//This class is abreviated for this sample
public class Order extends AnotherClassWithId{
   protected List<Item> items = new ArrayList<Item>();

   public List<ItemBorrador> getItems() {
      return items;
   }

   public void setItems(List<ItemBorrador> items) {
// I have done that because Hibernate used another implementation of
// List, and I need a standar implementation because is serialized by
// GWT, I think this is not my problem because I have the problem
// creating the Order, in wich case the list will naturally not e an Hibernate
// PersistantList, it wil be an arrayList also.
      this.items.clear();
      this.items.addAll(items);
//      this.items = items;
   }
}

//This class is abreviated for this sample
public class Item extends  AnotherClassWithId{
   protected String data;

   public String getData() {
      return data;
   }
   public void setData(String data) {
      this.data = data;
   }
}

<?xml version="1.0"?>
<!DOCTYPE hibernate-mapping PUBLIC
        "-//Hibernate/Hibernate Mapping DTD 3.0//EN"
        "http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd">

<hibernate-mapping default-lazy="false">
    <class name="path.to.Order" table="orders">
   
        <id name="id" column="id">
          <generator class="sequence">
              <param name="sequence">orders_id_seq</param>
          </generator>
        </id>
      
        <list name="items"    table="items_orders" cascade="all">
           <key column="id_order"/>
           <list-index column="position"/>
           <composite-element class="path.to.Item">
              <property name="dato" length="10"/>
           </composite-element>
       </list>
    </class>
</hibernate-mapping>

      Transaction tr = HibernateUtil.getSession().beginTransaction();

      //Supose the Order already created and received by GWT RPC
                //Every thing at this point is correct the Order has the Items I
                //wish to be saved
   
      HibernateUtil.getSession().save(order);
      HibernateUtil.getSession().flush();
      tr.commit();