Is your Person class in a package? If so you will need to modify the mapping.xml file to include the package name before the class name. For example if you put the Person class in a package called MyPackage you would make the following changes to the mapping.xml file:
* change <class name="Person"> to:
* change <field name="persons" type="Person" collection="collection" /> to:
<field name="persons" type="MyPackage.Person" collection="collection" />
Hope this helps.