JPA：双向m：n关系中的数据访问

Question

I want to model a sample m:n relationship between the entities 'order' and 'items'

The corresponding mapping sections are:

Order.java:

...
@Id
@GeneratedValue(strategy = GenerationType.IDENTITY)
@Column(name="ORDER_ID")
private Long orderId;

@Column(name="ORDER_DESCRIPTION")
private String description;

@JoinTable(name ="ORDER_ITEMS",
    joinColumns=@JoinColumn(name="ORDER_ID", referencedColumnName="ORDER_ID"),
    inverseJoinColumns=@JoinColumn(name="ITEM_ID", referencedColumnName="ITEM_ID"))
private Set<Item> items;
...

Item.java:

...
@Id
@GeneratedValue(strategy = GenerationType.IDENTITY)
@Column(name = "ITEM_ID")
private Long itemId;

@Column(name = "ITEM_DESCRIPTION")
private String description;

@Column(name = "ITEM_PRICE")
private Long price;

@ManyToMany(mappedBy="items")
private Set<Order> orders;
...

Do you know why I am not able to inversely get the orders referenced with a specific item? The part @ManyToMany(mappedBy="items") gives me an "invalid mapping type for this relationship". I thought I could just create arbitrary items and orders, add the items to the orders, and get subsequently all the orders regarding to an item.

In my case, an item.getOrders() gives me a NullPointerException.

Thanks a lot!

Answer 1

Simply because you forgot to add the ManyToMany annotation on items:

@ManyToMany
@JoinTable(name ="ORDER_ITEMS",
    joinColumns=@JoinColumn(name="ORDER_ID", referencedColumnName="ORDER_ID"),
    inverseJoinColumns=@JoinColumn(name="ITEM_ID", referencedColumnName="ITEM_ID"))
private Set<Item> items;