Spring MVC，Hibernate：延迟初始化异常

Question

我正在开发一个Spring-MVC应用程序，其中我有3个类，组扫描，组节，组注释。组扫描器具有与组节的一对多映射，组节与组注释具有一对多映射。我正在尝试根据 GroupCanvas 的主键检索注释，但我收到Hibernate延迟初始化异常。我在网上尝试了这些建议，主要是SO，但似乎没有帮助。这是代码。

DAO 方法抛出错误：

 @Override
    public List<GroupNotes> searchNotesByDays(int days, int mcanvasid) {
        Session session = this.sessionFactory.getCurrentSession();
        Calendar cal = Calendar.getInstance();
        cal.add(Calendar.DAY_OF_YEAR, -days);
        long daysAgo = cal.getTimeInMillis();
        Timestamp nowMinusDaysAsTimestamp = new Timestamp(daysAgo);
        Query query = session.createQuery("from GroupSection as n where n.currentcanvas.mcanvasid=:mcanvasid");
        query.setParameter("mcanvasid", mcanvasid);
        List<GroupSection> sectionList = query.list();
        List<GroupNotes> notesList = new ArrayList<GroupNotes>();
        for (GroupSection e : sectionList) {
            Query query1 = session.createQuery("from GroupNotes as n where n.ownednotes.msectionid=:msectionid and n.noteCreationTime >:limit");
            query1.setParameter("limit", nowMinusDaysAsTimestamp);
            query1.setParameter("msectionid",e.getMsectionid());
            notesList.addAll(query1.list());
        }
        return notesList;
    }

GroupCanvas模型:

@Entity
@Table(name = "membercanvas")
public class GroupCanvas{

variables, getters, setters ignored
    @OneToMany(mappedBy = "currentcanvas",fetch=FetchType.LAZY, cascade = CascadeType.REMOVE)
    @JsonIgnore
    private Set<GroupSection> ownedsection = new HashSet<>();

    public Set<GroupSection> getOwnedsection() {
        return this.ownedsection;
    }

    public void setOwnedsection(Set<GroupSection> ownedsection) {
        this.ownedsection = ownedsection;
    }
}

组段模型类：

@Entity
@Table(name = "membersection")
public class GroupSection {
 @ManyToOne
    @JoinColumn(name = "groupcanvasid",nullable = false)
    @JsonIgnore
    private GroupCanvas currentcanvas;

    public GroupCanvas getCurrentcanvas() {
        return this.currentcanvas;
    }

    public void setCurrentcanvas(GroupCanvas currentcanvas) {
        this.currentcanvas = currentcanvas;
    }

    public int getCurrentCanvasId(){
        return this.currentcanvas.getMcanvasid();
    }


    @OneToMany(mappedBy = "ownednotes", fetch = FetchType.EAGER,cascade = CascadeType.REMOVE)
    @JsonIgnore
    private Set<GroupNotes> sectionsnotes = new HashSet<>();

    public Set<GroupNotes> getSectionsnotes(){
        return this.sectionsnotes;
    }

    public void setSectionsnotes(Set<GroupNotes> sectionsnotes){
        this.sectionsnotes=sectionsnotes;
    }
}

群注释：

@Entity
@Table(name="groupnotes")
public class GroupNotes{
   @ManyToOne
    @JoinColumn(name = "msectionid")
    @JsonIgnore
    private GroupSection ownednotes;

    public GroupSection getOwnednotes(){return this.ownednotes;}

    public void setOwnednotes(GroupSection ownednotes){this.ownednotes=ownednotes;}
}

错误日志：

org.springframework.http.converter.HttpMessageNotWritableException: Could not write JSON: failed to lazily initialize a collection of role: com.journaldev.spring.model.GroupCanvas.ownedsection, could not initialize proxy - no Session (through reference chain: java.util.ArrayList[0]->com.journaldev.spring.model.GroupNotes["ownednotes"]->com.journaldev.spring.model.GroupSection["currentcanvas"]->com.journaldev.spring.model.GroupCanvas["ownedsection"]); nested exception is com.fasterxml.jackson.databind.JsonMappingException: failed to lazily initialize a collection of role: com.journaldev.spring.model.GroupCanvas.ownedsection, could not initialize proxy - no Session (through reference chain: java.util.ArrayList[0]->com.journaldev.spring.model.GroupNotes["ownednotes"]->com.journaldev.spring.model.GroupSection["currentcanvas"]->com.journaldev.spring.model.GroupCanvas["ownedsection"])
    org.springframework.http.converter.json.MappingJackson2HttpMessageConverter.writeInternal(MappingJackson2HttpMessageConverter.java:256)

我做错了什么，请告诉我。如果需要更多信息，请发表评论。

Answer 1

JSON 转换器在Hibernate会话完成后执行。JSON转换器正在盲目地访问所有获取器和设置器，甚至是懒惰的获取器和设置器。因此，当Hibernate尝试初始化组扫描#拥有部分时，没有可用的会话，因此会引发此异常。

可能的解决方案：

>

不要直接在Hibernate托管对象上执行JSON转换器。创建DTO对象来完成这项工作。DTO对象没有逻辑，是纯java bean，非常适合这个角色。但是缺点是你必须维护另一个类层次结构。利大于弊。以下帖子可以帮助这种方法：

DTO模式：在两个对象之间复制属性的最佳方式

使用注释将某些字段标记为不可序列化。例如，JsonIgnore。这样做的缺点是，如果在不同的API中需要此字段，则不能使用此字段。

如果可以从您的模型中删除一个反向参考（注释-