help on Unable to create javax script engine for jython

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I use ant, and build and xml file like this:
<project name="gen" default="gen" basedir=".">
<target name="gen">
<script language="jython">

I have jython2.2.1.jar, and it always fails at <script
language="jython"> with this error: Unable to create javax script
engine for jython.

Anyone can help?

THanks