Spring JPA 一对多无限递归

Question

我试图返回一个具有@OneToMany 和@ManyToOne 双向关系的JPA 数据（当然，转换为DTO）。我目前申请的东西fix。问题是输出是可恢复的。 cmets 有帖子有 cmets 然后有帖子（cmets -> 帖子 -> 评论 -> 等等..）。

我只想拥有这样的东西

{
    "post_id": 1
    "user": {
        // user data
    },
    "description": "some description",
    "images": "{images,images}",
    "tags": "{tags, tags}",
    "comments": [
     {
        //some comments data

     },
     {
        //some comments data
     }
    ]
    "lastUpdated": "2020-04-08T14:23:18.000+00:00"
}

这是我的代码

这是我的 Posts.class

@Data
@Entity
@Table(name = "posts")
@EntityListeners(AuditingEntityListener.class)
public class Posts {

    @Id
    @GeneratedValue(strategy = GenerationType.AUTO)
    @Column(name = "post_id")
    private Long post_id;

    @ManyToOne
    @JoinColumn(name = "user_id", nullable = false)
    private Users user;

    @Column(name = "description")
    private String description;

    @Column(name="images")
    private String images;

    @Column(name="tags")
    private String tags;

    @OneToMany(
        fetch = FetchType.LAZY,
        mappedBy = "post",
        cascade = CascadeType.ALL,
        orphanRemoval = true
    )
    @JsonIgnoreProperties(value = { "comments" ,"hibernateLazyInitializer", "handler" }, allowSetters = true)
    //@JsonManagedReference
    private List<Comments> comments;


    @LastModifiedDate
    @Column(name = "last_updated")
    private Date lastUpdated;


    public void addComments(Comments comment) {
        this.comments.add(comment);
    }
}

这是我的 PostDTO.class

@Data
public class PostDTO {

    private Long post_id;
    private UserDTO user;
    private String description;
    private String images;
    private String tags;
    private List<CommentsDTO> comments;
    private Date lastUpdated;


}

这是我的 Comments.class

@Data
@Entity
@Table(name = "comments")
@EntityListeners(AuditingEntityListener.class)
public class Comments {

    @Id
    @GeneratedValue(strategy = GenerationType.AUTO)
    @Column(name="comment_id")
    private Long comment_id;

    @OneToOne
    @JoinColumn(name = "user_id")
    private Users user;

    @Column(name = "description")
    private String description;

    @Column(name="images")
    private String images;

    @ManyToOne
    @JoinColumn(name ="post_id" , nullable = false)
    @JsonIgnoreProperties(value = { "post" ,"hibernateLazyInitializer", "handler" }, allowSetters = true)
    //@JsonBackReference
    private Posts post;

    @Column(name="last_updated")
    @LastModifiedDate
    private Date lastUpdated;

}

这是我的 CommentsDTO.class

@Data
public class CommentsDTO {

    private Long comment_id;

    private UserDTO user;

    private String description;

    private PostDTO post;

    private String images;

    private Date lastUpdated;
}

这是我的 REST 控制器

@GetMapping
public @ResponseBody ResponseEntity<List<PostDTO>>  getAll() throws Exception {

    return new ResponseEntity<List<PostDTO>>(service.getAll(), HttpStatus.OK);
}

这是我的服务

public List<PostDTO> getAll() throws Exception  {
    return repo.findAll()
               .stream()
               .map(this::convert)
               .collect(Collectors.toList());
}

private PostDTO convert(Posts e) {
    return  mapper.map(e, PostDTO.class);
}

希望有人能阐明我的问题。到现在为止有点迷失了。

Answer 1

问题是当您将Post 转换为PostDTO 时，然后通过使用ModelMapper，它会为PostDTO 调用Post 的所有字段的getter。所以这递归地发生了 mapper.map(e, PostDTO.class)

所以，只需从 CommentDTO 中删除 private PostDTO post ， 然后 modelmapper 不要尝试设置 PostDTO->comment-> post 字段。 而且您不需要 DTO 中的双向关系。 DTO 就是您想要在响应中显示的内容。

Answer 2

您在错误的类上打破了 json 序列化周期。

您正在发送PostDTO 的列表，但将JsonBackReference 应用于Comments 和JsonManagedReference 应用于Posts

更新

请注意，ObjectMapper 类、JsonManagedReference 和 JsonBackReference 可能来自 2 个包

• com.fasterxml.jackson.databind.ObjectMapper
• com.fasterxml.jackson.annotation.JsonManagedReference
• com.fasterxml.jackson.annotation.JsonBackReference

或：

• org.codehaus.jackson.map.ObjectMapper
• org.codehaus.jackson.annotate.JsonManagedReference
• org.codehaus.jackson.annotate.JsonBackReference

如果不一致，不匹配的注解会被忽略，序列化时仍然会出现死循环。

Answer 3

将 JsonIgnoreProperties 与 Manage 和反向引用一起使用。

 @Data
    @Entity
    @Table(name = "comments")
    @EntityListeners(AuditingEntityListener.class)
    public class Comments {

        @Id
        @GeneratedValue(strategy = GenerationType.AUTO)
        @Column(name="comment_id")
        private Long comment_id;

        @OneToOne
        @JoinColumn(name = "user_id")
        private Users user;

        @Column(name = "description")
        private String description;

        @Column(name="images")
        private String images;

        @ManyToOne
        @JoinColumn(name ="post_id" , nullable = false)
        @JsonIgnoreProperties(value = { "comments" ,"hibernateLazyInitializer", "handler" }, allowSetters = true)
       @JsonBackReference
        private Posts post;

        @Column(name="last_updated")
        @LastModifiedDate
        private Date lastUpdated;

    }

你的 Post 类应该是

@Data
@Entity
@Table(name = "posts")
@EntityListeners(AuditingEntityListener.class)
public class Posts {

    @Id
    @GeneratedValue(strategy = GenerationType.AUTO)
    @Column(name = "post_id")
    private Long post_id;

    @ManyToOne
    @JoinColumn(name = "user_id", nullable = false)
    private Users user;

    @Column(name = "description")
    private String description;

    @Column(name="images")
    private String images;

    @Column(name="tags")
    private String tags;

    @OneToMany(
        fetch = FetchType.LAZY,
        mappedBy = "post",
        cascade = CascadeType.ALL,
        orphanRemoval = true
    )
    @JsonIgnoreProperties(value = { "post" ,"hibernateLazyInitializer", "handler" }, allowSetters = true)
    @JsonManagedReference
    private List<Comments> comments;


    @LastModifiedDate
    @Column(name = "last_updated")
    private Date lastUpdated;


    public void addComments(Comments comment) {
        this.comments.add(comment);
    }
}

另外我建议使用 Getter 和 Setter 注释而不是 @Data 。因为 Tostring() 方法也会导致递归。