Most people used an internal javafx FontMetrics class, which has been deprecated in version 9 of the jdk. That means that your app that relied on this will simply not work anymore. Below is a simple replacement that will provide the computeStringWidth as well as ascent, descent and lineHeight. The produiced values are exactly the same as if they were called from the FontMetrics class itselve.

import javafx.geometry.Bounds;
import javafx.scene.text.Font;
import javafx.scene.text.Text;

public class FontMetrics
{
 final private Text internal;
 public float ascent, descent, lineHeight;
 public FontMetrics(Font fnt)
 {
 internal =new Text();
 internal.setFont(fnt);
 Bounds b= internal.getLayoutBounds();
 lineHeight= (float) b.getHeight();
 ascent= (float) -b.getMinY();
 descent=(float) b.getMaxY();
 }

 public float computeStringWidth(String txt)
 {
 internal.setText(txt);
 return (float) internal.getLayoutBounds().getWidth();
 }
}