Just dont forget to put the jdbc  driver of your database at <tomcat_home>/common/lib it is because of the tomcat  class loader. (Though it is not a good idea to bump up <tomcat_home>/common/lib with not necessary jar files. keep all required jar files for a web  application in WEB-INF/lib)
and also as a second guideline do not mess up server.xml of tomcat; instead put the datasource  definitions into META-INF/context.xml