public class ChangeListener implements PostUpdateEventListener
{
   private static final ChangeGroupStore store = new ChangeGroupStore();

   public void onPostUpdate(PostUpdateEvent event) throws HibernateException
   {
      Object entity = event.getEntity();
      if(isLoggable(entity))
      {
         ChangeGroup change = createChangeGroup(entity, event.getId());
         change.setType(ChangeGroupType.UPDATE);

         EntityPersister persister = event.getPersister();
         String[] propertyNames = persister.getPropertyNames();
         Object[] oldValues = event.getOldState();
         Object[] currentValues = event.getState();
         for (int i = 0; i < propertyNames.length; i++)
            if (!DomainUtils.equals(oldValues[i], currentValues[i]))
               change.addChangeItem(new ChangeItem(propertyNames[i], oldValues[i].toString(), currentValues[i].toString()));

         store.save(change);
      }
   }

   private boolean isLoggable(Object o)
   {
      System.out.println("Should we log " + o.getClass());
      return o instanceof Loggable;
   }

   private ChangeGroup createChangeGroup(Object entity, Serializable id)
   {
      ChangeGroup change = new ChangeGroup();
      change.setEntityClass(entity.getClass());
      change.setEntityId(id);

      Loggable changedObject = (Loggable)entity;
      change.setCategoryClass(changedObject.getCategoryClass());
      change.setCategoryId(changedObject.getCategoryId());

      String user = UserContext.getUser();
      change.setUser(user == null ? "Unknown User" : user);
      return change;
   }
}