Jackson mapping JSON to Java POJO - MismatchedInstanceException

Question

I´m using spring rest template and want to parse the JSON response into the corresponding java POJO. I get every time a MismatchedInputException - Cannot deserialized instance of out of start array token.

I´ve looked at several questions relating to jackson but didn´t find the suitable solution. I´m not sure if my POJO structure is correct.

First the related JSON:

{
   "days":[
      [
         {
            "day":"2019-04-29",
            "time":"08:00-09:30",
            "room":"room1",
            "name":"lecture1",
            "lecturer":"prof1",
            "eventId":332713,
            "modified":false
         }
      ],
      [

      ],
      [

      ],
      [
         {
            "day":"2019-05-02",
            "time":"08:00-10:15",
            "room":"room2",
            "name":"lecture2",
            "lecturer":"prof2",
            "eventId":332714,
            "modified":false
         },
         {
            "day":"2019-05-02",
            "time":"10:45-13:00",
            "room":"room3",
            "name":"lecture3",
            "lecturer":"prof3",
            "eventId":332721,
            "modified":false
         }
      ],
      [

      ],
      [

      ],
      [
      ]
   ]
}

POJO Structure:

public class Main{

    ArrayList<Day> days = new ArrayList<>();

    public ArrayList<Day> getDays() {
        return days;
    }

    public void setDays(ArrayList<Day> days) {
        this.days = days;
    }

    public class Day {

        private String day;
        private String time;
        private String room;
        private String name;
        private String lecturer;
        private Integer eventId;
        private Boolean modified;

        public Day() {
        }
       // Getter and setter of Day Object
    }
}

Rest-Template Method in API Object:

public String getInfoByEventsPeriods(String eventIDs) {
        String url = APIURL + "/info?from="2019-04-29"&to="2019-05-03"&eventId=" + eventIDs;

        return rest.getForObject(url, String.class);
    }

Processing with ObjectMapper in Main Object:

public Main getLectures(String eventIDs) throws IOException {
        API api = new API();
        ObjectMapper mapper = new ObjectMapper();
        return mapper.readValue(api.getInfoByEventsPeriods(eventIDs), Main.class);

    }

Answer 1

Yes, you'r right,the wrong type is used while deserializing.
I'm not sure exactly, but your days array contains another arrays with day objects, but in your structure ArrayList consists of objects day, not array of them.

Correct JSON would be:

{
   "days":[
         {
            "day":"2019-04-29",
            "time":"08:00-09:30",
            "room":"room1",
            "name":"lecture1",
            "lecturer":"prof1",
            "eventId":332713,
            "modified":false
         },
         {
            "day":"2019-05-02",
            "time":"08:00-10:15",
            "room":"room2",
            "name":"lecture2",
            "lecturer":"prof2",
            "eventId":332714,
            "modified":false
         },
         ......
   ]
}   

So, change response structure or pojo object depending on what needs to be fixed.
If you need to modify POJO, your ArrayList must contains List of days.

Answer 2

Can you have your POJO like below. I tried to deserialize with your JSON and I was able to deserialize successfully.

class Day {

  private String day;
  private String time;
  private String room;
  private String name;
  private String lecturer;
  private Integer eventId;
  private Boolean modified;

  public void setDay(String day) {
    this.day = day;
  }

  public void setTime(String time) {
    this.time = time;
  }

  public void setRoom(String room) {
    this.room = room;
  }

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

  public void setLecturer(String lecturer) {
    this.lecturer = lecturer;
  }

  public void setEventId(Integer eventId) {
    this.eventId = eventId;
  }

  public void setModified(Boolean modified) {
    this.modified = modified;
  }
}

class Main{
  private List<List<Day>> days;
  public void setDays(List<List<Day>> days) {
    this.days = days;
  }
}