JavaFX AnimationTimer renders

Question

My goal is to completely sync each animation frame to the monitor device vsync at perfectly 60fps. I googled online, and people suggested that i could use JavaFX AnimationTimer. I wrote a simple ball JavaFX AnimationTimer, but i could sometimes see a very noticeable jittery in the animation. Could you guys please help me?

I have attached the code here:

import javafx.animation.AnimationTimer;
import javafx.application.Application;
import javafx.event.EventHandler;
import javafx.scene.Group;
import javafx.scene.Scene;
import javafx.scene.input.KeyCode;
import javafx.scene.input.KeyEvent;
import javafx.scene.paint.Color;
import javafx.scene.shape.Box;
import javafx.scene.shape.Circle;
import javafx.stage.Stage;

public class Main extends Application
        implements EventHandler <KeyEvent>
{
    final int WIDTH = 600;
    final int HEIGHT = 400;

    double ballRadius = 40;
    double ballX = 100;
    double ballY = 200;
    double xSpeed = 4;

    public static void main(String[] args) {

        launch(args);
    }

    @Override
    public void start(Stage stage) throws Exception {

        stage.setTitle("Basic JavaFX demo");

        Group root = new Group();
        Scene scene = new Scene(root, WIDTH, HEIGHT);

        // Bouncing Ball
        Circle circle = new Circle();
        circle.setCenterX(ballX);
        circle.setCenterY(ballY);
        circle.setRadius(ballRadius);
        circle.setFill(Color.BLUE);
        root.getChildren().add(circle);

        // need to attach KeyEvent caller to a Node of some sort.
        // How about an invisible Box?
        final Box keyboardNode = new Box();
        keyboardNode.setFocusTraversable(true);
        keyboardNode.requestFocus();
        keyboardNode.setOnKeyPressed(this);

        root.getChildren().add(keyboardNode);
        stage.setScene(scene);
        stage.show();

        AnimationTimer animator = new AnimationTimer(){

            @Override
            public void handle(long arg0) {
                long startTime = System.currentTimeMillis();


                // UPDATE
                ballX += xSpeed;

                if (ballX + ballRadius >= WIDTH)
                {
                    ballX = WIDTH - ballRadius;
                    xSpeed *= -1;
                } else if (ballX - ballRadius < 0) {
                    ballX = 0 + ballRadius;
                    xSpeed *= -1;
                }

                // RENDER
                circle.setCenterX(ballX);
                long stopTime = System.currentTimeMillis();
                long elapsedTime = stopTime - startTime;
                System.out.println(elapsedTime);
            }
        };

        animator.start();
    }

    @Override
    public void handle(KeyEvent arg0) {

        if (arg0.getCode() == KeyCode.SPACE )
        {
            xSpeed *= -1;
        }
    }
}