Xmonad and Java swing application's don't play well with the default config. You'll need to set your WM name to "LG3D" for Java Swing apps to work. The easiest way to add a call to setWMName "LG3D" in your startupHook like this:
import XMonad.Hooks.SetWMName
xmonad $ defaultConfig { startupHook = setWMName "LG3D" }
