpl.java:675)
    at
  com.sun.javafx.application.LauncherImpl.launchApplicationWithArgs(LauncherImpl.java:337)
    at
  com.sun.javafx.application.LauncherImpl.launchApplication(LauncherImpl.java:328)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)  at
  sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
    at
  sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.lang.reflect.Method.invoke(Method.java:498)   at
  sun.launcher.LauncherHelper$FXHelper.main(LauncherHelper.java:767)
  Exception in thread "main" java.lang.reflect.InvocationTargetException
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)  at
  sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
    at
  sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.lang.reflect.Method.invoke(Method.java:498)   at
  sun.launcher.LauncherHelper$FXHelper.main(LauncherHelper.java:767)
  Caused by: java.lang.NullPointerException   at
  com.sun.glass.ui.Application.run(Application.java:146)  at
  com.sun.javafx.tk.quantum.QuantumToolkit.startup(QuantumToolkit.java:257)
    at
  com.sun.javafx.application.PlatformImpl.startup(PlatformImpl.java:211)
    at
  com.sun.javafx.application.LauncherImpl.startToolkit(LauncherImpl.java:675)
    at
  com.sun.javafx.application.LauncherImpl.launchApplicationWithArgs(LauncherImpl.java:337)
    at
  com.sun.javafx.application.LauncherImpl.launchApplication(LauncherImpl.java:328)
    ... 5 more

And the message cannot load the glass factory class at the end of the stack trace

I also tried to add JavaFX as a maven dependency and add this jar to the classpath I use idea for my project

to update:

My code is simple hello JavaFX exmaple:

public class Application extends javafx.application.Application {

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

    @Override
    public void start(Stage primaryStage) {
        Button btn = new Button();
        btn.setText("Say 'Hello World'");
        btn.setOnAction(new EventHandler<ActionEvent>() {

            public void handle(ActionEvent event) {
                System.out.println("Hello World!");
            }
        });

        StackPane root = new StackPane();
        root.getChildren().add(btn);

        Scene scene = new Scene(root,300,250);

        primaryStage.setTitle("Hello World!");
        primaryStage.setScene(scene);
        primaryStage.show();
    }
}

Update 1:

I used debug and found ThreadPoolExecutor. On line 1057 Excsession in Java is thrown In addition:

yes! After executing this code snippet, the first exception appears in the project console

Solution

You need to download and install openjfx lib

Sudo apt install openjfx should solve the problem