based on the mailing list archive, it seems new users often run more than one jackrabbit instance with the same configuration. I propose to lock the repository by creating an empty file called ".lock" at the repository home on startup and remove it on shutdown.
If the lock file is found on jackrabbit startup the following message will be logged:
"The repository home at " + home.getAbsolutePath() + " appears to be in use. If you are sure it's not in use please delete the file at " + lock.getAbsolutePath() + ". Probably the repository was not shutdown properly."