This framework has been embedded in a number of open source projects, including the XSL processor Xalan and the XML/Java build tool Ant.
You can download a version of bsf.jar from http://oss.software.ibm.com/developerworks/projects/bsf that includes the com.ibm.bsf.engines.javascript.JavaScriptEngine class. The current version is 2.2 release candidate at the time of this writing. This version thus supports JavaScript through Rhino when used with the js.jar file from either rhino15R1.zip or rhinoTip.zip.
See Xalan-Java Extensions for more information on adding JavaScript to XSL and the Script task for using scripting in Ant build files.