Another possibility is to use JSyn by Phil Burke:
http://www.softsynth.com/jsyn/
I don't believe you can use it to connect to other softsynths, but it
allows you to set up your own high-quality synths within java. I've
used it for a few projects:
http://music.columbia.edu/~brad/jlooch/
http://music.columbia.edu/~brad/software/downloads/My_Music_Book.html
for some examples. These are both pretty old, but they still seem to
run ok.
brad
http://music.columbia.edu/~brad/