We're talking about Java (SWT/Eclipse) being able to display/run on Wayland, not whether a GUI-less java app can run on a computer with Wayland.
Well, We can include swing at the discussion too. Im concerned when we'll be able to run IntelliJ (I think it uses Swing) on Wayland.
Is Java Swing use GTK to render GUI components, or relys on a low level graphic server API to render?
IMO no because Eclipse is a Java app and Java must support Wayland too
Actually, no. "Java" doesn't need to support Wayland, because while Eclipse is written in Java, it doesn't use the standard Java APIs for GUI apps. All that's needed is for SWT (the API used by Eclipse) to work on Wayland, and obviously, porting that API to Gtk+3 is a bit step.
That said, people talk hopefully about Gtk3 ports, asking "does that mean it will run on Wayland?". And the answer to that is "probably not". Sure, if you're got perfect code that uses only Gtk and never references X11 libraries, then yes, it'll run on Wayland without further modification. But "perfect code" is produced only by perfect coders, a mythical species not encountered in the real world. As such, porting to Gtk3 will get you closer to Wayland support (because staying with Gtk2 will never get there), but it's only a first step.