HibernateException：非法尝试将集合与两个打开的会话相关联

Here is my tables:

这是我的表格:

Orders.java

@Entity
public class Orders implements Serializable {

@Id
@GeneratedValue
private Integer id;
@Column(nullable = false)
private Date orderDate;
@ManyToOne(cascade = CascadeType.ALL)
private User user;

@OneToMany(mappedBy = "orders", cascade = CascadeType.ALL)
private Set<OrderItem> orderItems = new HashSet<OrderItem>();
//getter/setters
@E