MaskTest.java

import javafx.application.Application;
import javafx.scene.Scene;
import javafx.scene.layout.Pane;
import javafx.scene.paint.Color;
import javafx.scene.paint.CycleMethod;
import javafx.scene.paint.RadialGradient;
import javafx.scene.paint.Stop;
import javafx.scene.shape.Circle;
import javafx.stage.Stage;

public class MaskTest extends Application {

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

    public void start(Stage stage) {
        Pane layout = new Pane();
        Circle red = new Circle();
        red.setCenterX(200);
        red.setCenterY(200);
        red.setRadius(50);
        red.setFill(Color.RED);
        layout.getChildren().addAll(red);

        Circle mask = new Circle();
        mask.setCenterX(200);
        mask.setCenterY(200);
        mask.setRadius(50);
        mask.setFill(new RadialGradient(0, .1, 200, 200, 20, false, CycleMethod.NO_CYCLE, new Stop(0, Color.BLACK), new Stop(1, Color.TRANSPARENT)));
        
        red.setClip(mask);

        Scene scene = new Scene(layout);
        stage.setScene(scene);
        stage.show();
    }
}