Jackson：将属性反序列化为另一个属性（如果为null）

Question

I want to deserialize one property to another property if it is null.

Let's say I have a class:

public class User {
    private String login;
    private String nickname;
    // getters, setters
}

Every time nickname comes as null I'd like to return login as default value.

Q: Is there a general way to achieve this playing around with Jackson deserializers?

Note: There is a strait-forward way, getter like this will solve the problem:

public String getNickname() {
    return nickname == null ? login : nickname;
}

But it isn't elegant, and I don't want to put any logic into my DTO objects.

Answer 1

You can custom the serialization

public class CustomFooSerialize extends JsonSerializer<Foo> {

    @Override
    public void serialize(Foo foo, JsonGenerator jsonGenerator, SerializerProvider serializerProvider) throws IOException, JsonProcessingException {

        if (foo.getNickname() == null) {
            jsonGenerator.writeStartObject();
            jsonGenerator.writeStringField("nickname", foo.getLogin());
            jsonGenerator.writeEndObject();
        }
    }
}

register the serializer directly on the class

@JsonSerialize(using = CustomFooSerialize.class)
public class Foo

Answer 2

During the deserialization jackson calls setters of the class. So, I would try:

public void setNickname(String nickname) {
    this.nickname == nickname;
    if (nickname != null) {
        login = nickname;
    }
}

The property login probably needs @JsonIgnore in this case. Anyway, this code is confusing, because setter has a side effect.

Returning to deserializer. In such case, you need to write a custom deserializer:

import com.fasterxml.jackson.core.JsonParser;
import com.fasterxml.jackson.core.JsonProcessingException;
import com.fasterxml.jackson.core.ObjectCodec;
import com.fasterxml.jackson.databind.DeserializationContext;
import com.fasterxml.jackson.databind.JsonDeserializer;
import com.fasterxml.jackson.databind.JsonNode;
import java.io.IOException;

public class UserDeserializer extends JsonDeserializer<User> {

    @Override
    public User deserialize(JsonParser jp, DeserializationContext ctxt) throws IOException, JsonProcessingException {

        ObjectCodec oc = jp.getCodec();

        JsonNode node = oc.readTree(jp);

        //final Long id = node.get("id").asLong();
        final String nickname = node.get("nickname").asText();
        final String login = null;
        if (nickname != null) {
            login = nickname;
        }
        User user = new User();
        user.setNickname(nickname);
        user.setLogin(login);
        return user;
    }
}

Sorry for possibly incorrect indentation.