使用Jackson参考现有对象进行反序列化

问题描述

JSON

{
  "schools": [
    {
      "id": 1,
      "name": "School A"
    },
    {
      "id": 2,
      "name": "School B"
    }
  ],
  "students": [
    {
      "id": 1,
      "name": "Bobby",
      "school": 1
    }
  ]

}

如何将JSON映射到以下类中，以便将Bobby的学校映射到已经实例化的学校A.

How would I map the JSON into the following classes such that Bobby's school is mapped to the already instantiated School A.

public class School {
  private Integer id;
  private String name;
}

public class Student {
  private Integer id;
  private String name;
  private School school;
}

我在学生班上尝试了一些奇怪的东西...

I've tried some weird stuff with the Student class...

public class Student {
  private Integer id;
  private String name;
  private School school;

  @JsonProperty("school")
  public void setSchool(Integer sid) {
    for (School school : getSchools()) {
      if (school.id == sid) {
        this.school = school;
        break;
      }
    }
  }
}

我遇到的问题是学校和学生同时都是从JSON解析的，所以我不知道如何获得学校名单。也许我应该单独解析这些，所以我先得到学校名单？

The problem I'm having is that both the schools and the students are being parsed from the JSON at the same time, so I'm not sure how to get a list of the schools. Maybe I should parse these separately so I have the list of schools first?

推荐答案

杰克逊会为你做这件事。只需使用 @JsonIdentityInfo 注释您的对象：

Jackson will do it for you. Just annotate your objects with @JsonIdentityInfo:

@JsonIdentityInfo(scope=School.class, generator=ObjectIdGenerators.PropertyGenerator.class, property="id")
public class School {
    private Integer id;
    private String name;

    public School() {
    }

    public School(Integer id, String name) {
        this.id = id;
        this.name = name;
    }

    public Integer getId() {
        return id;
    }

    public void setId(Integer id) {
        this.id = id;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }
}

@JsonIdentityInfo(scope=Student.class, generator=ObjectIdGenerators.PropertyGenerator.class, property="id")
public class Student {
    private Integer id;
    private String name;
    private School school;

    public Student() {
    }

    public Student(Integer id, String name, School school) {
        this.id = id;
        this.name = name;
        this.school = school;
    }

    public Integer getId() {
        return id;
    }

    public void setId(Integer id) {
        this.id = id;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public School getSchool() {
        return school;
    }

    public void setSchool(School school) {
        this.school = school;
    }
}

public static void main(String[] args) throws IOException {
    School school = new School(1, "St Magdalene's");
    Student mary = new Student(1, "Mary", school);
    Student bob = new Student(2, "Bob", school);
    Student[] students = new Student[] {mary, bob};

    // Write out
    String serialized = mapper.writeValueAsString(students);
    System.out.println("Serialized: " + serialized);
    // Read in
    Student[] deserialized = mapper.readValue(serialized, Student[].class);
}

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