sun.font.FontManagerFactory uses reflection to load platform specific sun.font.FontManager class from "sun.font.fontmanager" system property.
https://github.com/openjdk/jdk/blob/c81690d7166c32caff6ef3a55fe9b157049e2b56/src/java.desktop/share/classes/sun/font/FontManagerFactory.java#L79
The "sun.font.fontmanager" property can be one of:
"sun.awt.Win32FontManager"
"sun.font.CFontManager"
"sun.awt.X11FontManager"
sun.font.FontManager is internal JDK class and JDK modularization feature prevents a user from providing custom FontManager class.
It would be good to avoid using reflection in this case and create corresponding platform specific classes directly.