Castor unmarshalling exception!

[Xpresso]

I'm a freshman. When I tried to run the demo in spring reference chepter 14.3 Using Marshaller and Unmarshaller,I met the problem as follows:

Exception in thread "main" org.springframework.oxm.UnmarshallingFailureExcept ion: Castor unmarshalling exception; nested exception is org.exolab.castor.xml.MarshalException: The class for the root element 'settings' could not be found.{File: [not available]; line: 2; column: 32}

This problem seems to emerge when using spring 3.0.5 and 3.1.0 .

[Tom08]

Has anyone found a solution for this problem? is it namespace missing...