PsychJavaTrouble

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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 Psychtoolboxs Java based functions, e.g., ListenChar, CharAvail,
GetChar or FlushEvents.

There are four common reasons for failure of Java based Psychtoolbox
functions:

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 a very old Matlab version which doesn't have Java support
(Matlab 5) or whose Java implementation is too old and incompatible with
Psychtoolbox. In that case you need to upgrade to a more recent Matlab
version. If you run GNU/Octave you are also out of luck, due to Octaves
lack of Java support.
Your Matlab needs to support at least java version 1.4. This means that:

On M$-Windows and GNU/Linux, Matlab version 7.0.0 and later should work,
because V7 is bundled with java 1.4.2 or 1.5. Matlab 5.x or 6.x won't work,
unless you manually upgrade Matlab with a new version of Java. See Mathworks
support knowledgebase for instructions on how to do that.

On MacOS-X 10.3.9 or later, all Matlab versions should work, as Matlab uses
the java which is installed as part of the operating system. OS-X 10.3.9 provides
Java 1.4.2, OS-X 10.4.x provides Java 1.5.

3. The Psychtoolbox/PsychJava/ subfolder of your working copy of
Psychtoolbox isn't included in Matlabs static Java classpath. The
Psychtoolbox installer/upgrader (DownloadPsychtoolbox or
UpdatePsychtoolbox) usually tries to edit the 'classpath.txt' file 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.

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
scripts.

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

5. 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.

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

Good luck!