Java-Sound-on-Linux

Sat, 05 Apr 2014 22:30:55 -0700
tags/rockets

Java Sound on Linux

I'm often in the position of having my favorite Java program (AltosUI) unable to make any sounds. Here's a history of the various adventures I've had.

Java and PulseAudio ALSA support

When we started playing with Java a few years ago, we discovered that if PulseAudio were enabled, Java wouldn't make any sound. Presumably, that was because the ALSA emulation layer offered by PulseAudio wasn't capable of supporting Java.

The fix for that was to make sure pulseaudio would never run. That's harder than it seems; pulseaudio is like the living dead; rising from the grave every time you kill it. As it's nearly impossible to install any desktop applications without gaining a bogus dependency on pulseaudio, the solution that works best is to make sure dpkg never manages to actually install the program with dpkg-divert:

# dpkg-divert --rename /usr/bin/pulseaudio

With this in place, Java was a happy camper for a long time.

Java and PulseAudio Native support

More recently, Java has apparently gained some native PulseAudio support in some fashion. Of course, I couldn't actually get it to work, even after running the PulseAudio daemon but some kind Debian developer decided that sound should be broken by default for all Java applications and selected the PulseAudio back-end in the Java audio configuration file.

Fixing that involved learning about said Java audio configuration file and then applying patch to revert the Debian packaging damage.

$ cat /usr/lib/jvm/java-7-openjdk-amd64/jre/lib/sound.properties
...
#javax.sound.sampled.Clip=org.classpath.icedtea.pulseaudio.PulseAudioMixerProvider
#javax.sound.sampled.Port=org.classpath.icedtea.pulseaudio.PulseAudioMixerProvider
#javax.sound.sampled.SourceDataLine=org.classpath.icedtea.pulseaudio.PulseAudioMixerProvider
#javax.sound.sampled.TargetDataLine=org.classpath.icedtea.pulseaudio.PulseAudioMixerProvider

javax.sound.sampled.Clip=com.sun.media.sound.DirectAudioDeviceProvider
javax.sound.sampled.Port=com.sun.media.sound.PortMixerProvider
javax.sound.sampled.SourceDataLine=com.sun.media.sound.DirectAudioDeviceProvider
javax.sound.sampled.TargetDataLine=com.sun.media.sound.DirectAudioDeviceProvider

You can see the PulseAudio mistakes at the top of that listing, with the corrected native interface settings at the bottom.

Java and single-open ALSA drivers

It used to be that ALSA drivers could support multiple applications having the device open at the same time. Those with hardware mixing would use that to merge the streams together; those without hardware mixing might do that in the kernel itself. While the latter is probably not a great plan, it did make ALSA a lot more friendly to users.

My new laptop is not friendly, and returns EBUSY when you try to open the PCM device more than once.

After downloading the jdk and alsa library sources, I figured out that Java was trying to open the PCM device multiple times when using the standard Java sound API in the simplest possible way. I thought I was going to have to fix Java, when I figured out that ALSA provides user-space mixing with the 'dmix' plugin. I enabled that on my machine and now all was well.

$ cat /etc/asound.conf
pcm.!default {
    type plug
    slave.pcm "dmixer"
}

pcm.dmixer  {
    type dmix
    ipc_key 1024
    slave {
        pcm "hw:1,0"
        period_time 0
        period_size 1024
        buffer_size 4096
        rate 44100
    }
    bindings {
        0 0
        1 1
    }
}

ctl.dmixer {
    type hw
    card 1
}

ctl.!default {
    type hw
    card 1
}

As you can see, my sound card is not number 0, it's number 1, so if your card is a different number, you'll have to adapt as necessary.