Exception Description: Invalid composite primary key specification. The names of the primary key fields or properties in the primary key class [entitys.OrderItemPK] and those of the entity bean class [class entitys.OrderItem] must correspond and their types must be the same. Also, ensure that you have specified ID elements for the corresponding attributes in XML and/or an @Id on the corresponding fields or properties of the entity class.

@Entity
public class CustomerOrder implements Serializable {
    private static final long serialVersionUID = 1L;
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    @Basic(optional = false)
    @Column(name = "idOrder")
    private Integer idOrder;

    @Basic(optional = false)
    @Column(name = "orderText")
    private String orderText;

    @OneToMany(cascade = CascadeType.ALL, mappedBy = "customerOrder")
    private Collection<OrderItem> orderItemCollection;

    public CustomerOrder() {
    }
}

@Entity
@IdClass(OrderItemPK.class)
public class OrderItem implements Serializable {
    private static final long serialVersionUID = 1L;
    @EmbeddedId
    protected OrderItemPK orderItemPK;

    @Basic(optional = false)
    @Column(name = "itemDesc")
    private String itemDesc;

     @Id
     @ManyToOne(optional = false)    
     @JoinColumns({
         @JoinColumn(name="idOrder", referencedColumnName="idOrder"),
         @JoinColumn(name="ItemNumber", referencedColumnName="ItemNumber")
     })      
    private CustomerOrder customerOrder;
    private CustomerOrder customerOrder;

    public OrderItem() {
        this.orderItemPK = new OrderItemPK();
   }
}

@Embeddable
public class OrderItemPK implements Serializable {
    @Basic(optional = false)
    @Column(name = "idOrder")
    private int idOrder;

    @Basic(optional = false)
    @Column(name = "itemNumber")
    private int itemNumber;

    public OrderItemPK() {
    }
}

public static void main(String[] args) {
    factory = Persistence.createEntityManagerFactory(PERSISTENCE_UNIT_NAME);
    EntityManager em = factory.createEntityManager();

    em.getTransaction().begin();

    // Fill the items fileds
    OrderItem item = new OrderItem();
    item.setItemDesc("Item Text");

    // Fill the orders fields
    CustomerOrder order = new CustomerOrder();
    order.setOrderText("Order text");

    // Fill the relationship fields
    order.getOrderItemCollection().add(item);
    item.setCustomerOrder(order);        

    em.persist(order);
    em.getTransaction().commit();
}