[FIXED] Jackson cannot map JSON to Object in Java

Issue

I am trying to map the following JSON string to a pojo class:

{
    "Data": {
        "brand": "Porsche",
        "model": 2020,
        "color": "gray"
    },
    "Section": {
        "location": "UK",
        "service": "London"
    },
    // other elements 
}

I am only interested in 2 fields (brand, model) in the Data part and want to map these 2 fields to the following model:

Data:

@Getter
@Setter
@NoArgsConstructor
@AllArgsConstructor
@JsonIgnoreProperties(ignoreUnknown = true)
public class Data {

    @JsonProperty("brand")
    private String brand;

    @JsonProperty("model")
    private int model;
}

However, although jsonString has a valid JSON, the fields in Data are null or empty.

// jsonString is a valid JSON string read from a REST API via WebClient
String jsonString = request.block(); 
ObjectMapper mapper = new ObjectMapper();
Data data = mapper.readValue(value, Data.class);

So, what is missing? Is there any problem regarding to pojo fields or annotations in it? Could you help me pls?

Solution

You have not specified any class to catch the response. You have only created 1 model to catch the Data object. You can do it like this,

i) Create a class to catch the JSON,

@Getter
@Setter
@NoArgsConstructor
@AllArgsConstructor
@JsonIgnoreProperties(ignoreUnknown = true)
public class Payload {

    @JsonProperty("Data")
    private Data data;

  
}

You have to first catch the JSON using the Payload class and then parse it accordingly,

String jsonString = request.block(); 
ObjectMapper mapper = new ObjectMapper();
Payload payload = mapper.readValue(jsonString, Payload.class);
Data data = payload.getData();

ii) Without any extra class,

String payloadString = "{\n" +
                "    \"Data\": {\n" +
                "        \"brand\": \"Porsche\",\n" +
                "        \"model\": 2020,\n" +
                "        \"color\": \"gray\"\n" +
                "    },\n" +
                "    \"Section\": {\n" +
                "        \"location\": \"UK\",\n" +
                "        \"service\": \"London\"\n" +
                "    }\n" +
                "}";
        ObjectMapper objectMapper = new ObjectMapper();
        Map<String, Object> objectMap = objectMapper.readValue(payloadString, HashMap.class);
        String dataString = objectMapper.writeValueAsString(objectMap.get("Data"));
        Data data = objectMapper.readValue(dataString, Data.class);
        System.out.println(data);

Answered By - Akhil
Answer Checked By - Terry (JavaFixing Volunteer)