有两个实体类:

@Entity
public class Place implements Serializable {
    @Id @GeneratedValue(strategy = GenerationType.IDENTITY) private Long id;
    @OneToMany(mappedBy = "place")
    private List<Event> events = new ArrayList<Event>();
    private String name;
//getters setters
}
@Entity
public class Event implements Serializable {
    @Id @GeneratedValue(strategy=GenerationType.IDENTITY) private Long id;
    @OneToMany(mappedBy = "event" , cascade={CascadeType.PERSIST})
    private List<Participant> participants = new ArrayList<Participant>();
    @ManyToOne(optional=false,fetch=FetchType.EAGER) private Place place;
    private String name;
// getters setters
}


生成的表:
mysql> desc EVENT;
+----------+--------------+------+-----+---------+----------------+
| Field    | Type         | Null | Key | Default | Extra          |
+----------+--------------+------+-----+---------+----------------+
| ID       | bigint(20)   | NO   | PRI | NULL    | auto_increment |
| NAME     | varchar(255) | YES  |     | NULL    |                |
| PLACE_ID | bigint(20)   | YES  | MUL | NULL    |                |
+----------+--------------+------+-----+---------+----------------+
mysql> desc PLACE;
+----------+--------------+------+-----+---------+----------------+
| Field    | Type         | Null | Key | Default | Extra          |
+----------+--------------+------+-----+---------+----------------+
| ID       | bigint(20)   | NO   | PRI | NULL    | auto_increment |
| LOCATION | varchar(255) | YES  |     | NULL    |                |
| NAME     | varchar(255) | YES  |     | NULL    |                |
+----------+--------------+------+-----+---------+----------------+

现在,我将编写一些Java代码,以便Place和Event表以这种方式显示:

+----+--------+
| ID | NAME   |
+----+--------+
|  1 | Prague |
+----+--------+
+----+-------------+----------+
| ID | NAME        | PLACE_ID |
+----+-------------+----------+
|  1 | Programming |        1 |
+----+-------------+----------+


现在,我想选择一个地方并查看其所有事件:
// there's an ejb talking with persistence layer
Place p = ejb.getPlaceWithId(1);

并且我认为p.events中有比空集合更多的东西,但是根据我的观察,没有。为什么?

最佳答案

现在,我想选择一个位置并查看其所有事件(...),我认为p.events中除了空集合之外还有其他东西,但是根据我的观察,没有。为什么?


我的观察结果是,在显示的代码中eventsLAZY

@OneToMany(mappedBy = "place")
private List<Event> events = new ArrayList<Event>();


那么,如何得出这样的结论:确切地加载给定的Place时,它们为空?您是否尝试在集合上调用size()来强制加载(或对集合项进行迭代)?

不管上述问题的答案如何,我的建议都是


激活SQL日志记录(并在必要时在SQL客户端中运行查询)以查看正在发生的情况。
如果您想尽快加载fetch join,则可能实际上使用了events

SELECT p FROM Place p join fetch p.events WHERE p.id = :id

10-07 19:31
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