Spring，Hibernate：多對多映射的延遲初始化異常

Question

我正在開發一個Spring-MVC應用程序，其中我試圖為兩個實體之間已經存在的一對多映射添加多對多映射。 我們在項目中擁有的兩個實體是GroupSection和GroupNotes 。

對於項目中的一項任務，我不得不在GroupSection和GroupNotes之間引入多對多映射，但是我遇到了一個懶惰的初始化異常。

錯誤：

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"])

這是被稱為的控制器方法：

@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方法僅調用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<>();
}

組注釋：

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<>();
}

我究竟做錯了什么？ 是否可以同時在2個類之間進行一對多和多對多映射。 如果是，那么該錯誤怎么處理。 請讓我知道。 謝謝。 :-)

Answer 1

調用listGroupSectionByCanvasid時，您打開一個事務和一個會話。 呼叫返回時，會話關閉。 當spring返回值時，它嘗試讀取您的對象，並且由於延遲加載了許多關系groupNotesSet和groupSections是需要會話的休眠集合。 在您的情況下，會話將不再存在。