Class Application

Life-cycle

The entry point for JavaFX applications is the Application class. The JavaFX runtime does the following, in order, whenever an application is launched:

1. Starts the JavaFX runtime, if not already started (see Platform.startup(Runnable) for more information)
2. Constructs an instance of the specified Application class
3. Calls the init() method
4. Calls the start(javafx.stage.Stage) method
5. Waits for the application to finish, which happens when either of the following occur:
   • the application calls Platform.exit()
   • the last window has been closed and the implicitExit attribute on Platform is true
6. Calls the stop() method

Note that the start method is abstract and must be overridden. The init and stop methods have concrete implementations that do nothing.

The Application subclass must be declared public and must have a public no-argument constructor.

Calling Platform.exit() is the preferred way to explicitly terminate a JavaFX Application. Directly calling System.exit(int) is an acceptable alternative, but doesn't allow the Application stop() method to run.

A JavaFX Application should not attempt to use JavaFX after the FX toolkit has terminated or from a ShutdownHook, that is, after the stop() method returns or System.exit(int) is called.

Note: The JavaFX classes must be loaded from a set of named javafx.* modules on the module path. Loading the JavaFX classes from the classpath is not supported. See Platform.startup for more information.

Deploying an Application as a Module

If the Application subclass is in a named module then that class must be accessible to the javafx.graphics module. Otherwise, an exception will be thrown when the application is launched. This means that in addition to the class itself being declared public, the module must export (or open) the containing package to at least the javafx.graphics module.

For example, if com.foo.MyApplication is in the foo.app module, the module-info.java might look like this:

module foo.app {
    exports com.foo to javafx.graphics;
}

Parameters

Application parameters are available by calling the getParameters() method from the init() method, or any time after the init method has been called.

Threading

JavaFX creates an application thread for running the application start method, processing input events, and running animation timelines. Creation of JavaFX Scene and Stage objects as well as modification of scene graph operations to live objects (those objects already attached to a scene) must be done on the JavaFX application thread.

The Java launcher loads and initializes the specified Application class on the JavaFX Application Thread. If there is no main method in the Application class, or if the main method calls Application.launch(), then an instance of the Application is then constructed on the JavaFX Application Thread.

The init method is called on the launcher thread, not on the JavaFX Application Thread. This means that an application must not construct a Scene or a Stage in the init method. An application may construct other JavaFX objects in the init method.

All the unhandled exceptions on the JavaFX application thread that occur during event dispatching, running animation timelines, or any other code, are forwarded to the thread's uncaught exception handler.

Example

The following example will illustrate a simple JavaFX application.

import javafx.application.Application;
import javafx.scene.Group;
import javafx.scene.Scene;
import javafx.scene.shape.Circle;
import javafx.stage.Stage;

public class MyApp extends Application {
    public void start(Stage stage) {
        Circle circ = new Circle(40, 40, 30);
        Group root = new Group(circ);
        Scene scene = new Scene(root, 400, 300);

        stage.setTitle("My JavaFX Application");
        stage.setScene(scene);
        stage.show();
    }
}
 

The above example will produce the following: