多双向映射

Question

我的实体如下所示：

package com.myhome.api.components.account.entity;

import com.fasterxml.jackson.annotation.JsonBackReference;
import com.myhome.api.components.house.entity.House;
import com.myhome.api.components.member.entity.Member;
import com.myhome.api.components.recipe.entity.Recipe;
import lombok.Data;

import javax.persistence.*;
import java.util.List;
import java.util.Set;

@Table(name = "account")
@Entity
@Data
public class Account {

    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    @Column(name = "id", nullable = false)
    private Integer id;

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

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

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

    @OneToMany(
            cascade = {CascadeType.ALL},
            orphanRemoval = true,
            mappedBy = "fkAccountId")
    @JsonBackReference
    private Set<Member> members;
}

package com.myhome.api.components.member.entity;

import com.fasterxml.jackson.annotation.JsonBackReference;
import com.fasterxml.jackson.annotation.JsonBackReference;
import com.fasterxml.jackson.annotation.JsonManagedReference;
import com.myhome.api.components.account.entity.Account;
import com.myhome.api.components.meal.entity.Meal;
import com.myhome.api.components.rating.entity.Rating;
import lombok.Data;

import javax.persistence.*;
import java.util.Set;

@Table(name = "member")
@Entity
@Data
public class Member {

    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    @Column(name = "id", nullable = false)
    private Long id;

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

    @Column(name = "icon")
    private Integer icon;

    @ManyToOne
    @JoinColumn(name = "fkAccountId", nullable = false)
    @JsonManagedReference
    private Account fkAccountId;

    @OneToMany(
            cascade = {CascadeType.ALL},
            orphanRemoval = true,
            mappedBy = "fkMemberId")
    @JsonBackReference
    private Set<Meal> meals;

    @OneToMany(
            cascade = {CascadeType.ALL},
            orphanRemoval = true,
            mappedBy = "fkMemberId")
    @JsonBackReference
    private Set<Rating> ratings;
}

我的问题是：

1. 为什么会这样，即使在我添加食谱和房子之间的联系之前没有发生？
2. 我怎样才能修好它？
3. 原因是什么？

Answer 1

为什么会这样

这是由于类中都存在toString（）方法并形成循环依赖关系造成的。这将导致帐户和成员的toString（）方法之间的无限递归调用。

考虑这两个简单的类。Lombok的@Data默认为所有字段生成toString方法。

class Member {
    String name;
    Account account;

    @Override
    public String toString() {
        return "Member{" +
                "name='" + name + '\'' +
                ", account=" + account +
                '}';
    }
}

class Account {
    String id;
    Member member;

    @Override
    public String toString() {
        return "Account{" +
                "id='" + id + '\'' +
                ", member=" + member +
                '}';
    }
}

public static void main(String[] args) {
        Account account = new Account();
        Member member  = new Member();
        account.id="1";
        account.member=member;

        member.name ="XYZ";
        member.account=account;

        System.out.println(account);
    }

当您在其中一个类上调用toString方法时，假设帐户，它将依次调用成员的toString方法，循环重复，直到您用完堆栈内存。这是堆栈跟踪：

Exception in thread "main" java.lang.StackOverflowError
    at java.lang.StringBuilder.append(StringBuilder.java:136)
    at com.javainuse.main.Member.toString(AnotherMain.java:23)
    at java.lang.String.valueOf(String.java:2994)
    at java.lang.StringBuilder.append(StringBuilder.java:131)
    at com.javainuse.main.Account.toString(AnotherMain.java:10)
    at java.lang.String.valueOf(String.java:2994)

我怎样才能修好它？

从lombok中排除其中一个类的toString生成，这样应该可以工作。参考此

原因是什么？

我想我不明白。你能详细说明一下吗？