In _jrubyvars.bat, the value of CLASSPATH environment variable increases exponentially. This is a fragment from batch file: – cut here – if not "%CLASSPATH%" == "" goto gotCP set CLASSPATH=%CP% goto doneCP :gotCP set CLASSPATH=%CP%;%CLASSPATH% :doneCP – cut here – Each execution of this script increases the value of the CLASSPATH variable exponentially by appending from the previous run. If this script gets executed in a new shell, then everything works.