消除 Java Spring 多对多关系中的循环 JSON

Question

我有一个 Spring 应用程序，其中包含 2 个具有多对多关系的实体。有学生和团体。一个学生可以是许多组的一部分，一个组可以有许多学生。

学生模特

@Entity
@Table(name = "STUDENTS")
@Inheritance(strategy = InheritanceType.TABLE_PER_CLASS)
@JsonIdentityInfo(
        generator = ObjectIdGenerators.PropertyGenerator.class,
        property = "id")

public class Student extends AbstractUser {

    //Fields

    @ManyToMany(fetch = FetchType.LAZY, targetEntity = Group.class)
    @JoinTable(name = "GROUPS_STUDENTS",
            joinColumns = { @JoinColumn(name = "student_id") },
            inverseJoinColumns = { @JoinColumn(name = "group_id") })
    private List<Group> groups = new ArrayList<Group>();

    //Constructors

    public Student(String password, String firstName, String lastName, String email){
        super(password, firstName, lastName, email);
    }

    public Student(){
    }

    //Setters and getters

    public List<Group> getGroups() {
        return groups;
    }

    public void setGroups(List<Group> groups) {
        this.groups = groups;
    }

}

团体模式

@Entity
@Table(name = "GROUPS")
@JsonIdentityInfo(
        generator = ObjectIdGenerators.PropertyGenerator.class,
        property = "id")
public class Group implements Item, Serializable {

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

    @Column(name = "name", nullable = false, unique = true)
    private String name;

    @Column(name = "year", nullable = false, length = 1)
    private int year;

    @ManyToMany(mappedBy = "groups", targetEntity = Student.class)
    private List<Student> students;

    public Group(String name, int yearOfStudy) {
        this.setName(name);
        this.setYear(yearOfStudy);
    }
...
}

问题是当我请求显示所有学生时，如果有 2 个学生在同一组中，他们会出现在一个层次结构中，而不是一个接一个。我的意思是 JSON 太深了。我不知道如何准确解释，但我会举一个例子。

它是如何出现的

@Entity
@Table(name = "STUDENTS")
@Inheritance(strategy = InheritanceType.TABLE_PER_CLASS)
@JsonIdentityInfo(
        generator = ObjectIdGenerators.PropertyGenerator.class,
        property = "id")

public class Student extends AbstractUser {

    //Fields

    @ManyToMany(fetch = FetchType.LAZY, targetEntity = Group.class)
    @JoinTable(name = "GROUPS_STUDENTS",
            joinColumns = { @JoinColumn(name = "student_id") },
            inverseJoinColumns = { @JoinColumn(name = "group_id") })
    private List<Group> groups = new ArrayList<Group>();

    //Constructors

    public Student(String password, String firstName, String lastName, String email){
        super(password, firstName, lastName, email);
    }

    public Student(){
    }

    //Setters and getters

    public List<Group> getGroups() {
        return groups;
    }

    public void setGroups(List<Group> groups) {
        this.groups = groups;
    }

}

它应该如何出现

@Entity
@Table(name = "GROUPS")
@JsonIdentityInfo(
        generator = ObjectIdGenerators.PropertyGenerator.class,
        property = "id")
public class Group implements Item, Serializable {

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

    @Column(name = "name", nullable = false, unique = true)
    private String name;

    @Column(name = "year", nullable = false, length = 1)
    private int year;

    @ManyToMany(mappedBy = "groups", targetEntity = Student.class)
    private List<Student> students;

    public Group(String name, int yearOfStudy) {
        this.setName(name);
        this.setYear(yearOfStudy);
    }
...
}

你知道如何解决这个问题吗？我不知道如何确切地描述我的问题，这就是为什么我还没有找到解决方案的原因。任何帮助将不胜感激。谢谢你。

Answer 1

我已经解决了。我制作了一个自定义序列化程序。因此，在 Group 我将通过设置自定义注释@JsonSerialize(using = CustomStudentSerializer.class) 来序列化学生

自定义学生序列化程序

public class CustomStudentSerializer extends StdSerializer<List<Student>> {

    public CustomStudentSerializer() {
        this(null);
    }

    public CustomStudentSerializer(Class<List<Student>> t) {
        super(t);
    }

    @Override
    public void serialize(
            List<Student> students,
            JsonGenerator generator,
            SerializerProvider provider)
            throws IOException, JsonProcessingException {

        List<Student> studs = new ArrayList<>();
        for (Student s : students) {
            s.setGroups(null);
            studs.add(s);
        }
        generator.writeObject(studs);
    }
}

对组做了同样的事情。当关系已经嵌套时，我刚刚删除了学生/组组件。现在它工作得很好。

我花了一段时间才弄清楚这一点，但我在这里发帖是因为它也可能对其他人有所帮助。

Answer 2

使用@JsonIgnoreProperties注释是另一种选择：

@Entity
public class Student extends AbstractUser {
    @ManyToMany(fetch = FetchType.LAZY, targetEntity = Group.class)
    @JoinTable(name = "GROUPS_STUDENTS",
            joinColumns = { @JoinColumn(name = "student_id") },
            inverseJoinColumns = { @JoinColumn(name = "group_id") })
    @JsonIgnoreProperties("students")
    private List<Group> groups = new ArrayList<Group>();
}

@Entity
public class Group implements Item, Serializable {
    @ManyToMany(mappedBy = "groups", targetEntity = Student.class)
    @JsonIgnoreProperties("groups")
    private List<Student> students;
}

之间找到比较@JsonManagedReference+ @JsonBackReference，@JsonIdentityInfo并@JsonIgnoreProperties在这里：http://springquay.blogspot.com/2016/01/new-approach-to-solve-json-recursive.html