如何通过杰克逊的注释定义通用列表反序列化器？

Question

Let's say I have an object which has list properties:

public class Citizen {
    name
    List<Tickets> tickets
    List<Fines> fines
}

I'd like to define a generic custom deserializer for lists through annotations:

public class Citizen {
    ...
    @JsonDeserializer(MyListDeserializer<Tickets>) // <-- generic deserializer
    public void setTickets(List<Tickets> tickets) {
        this.tickets = tickets;
    }

    @JsonDeserializer(MyListDeserializer<Fines>) // <-- how can I do that? 
    public void setFines(List<Fines> fines) {
        this.fines = fines;
    }
}

I'm looking for a way to create a "generic" deserializer — one that would be able to deserialize both types of lists, similar to ContextualDeserializer for mapping JSON to different types of maps with Jackson .

The final purpose is to implement custom deserializing logic in MyListDeserializer to deserialize empty strings "" as empty lists, but I'd like to know about a general approach, not just for empty strings.

Answer 1

You can specify the deserializer class with which to deserialize the elements of the list with the contentUsing attribute of the @JsonDeserializer annotation.

public class Citizen {
    ...
    @JsonDeserializer(contentUsing=MyListDeserializer.class) 
    public void setTickets(List<Tickets> tickets) {
        this.tickets = tickets;
    }
}

Make your deserializer extend JsonDeserializer<BaseClass> and have a attribute in the BaseClass that stores the actual type of the concrete class.

abstract class BaseTickets {
    String ticketType;
    public String getTicketType()
}

public class MyListDeserializer extends JsonDeserializer<BaseTickets> {

    @Override
    public BaseTickets deserialize(JsonParser jsonParser, DeserializationContext arg1) throws IOException, JsonProcessingException {
        ObjectCodec oc = jsonParser.getCodec();
        JsonNode node = oc.readTree(jsonParser);
        Iterator<JsonNode> elements = node.getElements();
        for (; elements.hasNext();) {
            String type = (String) elements.next().get("ticketType");

            if (type.equals()){
               //create concrete type here
            }
        }
     }

Or if you want a single deserializer for all List types with no common base class, then use the using attribute, have MyListDeserializer extend JsonDeserialize<Object> . For determining the type of list element you would have to write a custom serializer that adds the type information to the list which can then be used in the generic deserializer.

public class Citizen {
    ...
    @JsonDeserializer(using=MyListDeserializer.class)
    @JsonSerializer(using=MyListSerializer.class) 
    public void setTickets(List<Tickets> tickets) {
        this.tickets = tickets;
    }
}

public class MyListSerializer extends JsonSerializer<Object> {

    @Override
    public void serialize(Object list, JsonGenerator jgen, SerializerProvider provider) throws IOException, JsonProcessingException {
        @SuppressWarnings("rawtypes")
        jgen.writeStartObject();
        String type = getListType(list);
        jgen.writeStringField("listType", type);
        jgen.writeObjectField("list", list)
    }
}