How to validate an XML document against a xsd file ?

Hi all,
I have an xml file loaded in memory (Actually in a String but I can parse it).
I'd like to check the file against an .xsd document (the xml document must respect rules in the xsd file) Does anybody know how to do it in Java?
Thanks a lot
Francesco

This has been asked before, so a search on this forum would have helped immediately...
Anyway, here are a couple of resources to try out:
http://www.coderanch.com/t/125376/XML/Validating-XML-against-xsd-schemata
http://bobcat.webappcabaret.net/javachina/faq/xml_java_01.htm#jaxp_sax_Q60
http://www-106.ibm.com/developerworks/edu/x-dw-xvalid-i.html

Thanks for your answer. Regards
Francesco