Casting LinkedHashMap to Complex Object

I've got an application that stores some data in DynamoDB using Jackson to marshall my complex object into a JSON.

For example the object I'm marshalling might look like this:

private String aString;
private List<SomeObject> someObjectList;

Where SomeObject might look like this:

private int anInteger;
private SomeOtherObject;

and SomeOtherObject might look like this:

private long aLong;
private float aFloat; 

This is fine an the object gets marshalled no problem and stored in the DB as a JSON string.

When it comes time to retrieve the data from DynamoDB Jackson automatically retrieves the JSON and converts it back... EXCEPT that 'someObjectList' is returned as a List<LinkedHashMap> not as a List<SomeObject>! This is standard behaviour for Jackson, its not a mistake that this is happening.

So now this leads to a problem. My code base thinks its dealing with a List<SomeObject> but the reality is that its handling a List<LinkedHashMap>! My question is how do I get my LinkedHashMap back into a 'SomeObject'. Obviously this is a manual process but what I mean is I can't even extract the values.

If I do this:

for (LinkedHashMap lhm : someObjectList) {
    // Convert the values back
}

I get a compile error telling me that someObjectList is of type 'SomeObject' not LinkedHashMap.

If I do this:

for (SomeObject lhm : someObjectList) {
    // Convert the values back
}

I get a runtime error telling me that LinkedHashMap cannot be cast to 'SomeObject'.

You can use ObjectMapper.convertValue(), either value by value or even for the whole list. But you need to know the type to convert to:

POJO pojo = mapper.convertValue(singleObject, POJO.class);
// or:
List<POJO> pojos = mapper.convertValue(listOfObjects, new TypeReference<List<POJO>>() { });

this is functionally same as if you did:

byte[] json = mapper.writeValueAsBytes(singleObject);
POJO pojo = mapper.readValue(json, POJO.class);

but avoids actual serialization of data as JSON, instead using an in-memory event sequence as the intermediate step.

I had similar Issue where we have GenericResponse object containing list of values

 ResponseEntity<ResponseDTO> responseEntity = restTemplate.exchange(
        		redisMatchedDriverUrl,
                HttpMethod.POST,
                requestEntity,
                ResponseDTO.class
        );

Usage of objectMapper helped in converting LinkedHashMap into respective DTO objects

 ObjectMapper mapper = new ObjectMapper();

 List<DriverLocationDTO> driverlocationsList = mapper.convertValue(responseDTO.getData(), new TypeReference<List<DriverLocationDTO>>() { });

There is a good solution to this issue:

import com.fasterxml.jackson.databind.ObjectMapper;

ObjectMapper objectMapper = new ObjectMapper();

***DTO premierDriverInfoDTO = objectMapper.convertValue(jsonString, ***DTO.class); 

Map<String, String> map = objectMapper.convertValue(jsonString, Map.class);

Why did this issue occur? I guess you didn't specify the specific type when converting a string to the object, which is a class with a generic type, such as, User <T>.

Maybe there is another way to solve it, using Gson instead of ObjectMapper. (or see here Deserializing Generic Types with GSON)

Gson gson = new GsonBuilder().create();

Type type = new TypeToken<BaseResponseDTO<List<PaymentSummaryDTO>>>(){}.getType();

BaseResponseDTO<List<PaymentSummaryDTO>> results = gson.fromJson(jsonString, type);

BigDecimal revenue = results.getResult().get(0).getRevenue();

I use com.fasterxml.jackson.databind.ObjectMapper to mapping from LinkedHashMap to Json string first and convert from json string to Object

Map<String, String> mappingValue
OBJECTA a = objectMapper.readValue(toJson(mappingValue), OBJECTA.class);

public static String toJson(Object object) {
    ObjectMapper mapper = new ObjectMapper();
    mapper.configure(DeserializationFeature.FAIL_ON_UNKNOWN_PROPERTIES, false);
    String jsonString = "";

    try {
        jsonString = mapper.writeValueAsString(object);
    } catch (JsonProcessingException var4) {
        var4.printStackTrace();
        jsonString = "Can't build json from object";
    }

    return jsonString;
}