无法写入 JSON：无限递归答案

【问题标题】：Could not write JSON: Infinite recursion无法写入 JSON：无限递归
【发布时间】：2020-05-13 23:20:19
【问题描述】：

我在 Postman 或 Browser 中运行查询时遇到 StackOverflow 递归错误。

当我跑步时说：

.w.s.m.s.DefaultHandlerExceptionResolver：无法写入 JSON：无限递归 (StackOverflowError)；嵌套异常是 com.fasterxml.jackson.databind.JsonMappingException: Infinite recursion (StackOverflowError)

这里是模型类：

@Entity 
public class Product {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;
    @NotNull
    private String title;
    @NotNull
    private String description;
    @NotNull
    private double price;
    @OneToOne(fetch = FetchType.LAZY, optional = false)
    @JoinColumn(name = "category_id", nullable = false)
    private Category category;
    private boolean isSealed;
    @OneToOne(fetch = FetchType.LAZY, optional = false)
    @JoinColumn(name = "currency_id", nullable = false)
    private Currency currency;
    @OneToOne(fetch = FetchType.LAZY, optional = false)
    @JoinColumn(name = "user_id", nullable = false)
    private User user;
    @Nullable
    @OneToMany(mappedBy = "product",
            cascade = CascadeType.ALL, orphanRemoval = true)
    private List<Images> images;
    private Date createdDate = new Date();
    @OneToOne(fetch = FetchType.LAZY,
            cascade = CascadeType.ALL,
            mappedBy = "product")
    private View view;
    @OneToOne(fetch = FetchType.LAZY,cascade=CascadeType.ALL)
    @JoinColumn(name="type_id")
    private Type type;
    private Long viewCount; }

@Entity public class Images{
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;
    private String imagePath;
    @ManyToOne(fetch = FetchType.LAZY)
    @JoinColumn(name = "product_id")
    private Product product; }

@Entity public class User implements UserDetails, Serializable {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;
    @NotEmpty
    private String fullName;
    @NotEmpty
    @Email
    @Column(unique = true)
    private String email;
    @NotNull
    @Column(unique = true)
    private int phoneNumber;
    @NotEmpty
    @Size(min = 5)
    private String password;
    private Date createAt = new Date();
    @Nullable
    private String picPath;
    @Nullable
    private String token;
    @ManyToMany
    @JoinTable(name = "user_roles", joinColumns = {@JoinColumn(
            name = "user_id")},
            inverseJoinColumns = {@JoinColumn(name = "role_id")})
    private List<Role> roles;
    @OneToOne(fetch = FetchType.LAZY,
            cascade = CascadeType.ALL,
            mappedBy = "user")
    private Product product;
    @OneToOne(fetch = FetchType.LAZY,
    cascade = CascadeType.ALL,
    mappedBy = "user")
    private View view; }

@Entity 
public class Currency{
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;
    private String code;
    private String currency;
    private String region_country;
    @OneToOne(mappedBy = "currency", cascade = CascadeType.ALL, fetch = FetchType.LAZY)
    private Product product; }

@Entity 
public class Category {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;
    private String name;
    private String imagePath;
    @OneToOne(cascade = CascadeType.ALL, fetch = FetchType.LAZY,
    mappedBy = "category")
    private Product product;
    @OneToMany(mappedBy = "category", fetch = FetchType.LAZY,
            cascade = CascadeType.ALL)
    private Set<Brand> brands; }

@Entity public class Brand {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;
    private String name;
    @ManyToOne(fetch = FetchType.LAZY, cascade = CascadeType.ALL)
    @JoinColumn(name = "category_id", nullable = false)
    private Category category; }

@Entity public class View {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;
    @OneToOne(fetch = FetchType.LAZY, optional = false)
    @JoinColumn(name = "user_id", nullable = false)
    private User user;
    @OneToOne(fetch = FetchType.LAZY, optional = false)
    @JoinColumn(name = "product_id", nullable = false)
    private Product product; }

@Entity public class Type {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;
    @NotNull
    private String name;
    @OneToOne(fetch = FetchType.LAZY,
            cascade = CascadeType.ALL,
            mappedBy = "type")
    private Product product; }

    @Id
    private String role;
    @ManyToMany(mappedBy = "roles")
    private List<User> users; 
}

【问题讨论】：

标签： spring spring-mvc jpa spring-data-jpa hibernate-mapping

【解决方案1】：

您的多个实体本身具有彼此。例如，Product 的对象为 User，User 的对象为 Product。

要解决这个问题，你必须写

@JsonBackReference(value = "user-product")
private User user;

在Product 类中，和

@JsonManagedReference(value = "user-product")
private Product product;

在user 类中。在每个领域和每个相互调用的类中都这样做。

另外，看看这个 JPA: Having lists on both ends without infinite loop

【讨论】：

【解决方案2】：

您的数据模型中有循环。例如，Product 持有Images 和Images 指向Products。

这适用于面向对象的世界，因为只有指针引用存储在这些字段中。

然而，当序列化时，实际对象被写为 json 文本。您的Product 打印Images 对象，该对象又打印Product 对象，该对象再次打印Image 对象，依此类推。

您需要决定如何表示您的 json，将您的数据库模型映射到简单的普通 Java 对象，并将其用于序列化。这些 POJO 通常称为 View Model 或 Transport Objects。

【讨论】：