Solution

The relation from customer to order was not properly set before saving the order.

getSession().lock(customer, LockMode.UPGRADE);
order.setCustomer(customer);
getSession().save(order);

was used instead of

getSession().lock(customer, LockMode.UPGRADE);
customer.getOrders().add(order);
order.setCustomer(customer);
getSession().save(order); 

<hibernate-mapping package="de.laliluna.webstock">

  <class name="Order" table="torder">

    <id name="id">

      <generator class="sequence">

        <param name="sequence">torder_id_seq</param>

      </generator>

    </id>

    <many-to-one name="customer" class="Customer"></many-to-one>

  </class>

</hibernate-mapping>

Exception Message

Exception in thread "main" org.hibernate.PropertyValueException: not-null property references a null or transient value: de.laliluna.webstock.Order._ordersBackref
at org.hibernate.engine.Nullability.checkNullability(Nullability.java:72)
at org.hibernate.event.def.AbstractSaveEventListener.performSaveOrReplicate(AbstractSaveEventListener.java:265)
at org.hibernate.event.def.AbstractSaveEventListener.performSave(AbstractSaveEventListener.java:167)
at org.hibernate.event.def.AbstractSaveEventListener.saveWithGeneratedId(AbstractSaveEventListener.java:114)
at org.hibernate.event.def.DefaultSaveOrUpdateEventListener.saveWithGeneratedOrRequestedId(DefaultSaveOrUpdateEventListener.java:186)