Note that for this sequencer to provide stable timing, three
conditions have to be met:

1. System.currentTimeMillis() has to do what its name says:
provide the system time in milliseconds. Some lecacy operating
systems (Windows & co.) provide a timer resolution of only 10,
30 or 60 ms. For information on what you can expect on your
operating system, see the measurements of YIP Chi Lap [Beta]

2. A synthesizer or MIDI port implementation that responds
immediately.
Testing with the synthesizer of the Sun implementation resulted
in horrible timing. Timing with the hardware synthesizer of a
SB Life!, accessed via ALSA, was very good. Working with
WireProvider should be ok, too (assuming you manage to meet
the first point)

3. Small scheduling latencies. On the GNU/Linux system I tested,
even heavy file system traffic didn't lead to noticeable effects
on the timing. However, usage of the X server almost always led
to delays. This is due to the fact that the X server normally
runs with very high scheduling priority, stealing other processes
CPU time. Moving a window killed the timing. Even a running 'top'
was noticeable.

For testing with this sequencer, I recommend to use a recent
version of MidiPlayer from the Java Sound Examples.