Jackson用泛型反序列化JSON

Question

I've some issues with polymorphic (is it polymorphic?) deserialising with Jackson.

Suppose I have the following JSON structure

{
  "list": [
    "02/01/2018",
    "03/01/2018",
    "04/01/2018",
    "05/01/2018",
    "08/01/2018",
    "05/02/2018"
  ]
}

where list may contains different types of data. I've modelled the data structure with the following POJO, using generics.

public class GeneralResponseList<T> extends BaseResponse {

    @JsonProperty("list")
    private List<T> list;

    @JsonProperty("paging")
    private Paging paging;

    @JsonProperty("sorting")
    private List<Sorting> sorting;

    // [CUT]
}

How to specify the deserializer for type T ? I've looked at polymorphic deserialization but I think it doesn't solve my issue.

I can also create a specific LocalDateResponseList which extends GeneralResponseList<LocalDate> . How to specify the deserializer for the the specific response?

Can you suggest me a solution or hints to solve this issue.

Answer 1

Assuming that you have a class like:

public class GeneralResponseList<T> {

    @JsonProperty("list")
    private List<T> list;

    // Getters and setters
}

You can use TypeReference<T> :

GeneralResponseList<LocalDate> response = 
    mapper.readValue(json, new TypeReference<GeneralResponseList<LocalDate>>() {});

---

If you have multiple date formats, as you mentioned in the comments, you can write a custom deserializer to handle that:

public class LocalDateDeserializer extends StdDeserializer<LocalDate> {

    private List<DateTimeFormatter> availableFormatters = new ArrayList<>();

    protected LocalDateDeserializer() {
        super(LocalDate.class);
        availableFormatters.add(DateTimeFormatter.ofPattern("dd/MM/yyyy"));
        availableFormatters.add(DateTimeFormatter.ofPattern("yyyy-MM-dd"));
    }

    @Override
    public LocalDate deserialize(JsonParser p, DeserializationContext ctxt) 
            throws IOException {

        String value = p.getText();

        if (value == null) {
            return null;
        }

        for (DateTimeFormatter formatter : availableFormatters) {
            try {
                return LocalDate.parse(value, formatter);
            } catch (DateTimeParseException e) {
                // Safe to ignore
            }
        }

        throw ctxt.weirdStringException(value, LocalDate.class, "Unknown date format");
    }
}

Then add the deserializer to a module and register the Module in the ObjectMapper instance:

ObjectMapper mapper = new ObjectMapper();

SimpleModule module = new SimpleModule();
module.addDeserializer(LocalDate.class, new LocalDateDeserializer());
mapper.registerModule(module);