Over the past few years there have been a few times that I've alpha- and beta-tested java applications. Now of course I like to try applying tests outside of what I think the authors have thought of, and surprisingly enough, a lot of these applications fail to understand the concept of the remote X11 display. You know, where you use ssh, telnet, rsh or rlogin to connect to machine which does not provide your primary display, and you set the DISPLAY environment variable.
I recall one particular application's authors told me that using a remote display was explicitly not supported. This for a multi-user, networked backup system. Of course the (now-EOL'd) Motif version worked without a problem when I ran the gui on the server and displayed it on my workstation.
Anyway, I finally got a few spare cycles to google this problem. I searched for java x11 display hang
and found a link to Elliot Hughes' blog
where this issue was discussed. Elliot did a bit of investigation and discovered that if he set
$ AWT_TOOLKIT=XToolkit ; export AWT_TOOLKIT
and then re-ran his java application, everything displayed just fine
So I tried that fix, and it works for me too. I'm happy now!