PsychJavaTrouble – What to do if Java-based Psychtoolbox functions fail?
These are common reasons for failure of Java based Psychtoolbox
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.
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.
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
You didn’t restart Matlab after the Psychtoolbox installer asked you
to do so. -> Restart Matlab and retry.
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.
Other reasons: Post to the Psychtoolbox forum and ask for help.