I found this, but havnt tried it yethttp://sites.google.com/site/mumart/home/liquinth-polyphonicanalogue-stylemusicsynthesizerforjava
From the looks of it (comments are a bit ... non-existent) it's a virtual keyboard/instrument. It does synthesise single notes to PCM audio data when you press a key. Unfortunately it doesn't look like it supports predefined sequences like MIDI files.
Java has a MIDI playback API - but it will output it via the sound card and not to PCM data