使用Jackson将JSON反序列化为Java

Question

I am working with JSON for the very first time and I am trying to deserialise a JSON file into Java objects.

InputJSON

{

    "student_id" : "123",
     "courses":[
        {
         "course_id":"789",
         "professor":"abc"
        }
      ]
}
{
    "student_id":"234",
     courses:[
       {
         "course_id":"789",
         "professor":"pqr"
       },
       {
         "course_id":"789",
         "professor":"xyz"
       }
     ]
}
{
   "student_id" : "345",
     "courses":[
        {
         "course_id":"567",
         "professor":"lmn"
        }
      ]
}

Student.class

class Student {

   @JsonProperty("student_id")
   private String studentId;
   @JsonProperty("courses")
   private List<Courses> courses;

   //getters and setters

}

Courses.class

public class Courses {

    @JsonProperty("course_id")
    private String courseId;
    @JsonProperty("professor")
    private String professor;

    //getters and setters
}

My JsonHelper.class creates an object of Object Mapper class and uses readValue(new File("Input.json"),Student.class) to to map the json fields to Java objects.

What I want to do is create a map with studentId as a key and the list of courses as the value corresponding to each studentId.

I am not exactly getting how to achieve it since my list is just able to get 1 element and the list size is thus 1. And issue with creating the map too. Any help will be appreciated.

Answer 1

This java class seems wrong. because your variable name is not the same with your JSON object.

public class Courses {
    @JsonProperty("course_id")
    private String courseId;
    @JsonProperty("professor")
    private String professor;
    //getters and setters
}

it would be like this,

class Student {
   @JsonProperty("student_id")
   private String student_id;
   @JsonProperty("courses")
   private List<Courses> courses;
   //getters and setters

    }
    public class Courses {
            @JsonProperty("course_id")
            private String course_id;
            @JsonProperty("professor")
            private String professor;
            //getters and setters
        }

Answer 2

Here are some ways to do mapping with ObjectMapper:

ObjectMapper objectMapper = new ObjectMapper();

For parsing a single Student object:

Student student = objectMapper.readValue(inputJSONString, Student.class);

For parsing a list of Student object which is what you want:

List<Student> students = objectMapper.readValue(inputJSONString, objectMapper.getTypeFactory().constructCollectionType(List.class, Student.class));

For parsing a maps:

Map<String, Courses> maps = objectMapper.readValue(inputJSONString, objectMapper.getTypeFactory().constructMapType(Map.class, String.class, Courses.class));