Problem with mapping JSON to ArrayList with Jackson - "out of START_OBJECT token"

Question

I'm trying to map JSON to ArrayList and I'm getting "Cannot deserialize instance of java.util.ArrayList out of START_OBJECT token" error and I don't understand why. I found this article: Can not deserialize instance of java.util.ArrayList out of START_OBJECT token resolving similar problem (there was invalid JSON), but it seems like I have valid json file. I'm using maping method presented in this question: How to use Jackson to deserialise an array of objects

I found this article: Can not deserialize instance of java.util.ArrayList out of START_OBJECT token resolving similar problem (there was invalid JSON), but it seems like I have valid json file. I'm using maping method presented in this question: How to use Jackson to deserialise an array of objects

My Json looks like this:

[
    { 
        "id": "12345", 
        "name": "John" 
    }, 
    { 
        "id": "09876", 
        "name": "Desmond" 
    }
]

Data model:

public class Student {

    private int id;
    private String name;

    public Student() {}

    // getters, setters and tostring
}

Parsing code:

ObjectMapper objectMapper = new ObjectMapper();

Path pathToStudentsJson = Paths.get("src/main/resources/static/students.json");
File studentsFile = new File(pathToSingleStudentJson.toString());

List<Student> listOfStudents = objectMapper.readValue(studentsFile, new TypeReference<List<Student>>(){});

The error, that I'm getting:

Exception in thread "main" com.fasterxml.jackson.databind.exc.MismatchedInputException: Cannot deserialize instance of `java.util.ArrayList` out of START_OBJECT token
 at [Source: (File); line: 1, column: 1]
    at com.fasterxml.jackson.databind.exc.MismatchedInputException.from(MismatchedInputException.java:63)
    at com.fasterxml.jackson.databind.DeserializationContext.reportInputMismatch(DeserializationContext.java:1343)
    at com.fasterxml.jackson.databind.DeserializationContext.handleUnexpectedToken(DeserializationContext.java:1139)
    at com.fasterxml.jackson.databind.DeserializationContext.handleUnexpectedToken(DeserializationContext.java:1093)
    at com.fasterxml.jackson.databind.deser.std.CollectionDeserializer.handleNonArray(CollectionDeserializer.java:332)
    at com.fasterxml.jackson.databind.deser.std.CollectionDeserializer.deserialize(CollectionDeserializer.java:265)
    at com.fasterxml.jackson.databind.deser.std.CollectionDeserializer.deserialize(CollectionDeserializer.java:245)
    at com.fasterxml.jackson.databind.deser.std.CollectionDeserializer.deserialize(CollectionDeserializer.java:27)
    at com.fasterxml.jackson.databind.ObjectMapper._readMapAndClose(ObjectMapper.java:4013)
    at com.fasterxml.jackson.databind.ObjectMapper.readValue(ObjectMapper.java:2940)
    at io.json.JsonApp.main(JsonApp.java:46)

Process finished with exit code 1

Using version [2.9.8](https://mvnrepository.com/artifact/com.fasterxml.jackson.core/jackson-databind/2.9.8) it works properly. Which version do you use? — Michał Ziober, May 11 '19 at 22:15

score 0 · Answer 1 · answered May 11 '19 at 23:51

0

Your Data model shows Id is int whereas in the JSON it is string. Try changing this and use latest version of Jackson. It may resolve the issue.

answered May 11 '19 at 23:51

Niran

21
1

This is not the case, because I was trying with single student JSON file and id was mapped properly to int inside Student object. Anyway I can fix that to keep data types consistent, but this is not solution for my problem. – s-kaczmarek May 12 '19 at 07:03

score 0 · Answer 2 · answered May 12 '19 at 07:32

All right, I have found my mistake :) It's here:

Path pathToStudentsJson = Paths.get("src/main/resources/static/students.json"); 
File studentsFile = new File(pathToSingleStudentJson.toString());

I'm creating File object referring to the incorrect path- I should pass pathToStudentsJson to the constructor instead of pathToSingleStudentJson

Problem with mapping JSON to ArrayList with Jackson - "out of START_OBJECT token"

2 Answers2