HibernateJavaTypeMapper

I have set my Ant task as:
   <hibernate 
            destination="${build.gen-src.dir}"
            package="com.test"
 
javaTypeMapper="middlegen.plugins.hibernate.HibernateJavaTypeMapper"
            genXDocletTags="true"
         />

I set -
javaTypeMapper="middlegen.plugins.hibernate.HibernateJavaTypeMapper"
in order to let middle-hibernate-plugins detect the nullable fields and
use Java Wrapper class instead of primitive types for those fields, but
the above line seems not make any difference???
 
Please help!
Thanks a lot.
John