To reproduce this, run any JFXPanel application with a recent JDK (e.g., jdk-9+117) such as HelloJFXPanel and click on the JFXPanel to give it focus.

$ java HelloJFXPanel
Exception in thread "AWT-EventQueue-0" java.lang.IllegalAccessError: failed to access class sun.awt.CausedFocusEvent from class javafx.embed.swing.JFXPanel
at javafx.embed.swing.JFXPanel.sendFocusEventToFX(javafx.swing@9-internal/JFXPanel.java:595)
at javafx.embed.swing.JFXPanel.processFocusEvent(javafx.swing@9-internal/JFXPanel.java:616)
at java.awt.Component.processEvent(java.desktop@9-internal/Component.java:6357)
at java.awt.Container.processEvent(java.desktop@9-internal/Container.java:2258)
...


The fix for JDK-8080395 introduced new public API for getting the cause of a FocusEvent. It also made the existing import sun.awt.CausedFocusEvent class package-private. JFXPanel references this class, so we get the above error at run time when clicking on a JFXPanel to give it focus.