The Jack4j package contains some example clients. They can be found in the net.sf.jack4j.examples package.
Detailed description is in the code.
plays WAV files - very simple implementation, does no resampling
very simple recorder that creates WAV files
sine wave synthesizer, controlled by MIDI
primitive "sequencer", produces MIDI events
Timebase master - informs other clients about current bar/beat/tick
Transport-aware and timebase-aware metronome.