Minor Description
When xalan.jar is not on the classpath, then org.apache.activemq.filter.XalanXPathEvaluator in evaluate() method tries to load org.apache.xpath.CachedXPathAPI, throws NoClassDefFoundError, that is catched as Throwable and false is returned instead of an error.
No clue is given to the client, that it lacks a jar on the classpath and the impression is, that XPath selectors doesn't work and return false whatever the message is.
I believe if we catch Exception instead of Throwable, then the problem would be fixed.