杰克逊在通用名单中读了json

Question

我正在使用杰克逊来阅读json消息。我试图解析的一个值是List，另一个值包含列表中数据的类型。这是我在java中创建的结构。

public class Message<T> {
   private Timestamp time;
   private RestAction action;
   private String type;
   private List<T> data;
}

通过Class.forName();我可以获得表示列表中数据的类。问题是如何阅读清单。

Answer 1

更短的东西：

mapper.readValue(jsonString, new TypeReference<List<EntryType>>() {});

Answer 2

如果您需要将传入的json映射到列表，您可以这样做

String jsonString = ...; //Your incoming json string
ObjectMapper mapper = new ObjectMapper();
Class<?> clz = Class.forName(yourTypeString);
JavaType type = mapper.getTypeFactory().constructCollectionType(List.class, clz);
List <T> result = mapper.readValue(jsonString, type);

修改

像这样的东西，完全未经测试，从未完成

public Message<T> deserialize(JsonParser jsonParser, DeserializationContext arg1)
    throws IOException, JsonProcessingException {
    ObjectMapper mapper = new ObjectMapper();
    ObjectCodec oc = jsonParser.getCodec();
    JsonNode node = oc.readTree(jsonParser);

    JsonNode timeStamp = node.get("time");
    Timestamp time = mapper.readValue(timeStamp, Timestamp.class);
    JsonNode restAction = node.get("action");
    RestAction action = mapper.readValue(restAction, RestAction.class);
    String type = node.get("type").getTextValue();
    Class<?> clz = Class.forName(type);
    JsonNode list = node.get("data");
    JavaType listType = mapper.getTypeFactory().constructCollectionType(List.class,   clz);
    List <T> data = mapper.readValue(list, listType);

    Message<T> message = new Message<T>;
    message.setTime(time);
    message.setAction(action);
    message.setType(type);
    message.setData(data);

    return message;
}

Answer 3

您需要使用@JsonDeserialize(using = MessageDeserializer.class)为您的班级添加注释并实现自定义反序列化器：

public class MessageDeserializer extends JsonDeserializer<Message> {
  @Override
  public Message deserialize(JsonParser jsonParser, DeserializationContext deserializationContext)
    throws IOException, JsonProcessingException {
    // YOU DESERIALIZER CODE HERE
  }
}

@see示例：How Do I Write a Jackson JSON Serializer & Deserializer?

Answer 4

自 2.11 版（2020 年 4 月）起，Jackson 具有 readerForListOf 方法 (documentation)：

ObjectMapper mapper = ObjectMapper().configure(...);
ObjectReader reader = mapper.readerForListOf(MyClass.class);
List<MyClass> list = reader.readValue(...);

Answer 5

如果你需要阅读通用列表中的json， 做点什么

{
    Employee:
    [
        {
            "key": "value",
            "key": "value"
        },
        {
            "key": "value",
            "key": "value"
        }
    ]
} 

你的json列表对象 和 你的bean类是

class person {
private String key;
private String value;
}

现在你可以使用HashMap

来阅读这个json

类似

TypeReference<HashMap<String,List<Person>>> personList =  new   TypeReference<HashMap<String, List<Person>>>() {};

ObjectMapper objectMapper = new ObjectMapper();
objectMapper.setSerializationInclusion(JsonInclude.Include.NON_NULL);
objectMapper.configure(DeserializationFeature.FAIL_ON_UNKNOWN_PROPERTIES, false);

HashMap<String, List<Person>> employees = objectMapper.readValue(request.getInputStream(), personList);