PsychJavaTrouble – What to do if Java-based Psychtoolbox functions fail?

You probably arrived at this help text because of an error in execution
of one of the Psychtoolbox Java based functions, e.g., ListenChar,
CharAvail, GetChar or FlushEvents.

These are common reasons for failure of Java based Psychtoolbox

  1. You run Matlab in nojvm mode, i.e., you started Matlab via ‘matlab
    -nojvm’. In that case, Matlabs Java virtual machine is disabled and so
    are all Java based Psychtoolbox functions. In that case you need to
    restart Matlab with its Java VM enabled.

  2. You run GNU/Octave, which doesn’t support our Java based keyboard
    functions. However, then you should not ever end here, as we use an
    alternate implementation on Octave.

  3. The Psychtoolbox/PsychJava/ subfolder of your working copy of
    Psychtoolbox isn’t included in Matlabs static Java classpath. The
    Psychtoolbox installer/upgrader (DownloadPsychtoolbox](DownloadAdditionsForLinux)) usually
    tries to edit the ‘classpath.txt’ file, or since Matlab 8.1,
    ‘javaclasspath.txt’, of your Matlab installation in order to add the
    Psychtoolbox/PsychJava subfolder to Matlabs classpath. This procedure may
    fail due to insufficient access permissions on your system. You can
    verify this by entering ‘type classpath.txt’ at the Matlab prompt. The
    printed file should contain the path to the PsychJava folder. If it
    doesn’t, you may want to edit the file yourself (‘which classpath.txt’
    tells you the location of the file) or ask a system administrator to do
    it for you. After editing the file you need to restart Matlab. Instead of
    manual editing you can also call this function as PsychJavaTrouble(1); -
    This will try to automatically modify the classpath.txt or
    javaclasspath.txt file if your Matlab runs with sufficient permissions,
    e.g., administrator permissions.

If you need a quick temporary fix for the problem, other than editing
classpath.txt and restarting Matlab, then type ‘PsychJavaTrouble’ at the
Matlab command prompt. The function will add the PsychJava folder to the
dynamic classpath to immediately enable Java based Psychtoolbox
functions. This fix is temporary however, it needs to be repeated after
each restart of Matlab. Executing the command will also clear all
variables and functions from Matlabs workspace (like ‘clear all’), so
adding it to experiment scripts may impair proper working of that

  1. You didn’t restart Matlab after the Psychtoolbox installer asked you
    to do so. -> Restart Matlab and retry.

  2. The versions of GetCharJava bundled with Psychtoolbox are incompatible
    with the version of Java installed on your machine or bundled with your
    version of Matlab. If you have a Java SDK installed on your machine,
    Psychtoolbox will try to compile a matching version of GetCharJava. This
    should succeed on OS-X but is unlikely to work on Windows, because that
    system does not have a javac compiler installed by default.

  3. Other reasons: Post to the Psychtoolbox forum and ask for help.

Good luck!

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