将 LinkedHashMap 转换为复杂对象

我有一个应用程序，它在 DynamoDB 中存储一些数据，使用 Jackson 将我的复杂对象封装成一个 JSON。

例如，我正在编组的对象可能如下所示:

private String aString;
private List<SomeObject> someObjectList;

有些对象可能看起来像这样:

private int anInteger;
private SomeOtherObject;

另一个对象可能是这样的:

private long aLong;
private float aFloat;

如果对象没有问题地进行编组，并以 JSON 字符串的形式存储在 DB 中，那么这样做很好。

当需要从 DynamoDB Jackson 检索数据时，Jackson 会自动检索 JSON 并将其转换回来... ... 除了‘ some ObjectList’作为 List<LinkedHashMap>而不是 List<SomeObject>返回！这是杰克逊的标准行为，这不是一个错误。

所以这就引出了一个问题。我的代码库认为它处理的是 List<SomeObject>，但实际上它处理的是 List<LinkedHashMap>！我的问题是如何让我的 LinkedHashMap 重新变成一个‘ Some Object’。显然，这是一个手工过程，但我的意思是，我甚至不能提取值。

如果我这样做:

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

我得到一个编译错误，它告诉我 some ObjectList 的类型是‘ Some Object’而不是 LinkedHashMap。

如果我这样做:

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

我得到一个运行时错误，告诉我 LinkedHashMap 不能被强制转换为‘ Some Object’。

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最佳答案

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.

小开

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();
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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>>() { });
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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;
}
小开

public static <T> List<T> getObjectList(final String json, final Class<T> cls) {

return objectMapper.readValue(json, objectMapper.getTypeFactory().constructCollectionType(ArrayList.class, cls));

}

小开

First you have to convert linkedhashmap to JsonString and then it can be converted into complex object.


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