Json Result 在 ManyToMany 中有重复项，在 Spring Boot 中有一个新实体

Posted 2023-03-16

【中文标题】Json Result 在 ManyToMany 中有重复项，在 Spring Boot 中有一个新实体

【英文标题】：Json Result has duplicates in ManyToMany with a New Entity in Spring Boot

【发布时间】：2021-11-19 21:56:46

【问题描述】：

我正在尝试使用关系表 EmployeeCourse 获取所有员工，并查看该员工有多少课程。当我在控制器中使用 employeeRepo.findAll() 方法时，我得到如下 JSON 结果：

["firstName":"Pera","lastName":"Peric","employeeCourses":[],"employeeId":1,"firstName":"Marko","lastName":"Markovic","employeeCourses":["id":1,"employee":"firstName":"Marko","lastName":"Markovic","employeeCourses":["id":1,"employee":"firstName":"Marko","lastName":"Markovic","employeeCourses":["id":1,"employee":"firstName":"Marko","lastName":"Markovic","employeeCourses":["id":1,"employee":"firstName":"Marko","lastName":"Markovic","employeeCourses":["id":1,"employee":"firstName":"Marko","lastName":"Markovic","employeeCourses":["id":1,"employee":"firstName":"Marko","lastName":"Markovic","employeeCourses":["id":1,"employee":"firstName":"Marko","lastName":"Markovic","employeeCourses":["id":1,"employee":"firstName":"Marko","lastName":"Markovic","employeeCourses":["id":1,"employee":"firstName":"Marko","lastName":"Markovic","employeeCourses":["id":1,"employee":"firstName":"Marko","lastName":"Markovic","employeeCourses":["id":1,"employee":"firstName":"Marko","lastName":"Markovic","employeeCourses":["id":1,"employee":"firstName":"Marko","lastName":"Markovic","employeeCourses":["id":1,"employee":"firstName":"Marko","lastName":"Markovic","employeeCourses":["id":1,"employee":"firstName":"Marko","lastName":"Markovic","employeeCourses":["id":1,"employee":"firstName":"Marko","lastName":"Markovic","employeeCourses":["id":1,"employee":"firstName":"Marko","lastName":"Markovic","employeeCourses":["id":1,"employee":"firstName":"Marko","lastName":"Markovic","employeeCourses":["id":1,"employee":"firstName":"Marko","lastName":"Markovic","employeeCourses":["id":1,"employee":"firstName":"Marko","lastName":"Markovic","employeeCourses":["id":1,"employee":"firstName":"Marko","lastName":"Markovic","employeeCourses":["id":1,"employee":"firstName":"Marko","lastName":"Markovic","employeeCourses":["id":1,"employee":"firstName":"Marko","lastName":"Markovic","employeeCourses":["id":1,"employee":"firstName":"Marko","lastName":"Markovic","employeeCourses":["id":1,"employee":"firstName":"Marko","lastName":"Markovic","employeeCourses":["id":1,"employee":"firstName":"Marko","lastName":"Markovic","employeeCourses":["id":1,"employee":"firstName":"Marko","lastName":"Markovic","employeeCourses":["id":1,"employee":"firstName":"Marko","lastName":"Markovic","employeeCourses":["id":1,"employee":"firstName":"Marko","lastName":"Markovic","employeeCourses":["id":1,"employee":"firstName":"Marko","lastName":"Markovic","employeeCourses":["id":1,"employee":"firstName":"Marko","lastName":"Markovic","employeeCourses":["id":1,"employee":"firstName":"Marko","lastName":"Markovic","employeeCourses":["id":1,"employee":"firstName":"Marko","lastName":"Markovic","employeeCourses":["id":1,"employee":"firstName":"Marko","lastName":"Markovic","employeeCourses":["id":1,"employee":"firstName":"Marko","lastName":"Markovic","employeeCourses":["id":1,"employee":"firstName":"Marko","lastName":"Markovic","employeeCourses":["id":1,"employee":"firstName":"Marko","lastName":"Markovic","employeeCourses":["id":1,"employee":"firstName":"Marko","lastName":"Markovic","employeeCourses":["id":1,"employee":"firstName":"Marko","lastName":"Markovic","employeeCourses":["id":1,"employee":"firstName":"Marko","lastName":"Markovic","employeeCourses":["id":1,"employee":"firstName":"Marko","lastName":"Markovic","employeeCourses":["id":1,"employee":"firstName":"Marko","lastName":"Markovic","employeeCourses":["id":1,"employee":"firstName":"Marko","lastName":"Markovic","employeeCourses":["id":1,"employee":"firstName":"Marko","lastName":"Markovic","employeeCourses":["id":1,"employee":"firstName":"Marko","lastName":"Markovic","employeeCourses":["id":1,"employee":"firstName":"Marko","lastName":"Markovic","employeeCourses":["id":1,"employee":"firstName":"Marko","lastName":"Markovic","employeeCourses":["id":1,"employee":"firstName":"Marko","lastName":"Markovic","employeeCourses":["id":1,"employee":"firstName":"Marko","lastName":"Markovic","employeeCourses":["id":1,"employee":"firstName":"Marko","lastName":"Markovic","employeeCourses":["id":1,"employee":"firstName":"Marko","lastName":"Markovic","employeeCourses":["id":1,"employee":"firstName":"Marko","lastName":"Markovic","employeeCourses":["id":1,"employee":"firstName":"Marko","lastName":"Markovic","employeeCourses":["id":1,"employee":"firstName":"Marko","lastName":"Markovic","employeeCourses":["id":1,"employee":

为简洁起见，删除了很多行，并且未加载其他实体员工。

我的实体如下所示：

@Entity
public class Employee

    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    @Column(name = "employee_id")
    private Integer id;
    private String firstName;
    private String lastName;
    @OneToMany(mappedBy = "employee", cascade = CascadeType.ALL)
    Set<EmployeeCourse> employeeCourses;
// getters and setters implemented
// dont have hashCode() and equals overridden

@Entity
public class EmployeeCourse 

    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Integer id;

    @ManyToOne
    @JoinColumn(name = "employee_id")
    Employee employee;
    @ManyToOne
    @JoinColumn(name = "course_id")
    Course course;
    private Date startDate;
    private Date endDate;
//have getter and setters and overridden hashCode() and equals

public class Course 

    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    @Column(name = "course_id")
    private Integer id;
    private String name;
    private String description;

    @OneToMany(mappedBy = "course")
    Set<EmployeeCourse> employeeCourses;

为什么我有这个双重结果，以及如何获取它们？

【参考方案1】：

读取您的 JSON 数据非常困难，但我猜想 double results 我假设您在谈论您的 JSON 响应中的大量 Employee 数据。这是由于双向关系：

Employee EmployeeCourse: Employee 引用了 Set<EmployeeCourse>，EmployeeCourse 引用了 Employee。 EmployeeCourse Course: EmployeeCourse 引用了 Course，Course 引用了 Set<EmployeeCourse>。

这意味着当 Jackson 序列化您的 Employees 时，它将序列化所有这些重复的信息。

话虽如此，您现在有三个选择：

您可以将双向关系转变为单向关系，例如，删除EmployeeCourse 中的employee 属性。 如果出于某种原因 1. 不可能，那么您可以使用 @JsonIgnore 以便 Jackson 不会序列化它们。 如果 2. 也不可能，您可以考虑使用 DTO。通过控制器返回数据时，通常建议使用 DTO 而不是普通实体。使用 DTO，您可以清楚地定义要包含在对调用者的响应中的任何内容。您甚至可以重新组织模型，使其更好地满足客户的需求。您可以将您的内部模型与您的 API 使用者所知道的模型解耦，从而可以重新设计您的内部模型并仍然保持相同的公共模型。

【讨论】：

我认为 3 可能会解决我的问题，谢谢。

太棒了！之后告诉我结果。

【参考方案2】：

只需将@JsonIgnor 放在您不想扩展的对象之上。前任。当然，如果您不想扩展 employeeCourses 的结果，那么就做这样的事情

@OneToMany(mappedBy = "course")
@JsonIgnore
    Set<EmployeeCourse> employeeCourses;

【讨论】：

这不是我的问题的解决方案，我需要显示所有数据。