hibernate - 实体管理器返回 @OneToMany 相关实体的重复副本

Question

我正在开发一个应用程序来帮助我的办公室跟踪和管理评论。在应用程序中，我使用 JPA 2.0 和 Hibernate 3.6.3 作为我的底层提供程序。我还使用 spring 将持久性上下文注入(inject)到我的 DAO 中。我已经构建了一个域，其中有一个评论、一个参与者和一个角色实体。

我遇到的问题是，当我从实体管理器收到评论时，如果参与者具有多个角色，则参与者列表中会存在同一参与者(即相同 ID)的重复副本。我还发现重复的数量与角色的数量直接相关(即，如果参与者有 3 个角色，则该参与者在审核的参与者列表中出现 3 次)

我以前直接使用过 Hibernate，但这是我第一次使用 JPA，所以我确信我配置了一些错误。我只是不知道那是什么。

这是代码:

评论:

@Entity
public class Review extends BaseModel{

@ManyToOne(cascade=CascadeType.ALL, fetch=FetchType.EAGER, optional=false)
private Item item;

@Column(name="ISNEW", nullable=false)
private boolean isNew;

@Enumerated(EnumType.STRING)
@Column(name="STATUS", nullable=false)
private ReviewStatus status;

@Enumerated(EnumType.STRING)
@Column(name="PHASE", nullable=false)
private Phase phase;

@Enumerated(EnumType.STRING)
@Column(name="REVIEW_TYPE", nullable=false)
private ReviewType reviewType;

@OneToMany( cascade=CascadeType.ALL, fetch=FetchType.EAGER)
private List<Participant> participants;

@OneToMany(cascade=CascadeType.ALL)
private List<Defect> defects;

@Column(name="START_DATE", nullable=false)
private Date startDate;

@Column(name="MEETING_DATE", nullable=false)
private Date meetingDate;

@Column(name="FINISH_DATE")
private Date finishDate;

@Column(name="DURATION", nullable=false)
private Double duration;

public Item getItem()
{
    return item;
}
public void setItem(Item item)
{
    this.item = item;
}   
public List<Participant> getParticipants() {
    return participants;
}
public void setParticipants(List<Participant> participants) {
    this.participants = participants;
}
public List<Defect> getDefects() {
    return defects;
}
public void setDefects(List<Defect> defects) {
    this.defects = defects;
}   
public boolean isNew() {
    return isNew;
}
public void setNew(boolean isNew) {
    this.isNew = isNew;
}
public Phase getPhase() {
    return phase;
}
public void setPhase(Phase phase) {
    this.phase = phase;
}
public Double getDuration() {
    return duration;
}
public void setDuration(Double duration) {
    this.duration = duration;
}
public ReviewStatus getStatus() {
    return status;
}
public void setStatus(ReviewStatus status) {
    this.status = status;
}
public Date getStartDate() {
    return startDate;
}
public void setStartDate(Date startDate) {
    this.startDate = startDate;
}
public Date getMeetingDate() {
    return meetingDate;
}
public void setMeetingDate(Date meetingDate) {
    this.meetingDate = meetingDate;
}
public Date getFinishDate() {
    return finishDate;
}
public void setFinishDate(Date finishDate) {
    this.finishDate = finishDate;
}
public ReviewType getReviewType()
{
    return reviewType;
}
public void setReviewType(ReviewType reviewType)
{
    this.reviewType = reviewType;
}

}

参与者:

@Entity
public class Participant extends BaseModel{


        private Double inspectionTime;

        @ManyToOne(cascade=CascadeType.ALL, optional=false)
        private Person person;

        @ManyToMany(cascade=CascadeType.ALL, fetch=FetchType.EAGER )
        private Set<Role> roles;

        public Double getInspectionTime() {
            return inspectionTime;
        }
        public void setInspectionTime(Double preInspectionTime) {
            this.inspectionTime = preInspectionTime;
        }
        public Person getPerson() {
            return person;
        }
        public void setPerson(Person person) {
            this.person = person;
        }
        public Set<Role> getRoles() {
            return roles;
        }
        public void setRoles(Set<Role> roles) {
            this.roles = roles;
        }

}

角色:

@Entity
public class Role extends BaseModel{
            @Column(nullable=false, unique=true)
            private String name;
            private String responsiblity;

            public String getName() {
                return name;
            }
            public void setName(String name) {
                this.name = name;
            }
            public String getResponsiblity() {
                return responsiblity;
            }
            public void setResponsiblity(String responsiblity) {
                this.responsiblity = responsiblity;
            }

}

基础:

@MappedSuperclass
public abstract class BaseModel {

    @Id
    @GeneratedValue
    private Long id;

    public Long getId() {
        return id;
    }

    public void setId(Long id) {
        this.id = id;
    }   

}

使用entityManager的DataAccessObject:

@Transactional(readOnly = false, propagation = Propagation.REQUIRED)
public class ReviewDaoJpaImpl implements ReviewDao
{


    @PersistenceContext
    private EntityManager em;

    public Review getReviewById(Long id)
    {
        return em.find(Review.class, id);
    }
}

接收重复项的调用:

    Review review = reviewDao.getReviewById(1776L);
    List<Participant> participants = review.getParticipants();
    for (Participant participant : participants)
    {
        System.out.println(participant.getPerson().getName());
    }

Answer 1

该问题称为(N+1 问题)，与参与者中角色的急切获取有关。

句柄最简单的方法是通过延迟加载替换急切获取。