There are a number of web sites devoted almost exclusively to MIDI.
www.midi.org This appears to be the official site for MIDI developers. It contains a number of reference materials.
OpenJDK - The Sound Group This group is responsible for the OpenJDK's group efforts in relation to the packages starting with javax.sound.
http://ardour.org Open source provider of digital audio workstation (DAW).
MIDI code A tutorial (PDF) on MIDI from Juan P. Bello at New York University.
DT08/DT079 Music Technology Course notes by Thomas Scarff at Dublin Institute of Technology
Introduction to Computer Music: Volume One by Indiana University Jacobs School of Music Center for Electronic and Computer Music
MIDI tutorial for programmers by the Carnegie Mellon School of Music.
Music Software Development - Information, Tools and Tutorials
Welcome to Karl Brown's MIDI Java Servlet and web programming home page
IAC stnds for Inter Application Communications.
CoreMidi4J uses the JNI (Java Native Interface) to conntect the standard Java MIDI API to the Core Audio framework on Mac OS X as opposed to using the Core Java Application.
midi-music ( Documentation) This is my personal project for trying to write some MIDI software. It is a work in progress.
Java Sound Resources A number of sample programs involving Java applications for sampled sound and MIDI.
The page Java Sound Resources: Examples: MIDI: Playback and Recording has a description of the programs. However, it appears that Java Sound Resources ( http://www.jsresources.org ) has not been updated since 2013. Many of the programs require modification and use libraries that are no longer supported.
Edisyn eclab/edisyn This is a Synthesizer Patch Editor by Sean Luke at George Mason University.
Playmidi: Play midi files via external midi devices or soft synth
Frinika - Free open source DAW, MIDI sequencer, software synthesizers