<hibernate-mapping package="com.model.category">

<class name="Category"
      lazy="true">

   <id name="id"
      type="integer"
      column="id"
      unsaved-value="null">
      <generator class="native"/>
   </id>

   <property   name="title"
            type="string"
            column="title"
            length="40"
            not-null="true"
            unique-key="UNIQUE_NAME_AT_LEVEL"/>

   <property   name="description"
            type="string"
            column="description"
            length="200"
            not-null="false"/>

   <!-- Parent can be null for root categories. -->
   <many-to-one name="parentCategory"
            cascade="none"
            outer-join="false"
            foreign-key="gatalents_user_category_fk_id_parent">
      <column name="id_parent"
            not-null="false"
            unique-key="UNIQUE_NAME_AT_LEVEL"/>
   </many-to-one>

   <!-- We can't change the creation time, so map it with update="false". -->
   <property   name="creationDate"
            column="creation_date"
            type="java.util.Date"
            update="false"
            not-null="true"/>   

   <union-subclass name="UserCategory" table="user_category" />
   <union-subclass name="ProfileCategory" table="profile_category" />
   <union-subclass name="ItemCategory" table="item_category" />
   <union-subclass name="ResourceCategory" table="resource_category" />


</class>

</hibernate-mapping>

public Collection<?> findAll(Class clazz, boolean onlyRootCategories)
         throws InfrastructureException {

      Collection<?> categories;
      try {
         if (onlyRootCategories) {
            Criteria crit = HibernateUtil.getSession().createCriteria(clazz);
            categories = crit.add(Expression.isNull("parentCategory")).list();
         } else {
            categories = HibernateUtil.getSession().createCriteria(clazz).list();
         }
      } catch (HibernateException ex) {
         throw new InfrastructureException(ex);
      }
      return categories;
   }   

com.exception.InfrastructureException: org.hibernate.WrongClassException: Object with id: 1 was not of the specified subclass: com.model.category.ProfileCategory (loaded object was of wrong class class com.model.category.UserCategory)
   at com.dao.CategoryDAO.findAll(CategoryDAO.java:64)
   at com.controller.servlet.UserForm.service(UserForm.java:68)
   at javax.servlet.http.HttpServlet.service(HttpServlet.java:803)
   at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:290)
   at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206)
   at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:233)
   at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:175)
   at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:128)
   at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:102)
   at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:109)
   at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:286)
   at org.apache.coyote.http11.Http11Processor.process(Http11Processor.java:844)
   at org.apache.coyote.http11.Http11Protocol$Http11ConnectionHandler.process(Http11Protocol.java:583)
   at org.apache.tomcat.util.net.JIoEndpoint$Worker.run(JIoEndpoint.java:447)
   at java.lang.Thread.run(Unknown Source)
Caused by: org.hibernate.WrongClassException: Object with id: 1 was not of the specified subclass: com.model.category.ProfileCategory (loaded object was of wrong class class com.model.category.UserCategory)
   at org.hibernate.loader.Loader.instanceAlreadyLoaded(Loader.java:1243)
   at org.hibernate.loader.Loader.getRow(Loader.java:1195)
   at org.hibernate.loader.Loader.getRowFromResultSet(Loader.java:580)
   at org.hibernate.loader.Loader.doQuery(Loader.java:701)
   at org.hibernate.loader.Loader.doQueryAndInitializeNonLazyCollections(Loader.java:236)
   at org.hibernate.loader.Loader.doList(Loader.java:2213)
   at org.hibernate.loader.Loader.listIgnoreQueryCache(Loader.java:2104)
   at org.hibernate.loader.Loader.list(Loader.java:2099)
   at org.hibernate.loader.criteria.CriteriaLoader.list(CriteriaLoader.java:94)
   at org.hibernate.impl.SessionImpl.list(SessionImpl.java:1569)
   at org.hibernate.impl.CriteriaImpl.list(CriteriaImpl.java:283)
   at com.dao.CategoryDAO.findAll(CategoryDAO.java:61)
   ... 14 more

List<? super Category> resourceCategoryList = (List<? super Category>) categoryDao.findAll(ResourceCategory.class, false);
      request.setAttribute("resourceCategoryList", resourceCategoryList);
      // UserCategory
      List<? super Category> userCategoryList = (List<? super Category>) categoryDao.findAll(UserCategory.class, false);
      request.setAttribute("userCategoryList", userCategoryList);
      // ProfileCategory
      List<? super Category> profileCategoryList = (List<? super Category>) categoryDao.findAll(ProfileCategory.class, false);
      request.setAttribute("profileCategoryList", profileCategoryList);

Hibernate: select this_.id as id1_0_, this_.title as title1_0_, this_.description as descript3_1_0_, this_.id_parent as id4_1_0_, this_.creation_date as creation5_1_0_ from resource_category this_
Hibernate: select this_.id as id1_0_, this_.title as title1_0_, this_.description as descript3_1_0_, this_.id_parent as id4_1_0_, this_.creation_date as creation5_1_0_ from user_category this_