为什么调用flush（）后id不刷新？

Question

我在mysql db中生成了值策略IDENTITY和auto_increment。据我所知，持久对象必须在flush（）调用后生成值。但我也一样。怎么了？感谢。

@Entity
@Table(name = "orderr")
public class Order implements Serializable, ConvertibleToDTO<OrderDTO>{
    @NotNull @GeneratedValue(strategy = GenerationType.IDENTITY) private Integer id;
    @NotNull @Size(min = 1, max = 20) private String status;
    @NotNull @Size(min = 1, max = 100) private String address;
    @NotNull private Date orderDate;
    private Manager manager;
    @NotNull private Date deliveryDate;
    @NotNull private Integer totalCost;
    @NotNull private Customer customer;
    @NotNull private DeliveryType deliveryType;
    @NotNull private PaymentMethod paymentMethod;
    private Set<Product> products = new HashSet<Product>();

    @Id  @Column(name = "id")  public Integer getId() { return id; }
    public void setId(Integer id) { this.id = id; }

    @Column(name = "address")  public String getAddress() { return address; }
    public void setAddress(String address) { this.address = address; }

    @Column(name = "order_date")  public Date getOrderDate() { return orderDate; }
    public void setOrderDate(Date orderDate) { this.orderDate = orderDate; }

    @Column(name = "delivery_date")  public Date getDeliveryDate() { return deliveryDate; }
    public void setDeliveryDate(Date deliveryDate) { this.deliveryDate = deliveryDate; }

    @ManyToOne(cascade = {CascadeType.REFRESH, CascadeType.DETACH})
    @JoinColumn(name = "delivery_type")
    public DeliveryType getDeliveryType() { return deliveryType; }
    public void setDeliveryType(DeliveryType deliveryType) { this.deliveryType = deliveryType; }

    @Column(name = "status")  public String getStatus() { return status; }
    public void setStatus(String status) { this.status = status; }

    @Column(name = "total_cost")  public Integer getTotalCost() { return totalCost; }
    public void setTotalCost(Integer totalCost) { this.totalCost = totalCost; }

    @ManyToOne(cascade = {CascadeType.REFRESH, CascadeType.DETACH})
    @JoinColumn(name = "customer_id")
    public Customer getCustomer() { return customer; }
    public void setCustomer(Customer customer) { this.customer = customer; }

    @ManyToOne(cascade = {CascadeType.REFRESH, CascadeType.DETACH})
    @JoinColumn(name = "manager_id")
    public Manager getManager() { return manager; }
    public void setManager(Manager manager) { this.manager = manager; }

    @ManyToMany(cascade = {CascadeType.MERGE,CascadeType.DETACH}, fetch = FetchType.EAGER, targetEntity = Product.class)
    @JoinTable(name = "order_product", joinColumns = {@JoinColumn(name = "order_id")},
            inverseJoinColumns = {@JoinColumn(name = "product_id")})
    public Set<Product> getProducts() {return products; }
    public void setProducts(Set<Product> products) { this.products = products; }

    @ManyToOne(cascade = {CascadeType.REFRESH, CascadeType.DETACH})
    @JoinColumn(name = "payment_method_id")
    public PaymentMethod getPaymentMethod() { return paymentMethod; }
    public void setPaymentMethod(PaymentMethod paymentMethod) { this.paymentMethod = paymentMethod; }
}

@Entity
@Table(name = "product")
public class Product implements Serializable, ConvertibleToDTO<ProductDTO>{
    @NotNull @GeneratedValue(strategy = GenerationType.IDENTITY) private Integer id;
    @Size(min = 1, max = 50) private String name;
    @Size(max = 255) private String description;
    @IntegerSize(min = 1, max = 11) private Integer showcasePrice;
    private Set<Order> orders = new HashSet<Order>();

    @Id  @Column(name = "id")  public Integer getId() { return id; }
    public void setId(Integer id) { this.id = id; }

    @Column(name = "description") public String getDescription() { return description; }
    public void setDescription(String description) { this.description = description; }

    @Column(name = "name")  public String getName() { return name; }
    public void setName(String name) { this.name = name; }

    @Column(name = "showcase_price") public Integer getShowcasePrice() { return showcasePrice; }
    public void setShowcasePrice(Integer showcasePrice) { this.showcasePrice = showcasePrice; }

    @ManyToMany(mappedBy = "products", cascade = {CascadeType.MERGE, CascadeType.DETACH}, fetch = FetchType.EAGER, targetEntity = Order.class)
    public Set<Order> getOrders() { return orders; }
    public void setOrders(Set<Order> orders) { this.orders = orders; }
}

 public Integer create(OrderDTO orderDTO) {
        Session session = HibernateUtil.getSessionFactory().getCurrentSession();
        Integer result = null;
        try {
            session.beginTransaction();
            session.clear();

            Order entity = orderDTO.createEntityFromDTO();
            Set<Product> savedProducts = entity.getProducts();

            entity.setProducts(null);

            session.persist(entity);
            session.flush();

            //!!!!!!!!!!!!!!!!!!!!!
            result = entity.getId();   //RETURN THE SAME VALUE
            //!!!!!!!!!!!!!!!!!!!!!

            entity.setProducts(savedProducts);
            session.update(entity);

            session.getTransaction().commit();
        } catch (Exception e) {
            LoggerFactory.getLogger(Application.class).error(e.getMessage(), e);
            session.getTransaction().rollback();
        } finally { return result; }
    }
}