Question

I have a JSON:

{
    "firstField": "Something One",
    "secondField": "Something Two",
    "thirdField": [
        {
            "thirdField_one": "Something Four",
            "thirdField_two": "Something Five"
        },
        {
            "thirdField_one": "Something Six",
            "thirdField_two": "Something Seven"
        }
    ],
    "fifthField": [
        {
            "fifthField_one": "Something… ",
            "fifthField_two": "Something...",
            "fifthField_three": 12345
        },
        {
            "fifthField_one": "Something",
            "fifthField_two": "Something",
            "fifthField_three": 12345
        }
    ]
}

I have my classes:

public static class MyClass {
        @JsonProperty
        private String firstField, secondField;
        @JsonProperty
        private ThirdField thirdField;
        @JsonProperty
        private FifthField fifthField;

        public static class ThirdField {
            private List<ThirdFieldItem> thirdField;
        }

        public static class ThirdFieldItem {
            private String thirdField_one, thirdField_two;
        }

        public static class FifthField {
            private List<FifthFieldItem> fifthField;
        }

        public static class FifthFieldItem {
            private String fifthField_one, fifthField_two;
            private int fifthField_three;
        }
    }

I’m deserializing them with Jackson library:

public void testJackson() throws IOException {
    JsonFactory factory = new JsonFactory();
    ObjectMapper mapper = new ObjectMapper(factory);
    File from = new File("text.txt"); // JSON I mentioned above
    mapper.readValue(from, MyClass.class);
}

but I’m getting the Exception:

org.codehaus.jackson.map.JsonMappingException: Can not deserialize
instance of Main$MyClass$ThirdField out of START_ARRAY token

Answer 1

You defined your thirdField and fifthField properties as arrays in your JSON. They need to be arrays or collections on your Java bean as well:

public static class MyClass {
    @JsonProperty
    private String firstField, secondField;

    @JsonProperty
    private Collection<ThirdField> thirdField;

    @JsonProperty
    private Collection<FifthField> fifthField;

    /// ...
}

As you are going through and converting an existing JSON object into beans, keep in mind that JSON data is very much like a map. If you envision how you would map the data from a map into your object it really helps. Your ThirdField and FifthField objects need to map the definitions in your JSON. This is what your JSON says a ThirdField is:

{
    "thirdField_one": "Something Four",
    "thirdField_two": "Something Five"
}

Literally converting that to a Java bean gives you:

public class ThirdField implements Serializable {
    private String thirdField_one;
    private String thirdField_two;

    // ...
}

You can add in your annotations etc, etc to get a full fledged bean. Do the same thing for your FifthField object.