The JavaFX runtime creates an instance of an Application class when launching an application. The Application subclass must be public with a public no-argument constructor.

In JDK 9, there is a further requirement that the package containing the main Application must be exported to the javafx.graphics module. For applications in the unnamed module or in an automatic module, all public classes of all packages are exported to all modules, so no additional steps are needed. Applications in an explicit named module, must "exports" the package containing their Application class to javafx.graphics in their module-info.java class.

We need to document this in the API docs for the Application class.