在 Hibernate 中使用惰性属性

hibernate 中属性标签的惰性属性允许按照链接延迟加载属性：http://docs.jboss.org/hibernate/orm/3.3/reference/en-US/html/mapping.html#mapping-declaration-property http://docs.jboss.org/hibernate/orm/3.3/reference/en-US/html/mapping.html#mapping-declaration-property

惰性（可选 - 默认为 false）：指定此属性 首次访问实例变量时应延迟获取。 它需要构建时字节码检测。

但是，当我尝试为我的一个属性设置lazy=true时，在本例中它并没有延迟加载：

休眠映射文件：

<hibernate-mapping package="org.hibernate.tutorial.domain">

    <class name="Event" table="EVENTS" select-before-update="true">
        <id name="id" column="EVENT_ID">
            <generator class="native" />
        </id>
        <property name="date" type="timestamp" column="EVENT_DATE" />
        <property name="title" lazy="true"/>

        <set name="participants" table="PERSON_EVENT" inverse="true">
            <key column="EVENT_ID" />
            <many-to-many column="PERSON_ID" class="Person" />
        </set>
    </class>

</hibernate-mapping>

Program:

public static void main(String[] args) {
        Session session = HibernateUtil.getSessionFactory().getCurrentSession();
        session.beginTransaction();
        Event event = (Event) session.get(Event.class, 135L);
        session.getTransaction().commit();
        System.out.println(event);
        HibernateUtil.getSessionFactory().close();
    }

hibernate生成的查询：

Hibernate: select event0_.EVENT_ID as EVENT1_0_0_, event0_.EVENT_DATE as EVENT2_0_0_, event0_.title as title0_0_ from EVENTS event0_ where event0_.EVENT_ID=?

请帮助我理解为什么懒惰者在这种情况下不起作用？

使用 Hibernate 5，这可以使用字节码增强轻松完成 https://vladmihalcea.com/how-to-lazy-load-entity-properties-with-hibernate/.

首先，您需要添加以下 Maven 插件：

<plugin>
    <groupId>org.hibernate.orm.tooling</groupId>
    <artifactId>hibernate-enhance-maven-plugin</artifactId>
    <version>${hibernate.version}</version>
    <executions>
        <execution>
            <configuration>
                <enableLazyInitialization>true</enableLazyInitialization>
            </configuration>
            <goals>
                <goal>enhance</goal>
            </goals>
        </execution>
    </executions>
</plugin>

然后，您可以简单地注释您的实体属性@Basic(fetch = FetchType.LAZY):

@Entity(name = "Event")
@Table(name = "event")
public class Event extends BaseEntity {

    @Type(type = "jsonb")
    @Column(columnDefinition = "jsonb")
    @Basic(fetch = FetchType.LAZY)
    private Location location;

    public Location getLocation() {
        return location;
    }

    public void setLocation(Location location) {
        this.location = location;
    }
}

当您获取实体时：

Event event = entityManager.find(Event.class, 
    eventHolder.get().getId());

LOGGER.debug("Fetched event");
assertEquals("Cluj-Napoca", event.getLocation().getCity());

Hibernate 将使用辅助选择来加载惰性属性：

SELECT e.id AS id1_0_0_
FROM   event e
WHERE  e.id = 1

-- Fetched event

SELECT e.location AS location2_0_
FROM   event e
WHERE  e.id = 1