[Research/Discussion] Audio Issues on Linux

[Lyonlancer5]

Due to the number of issues regarding LineUnavailableException on Linux and an attempt to "fix" it, I decided dive deeper into the possible causes of the exception.

During the development of a project's audio recording modules, I came across a tutorial for capturing audio. Finding out that AudioSystem has checks for enumerating Mixers and Mixer methods for enumerating source and target DataLines, I rechecked how MultiClip was written.

Here are some of the possible points of interest:

Pinging #326 and (possibly) #89

This comment and this comment, respectively.

Note that some lines, once closed, cannot be reopened. Attempts to reopen such a line will always result in a LineUnavailableException.

The information above was taken from the Clip Javadoc. It is possible that by using the same format causes a line to reopen, although I haven't been able to personally test this out yet.

If proven, it makes this change have no effect.

On another point, a line that is already opened in the first place may result in an IllegalStateException, but none of the audio issues tagged here have so far reported this exception on the audio-side, so it already makes said change have no effect.

AudioSystem.isLineSupported(Line) isn't used as a check when opening a Clip

This was an oversight from #371 where I missed checking line support instead of line open. It's possible that there may be no supported audio mixers installed in the current setup (or have no audio server running, if at all present).

This is further elaborated in the next point:

Checking for presence of audio devices

Although OpenAL does check for the presence of audio devices, Clip uses Java's built-in sound system, where devices may be used by OpenAL but not to Java's own system.

Enumerating a list of Mixer objects and checking if there are any present (i.e. AudioSystem.getMixerInfo().length != 0) may be able to prevent the exception altogether (but will have no hitsound/sfx played if there are no available/supported devices).

(This research does not include JRE/JDK 9+ issues due to it being a different set of problems, unless otherwise related to the LineUnavailableException being noticed)

I'll be updating this issue with more information as I go by the development of said project's audio modules (and testing on Fedora and Mint) and a PR for testing purposes.

[tpenguinltg]

Thanks for the research. Maybe we'll finally get to solving this.

The information in #266 might help (see also the last comment) if you haven't already seen it. I think we concluded there that it was because of a limit with open lines, and having more programs open that emit sound will make this error more likely to appear. I think ideally we'd want to mix the sound ourselves and output the mixed audio through one line, but none of us have had the time to really try.