Spring，Hibernate：多对多映射的延迟初始化异常

Question

我正在开发一个 Spring-MVC 应用程序，我在其中尝试为两个实体之间已经存在的一对多映射添加多对多映射。我们在项目中拥有的两个实体是GroupSection 和GroupNotes。

对于项目中的一项任务，我不得不在 GroupSection 和 GroupNotes 之间引入多对多映射，但我遇到了延迟初始化异常。

错误：

org.springframework.http.converter.HttpMessageNotWritableException: Could not write content: failed to lazily initialize a collection of role: com.tooltank.spring.model.GroupSection.groupSections, could not initialize proxy - no Session (through reference chain: java.util.ArrayList[0]->com.tooltank.spring.model.GroupSection["groupSections"]); nested exception is com.fasterxml.jackson.databind.JsonMappingException: failed to lazily initialize a collection of role: com.tooltank.spring.model.GroupSection.groupSections, could not initialize proxy - no Session (through reference chain: java.util.ArrayList[0]->com.tooltank.spring.model.GroupSection["groupSections"])

这里是被调用的控制器方法：

@RequestMapping(value = "/sections/get/{canvasid}")
    public @ResponseBody List<GroupSection> getAllSectionsForCanvas(@PathVariable("canvasid") int canvasid) {
        boolean value = this.personService.getCanvasValuesForCanvas(canvasid);
        return this.groupSectionService.listGroupSectionByCanvasid(canvasid, value);
    }

DAO方法（Service方法只调用DAO方法）：

   @Override
    @Transactional(readOnly = true)
    public List<GroupSection> listGroupSectionByCanvasid(int mcanvasid) {
        try {
            Session session = this.sessionFactory.getCurrentSession();
            org.hibernate.Query query = session.createQuery("From GroupSection as msection where " +
                    "msection.currentcanvas.mcanvasid=:mcanvasid and msection.sectionDisabled=false and msection.sectionInActive=false");
            query.setParameter("mcanvasid", mcanvasid);
            return query.list();
        }catch (Exception e){
            e.printStackTrace();
            return null;
        }
    }

GroupSection 模型：

import com.fasterxml.jackson.annotation.JsonIgnore;
import org.hibernate.annotations.Type;
import org.hibernate.annotations.TypeDef;

import javax.persistence.*;
import java.util.Date;
import java.util.HashSet;
import java.util.List;
import java.util.Set;
@Entity
@Table(name = "membersection")
public class GroupSection {


// Below is self-mapping, required for one of the task, works.
    @JsonIgnore
    @ManyToOne
    @JoinColumn(name = "owned_section_id", nullable = true)
    private GroupSection primarySection;

    @OneToMany(mappedBy = "primarySection", fetch = FetchType.LAZY, cascade = CascadeType.REMOVE)
    private Set<GroupSection> groupSections = new HashSet<>();


// Many to many mapping, I had lazy loading before, but tried Eager to see if error goes, it doesn't. :

    @ManyToMany(fetch = FetchType.LAZY,cascade = CascadeType.REMOVE)
    @JoinTable(name = "sectionjunction",joinColumns = {@JoinColumn(name = "msectionid")},
            inverseJoinColumns = {@JoinColumn(name = "mnoteid")})
    private Set<GroupNotes> groupNotesSet = new HashSet<>();


// One to many mapping below :

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

组笔记：

import com.fasterxml.jackson.annotation.JsonIgnore;
import org.hibernate.annotations.Type;
import org.hibernate.annotations.TypeDef;

import javax.persistence.*;
import java.util.Date;
import java.util.HashSet;
import java.util.List;
import java.util.Set;
@Entity
@Table(name = "groupnotes")
public class GroupNotes implements Serializable {


    @JsonIgnore
    @ManyToMany(mappedBy = "groupNotesSet",fetch = FetchType.EAGER)
    private Set<GroupSection> groupSectionSet = new HashSet<>();

    @ManyToOne
    @JoinColumn(name = "msectionid",nullable = true)
    @JsonIgnore
    private GroupSection ownednotes;

 // Self mappings :


    @JsonIgnore
    @ManyToOne
    @JoinColumn(name = "owned_note_id", nullable = true)
    private GroupNotes primaryNote;

    @OneToMany(mappedBy = "primaryNote", fetch = FetchType.LAZY, cascade = CascadeType.REMOVE)
    private Set<GroupNotes> groupNotesSet = new HashSet<>();
}

我做错了什么？是否可以同时在两个类之间进行一对多和多对多映射。如果是，那么该错误是怎么回事。请告诉我。谢谢。 :-)

Answer 1

调用 listGroupSectionByCanvasid 时，您会打开一个事务和一个会话。当呼叫返回时，会话关闭。当 spring 返回值时，它会尝试读取您的对象，并且由于延迟加载您的多对多关系 groupNotesSet 和 groupSections 是需要会话的休眠集合。在您的情况下，会话不再存在。