I have tried to find the difference between runs that work OK and runs that throw the IllegalArgumentExceptions.
The attached Infotest.java calls MidiSystem.getMidiDeviceInfo two times, then tries to get the MidiDevice from the first Info object and compares the two instances of the corresponding Info objects. When calling CoreMidiDeviceProvider.addNotificationListener() before calling MidiSystem.getMidiDeviceInfo() for the first time, getDevice will fail most of the time. And I saw that when it fails, the corresponding Info objects have the same content but different hashCode (i.e. they have been newly instantiated).
When no notificationListener was added, it succeeded 100% of the time, and the Info objects were identical. It almost drove me crazy.
Then I instrumented the MidiSystem class with some debug printfs, and found out that the MidiSystem.getMidiDeviceInfo calls from two threads (main and CoreMidi4J environment change scanner) were overlapping, and returned different Info objects for the same device. Later calls to getMidiDeviceInfo returned consistent Info objects with a constant hashCode. When trying to call getDevice() with a stale Info object (i.e. one with hashCode != the latest one) would not find the device and throw the IllegalArgumentException.
So the culprit seems to be that two threads are calling MidiSystem.getMidiDeviceInfo at the same time and building two versions of the device caches. I could also verify this later with a background thread that was repeatedly calling getMidiDeviceInfo() while the main thread was also doing it for the first time, even when CoreMidi4J was not involved at all.
One easy fix is to ensure that getMidiDeviceInfo() is called and completed before a second thread can interfere. I moved the first call to snapshotCurrentEnvironment out of the background thread into addNotificationListener before the thread is created, and was not able to reproduce the problem again. Will send a pull request for it.
But the same might happen later, when midi devices are added. So it might be a good idea to also synchronize CoreMidiDeviceProvider.getMidiDeviceInfo() to snapshotCurrentEnvironment.

infotest.zip
contents:

Infotest.java test program
MidiSystem.java.excerpt instrumented getMidiDeviceInfo method (from jdk1.8, later versions are very similar and show the same problem)
out_notify.txt test program output when the problem arises
out_no_notify.txt output when addNotifyListener() was commented out, problem does not appear
out_background.txt output without notifyListener, but with background thread calling getMidiDeviceInfo: Problem arises
out_fixed.txt output with modified CoreMidiDeviceProvider and addNotifyListener enabled (no problem).

IllegalArgumentException when using addNotificationListener on Windows and Linux #38

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