There are lot of XML processing APIs available in Java. I like JAXB (Java XML Binding) API, which actually creates a set of POJO classes for your XSD. By this means it is easy to marshall and unmarshall, java object to xml document and visa versa.
Here is the JAXB link -http://java.sun.com/webservices/docs/2.0/tutorial/doc/JAXBUsing.html
subject: how to generate xml file using xsd schema in java ?