JavaFX AnimationTimer renders - javafx

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;
}
}
}