java - OneToMany 关系不起作用

Question

我的表:

产品:ID、名称

Offer:id、value、product_id

实体:

@Entity
@Table(name="product")
public class Product implements Serializable {
    @OneToMany(mappedBy="product")
    private Set<Offer> offers;
    ...
}

@Entity
@Table(name="offer")
public class Offer implements Serializable {
    @ManyToOne
    @JoinColumn(name="PRODUCT_ID")
    private Product product;
    ...
}

当我尝试从表 Product 中获取一些数据时，我得到一个 java.lang.NullPointerException，并且这个代码:product.getOffers() 返回:

{IndirectSet: not instantiated}

如何解决这个问题？

Answer 1

这不是错误消息。打印指令导致在底层 IndirectSet 上调用 toString() .

TopLink will place an IndirectSet in the instance variable when the containing domain object is read from the datatabase. With the first message sent to the IndirectSet, the contents are fetched from the database and normal Set behavior is resumed.

IndirectCollection类型被具体实现为不在 toString() 上实例化:

For debugging purposes, #toString() will not trigger a database read.

但是，对间接集合的任何其他调用，例如 size() 或 isEmpty() 都会实例化该对象。

The database read is ultimately triggered when one of the "delegated" methods makes the first call to getDelegate(), which in turn calls buildDelegate(), which sends the message getValue() to the value holder. The value holder performs the database read. With the first message sent to the IndirectSet, the contents are fetched from the database and normal Set behavior is resumed.

另见 IndirectList: not instantiated