JavaFX FXML is an XML format used to compose JavaFX GUIs in a way similar to how you compose web GUIs with HTML.
The Java volatile tutorial has been updated, so the visibility and happens-before guarantee is more properly explained.
The biggest addition to Java 9 is the Java Platform Module System which brings better modularity to Java. This Java modules tutorial explains how modules work.
The JavaFX MenuBar adds a top menu to a JavaFX application, just like you are used to from desktop applications.