Java; Jackson; Parsing the array of array json string

Question

I see issues in parsing below json to java object using jackson api.

Json String
[
  [
    {
      "id": "6555",
      "fname": "hello",
      "lname": "test"
    },
    {
      "id": "6588",
      "fname": "world",
      "lname": "test"
    }
  ]
]

I have created below pojos (removed setters and getters).

public class Result {
   List<Student> studentList;
}

public class Student{
   private String id;
   private String fname;
   private String lname;
}

ObjectMapper mapper = new ObjectMapper();
List<Result > responseList = mapper.readValue(jsonStr, new TypeReference<List<Result>>(){});

Jackson has throwing exception

com.fasterxml.jackson.databind.JsonMappingException: Can not deserialize instance of out of START_ARRAY token at [Source:

Do I need to any specific jackson annotation to Result object?

I would recommend going the other way around, create a list of results and try writing it using your mapper :) — Roy Shahaf
– Roy Shahaf, Commented Dec 13, 2018 at 19:28

Emre Savcı · Accepted Answer · 2018-12-13 19:30:03Z

1

You don't need to Result class, it is redundant. You need to parse it as List<List<Student>> because your json structure starts as array and contains another arrays.

Here is how you can parse your :

ObjectMapper mapper = new ObjectMapper();
try {
    List<Result> responseList = mapper.readValue(
            Files.readAllBytes(Paths.get("test.json")),
            new TypeReference<List<List<Student>>>() {});

} catch (IOException e) {
    e.printStackTrace();
}

Output :

answered Dec 13, 2018 at 19:30

Emre Savcı

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1 Comment

skc Over a year ago

thank you.. Yup. I can go with List<List<Student>> and its worked.

Geoffrey Wiseman · Accepted Answer · 2018-12-13 19:33:38Z

1

If I were trying to write JSON to match the structure you've defined, I'd want:

[ 
    { 
        "studentList": 
        [
            {
                "id": "6555",
                "fname": "hello",
                "lname": "test"
            },
            {
                "id": "6588",
                "fname": "world",
                "lname": "test"
            }
        ]
    }
]

Based on the JSON you've laid out, I'd expect Jackson to see that as a List<List<Student>>.

answered Dec 13, 2018 at 19:33

Geoffrey Wiseman

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1 Comment

skc Over a year ago

Thank you Geoffrey. I dont have control and its rest api response.

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