OpenJPA-延迟获取不起作用

问题描述

使用嵌入式OpenEJB容器进行单元测试时，我遇到一个特定的问题.我在两个班级之间有双向关系.在一个方向上，该关系正常工作，但在相反的方向上，该关系仅在EAGER模式下有效.在LAZY模式下，字段section保持为空.代码片段如下:

I have a specific problem with an unit test using embedded OpenEJB container. I have a bi-directional relation between two classes. In one direction the relation works properly, but in the opposite direction the relation works only in EAGER-mode. In LAZY-mode the field section stays null. The code snipped follows:

@Entity
@Table(name="tracks")
class TrackEntity implements Track {
    @Id
    private int trackNumber;
    @OneToMany(mappedBy = "track")
    private HashSet<SectionEntity> sections;

    public TrackEntity() {
        sections = new HashSet<SectionEntity>();
    }

    @Override
    public Collection<HistoricalEvent> getEvents() {
        if (sections == null)
            throw new CommonError("number=" + trackNumber, AppErrors.TRACK_EMPTY);

        TreeSet<HistoricalEvent> set = new TreeSet<HistoricalEvent>();
        for (SectionEntity se : sections)
            set.addAll(se.getEvents());

        return set;
    }
 }

我的代码有点具体.该类仅在内部使用字段sections来合并所有子集合.我无法懒惰地填写章节.我想，容器希望客户端通过getter从外部访问该字段.

My code is little bit specific. The class uses the field sections just internally to merge all sub-collections. I'm unable to fill sections lazily. I thing, the container expects client to access the field externally via a getter.

推荐答案

这是实体生命周期的问题.必须将所有实体(轨道及其部分)重新附加到持久性上下文.收集事件的方法必须在使用EntityManager的类中. (实体不能使用管理器来重新附加自身.)更新的实体管理类的示例如下:

Its the problem with life cycle of enties. All enties (track and its sections) must be re-attached to the persistence context. The method collecting events must be in the class using EntityManager. (The entity cannot use the manager to re-attach itself.) Example of updated entity managing class follows:

public class EntityDataAccessor {
    @PersistenceUnit(unitName = "someUnit")
    private EntityManagerFactory emFactory;

    //gets one track
    public Track getTrack(int number) {
        EntityManager em = emFactory.createEntityManager();
        try {
            return (Track)em.find(TrackEntity.class, number);
        }
        finally {
            em.close();
        }
    }

    //the new method collecting events
    public Collection<HistoricalEvent> getEventsForTrack(TrackEntity te) {
        EntityManager em = emFactory.createEntityManager();
        te = em.merge(te); //re-attach to the context

        Set<SectionEntity> sections = te.getSections();
        TreeSet<HistoricalEvent> set = new TreeSet<HistoricalEvent>();
        for (SectionEntity se : sections) {
            se = em.merge(se); //re-attach to the context
            set.addAll(se.getEvents());
        }
        em.close();
        return set;
    }
}

请参阅问题了解更多信息.

See question What's the lazy strategy and how does it work? for more detail.

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