The type [interface java.util.List] for the attribute [zamowieniaList] on the entity class [class projekttest.Klienci] is not a valid type for a serialized mapping. The attribute type must implement the Serializable interface.
Thanks for advice, but it changes nothing. I am getting still the same exception without remove @Basic(optional = false).
However if I remove @Basic(optional = false) I have another exception:
Exception Description: [class projekttest.Klienci] uses a non-entity [class projekttest.Zamowienia] as target entity in the relationship attribute [private java.util.List projekttest.Klienci.zamowieniaList].
But I am sure that Zamowienia class is a entity, It was generated but netbeans and has a adnotation: