Json to POJO parsing using jackson - @JsonPropertyOrder for below Json vary based on #students?

Question

This is my json input

{
  "students_key": {
    "student_key_one": {
      "profile_root": "/profile/student_key_one/",
      "nickname": "sam1",
      "email": "sam1@gmail.com",
      "studentkey": "student_key_one"
    },
    "student_key_two": {
      "profile_root": "/profile/student_key_two/",
      "nickname": "sam2",
      "email": "sam2@gmail.com",
      "studentkey": "student_key_two"
    },
    "student_key_three": {
      "profile_root": "/profile/student_key_three/",
      "nickname": "sam3",
      "email": "sam3@gmail.com",
      "studentkey": "student_key_three"
    },
    "student_key_four": {
      "profile_root": "/profile/student_key_four/",
      "nickname": "sam4",
      "email": "sam4@gmail.com",
      "studentkey": "student_key_four"
    },
    "student_key_five": {
      "profile_root": "/profile/student_key_five/",
      "nickname": "sam5",
      "email": "sam5@gmail.com",
      "studentkey": "student_key_five"
    }
  }
}

As the number of students increases the properties will also increase (student_key_one, student_key_two, student_key_three, student_key_four, student_key_five, student_key_six.......) the property will dynamically increase or decrease based on #students.

As this is not an array of objects, how can I create the POJO for StudentsKey? Can someone help me write the pojo for deserialisation using jackson?

Answer 1

Breaking it down:

The outer wrapper is an object and should be its own class

+--------------------------+
|                          |
|  {                       |
|      "students_key": {   |
|           ...            |
|      }                   |
|  }                       |
|                          |       
+--------------------------+

The { } curly brackets signifies an object, and in mapping terms, a class. And the class has only one property students_key. So we can make a class, say Students, with one field students_key

public class Students {
    @JsonProperty("students_key")
    (???????) students;
}

What type should we make the students_key? Let's look at the structure

"students_key": {
    "student_key_one": {
       ...
    }
    "student_key_two": {
       ...
    }
    ...
 }

We should ask ourselves what data structure best supports the concept of keys/values. First thing that comes to mind is a Map. So if we made students_key a Map, the types would be

 //       (key)          (value)
 Map <    String     ,   Object    >
 //   "student_key_one": { ... }  

We could go even further and give Object a solid type, since we have more attributes. So we could make a class Student

public class Student {
    @JsonProperty("profile_root")
    private String profileRoot;
    @JsonProperty("nickname")
    private String nickname;
    @JsonProperty("email")
    private String email;
    @JsonProperty("studentkey")
    private String studentKey;
    // GETTERS and SETTERS

    @Override
    public String toString() {
        return "Student{" + "profileRoot=" + profileRoot + ", nickname=" 
            + nickname + ", email=" + email + ", studentKey=" + studentKey + '}';
    }
}

So out final mapping would look like

public class Students {
    @JsonProperty("students_key")
    Map<String, Student> students;
}

When we test it out, it works as expected

public class StudentsTest {
    public static void main(String[] args) throws Exception {
        ObjectMapper mapper = new ObjectMapper();

        File file = new File("test.json");
        Students students = mapper.readValue(file, Students.class);

        for (Student s : students.getStudents().values()) {
            System.out.println(s);
        }
    }
}

Result

Student{profileRoot=/profile/student_key_one/, nickname=sam1, email=sam1@gmail.com, studentKey=student_key_one}
Student{profileRoot=/profile/student_key_two/, nickname=sam2, email=sam2@gmail.com, studentKey=student_key_two}
Student{profileRoot=/profile/student_key_three/, nickname=sam3, email=sam3@gmail.com, studentKey=student_key_three}
Student{profileRoot=/profile/student_key_four/, nickname=sam4, email=sam4@gmail.com, studentKey=student_key_four}
Student{profileRoot=/profile/student_key_five/, nickname=sam5, email=sam5@gmail.com, studentKey=student_key_five}