Jackson XML Bind Element based on value of attributes

Question

I have the following XML structure:

<participants>
    <participant side="AWAY">
        <team id="18591" name="Orlando Apollos" />
    </participant>
    <participant side="HOME">
        <team id="18594" name="Memphis Express" />
    </participant>
</participants>

If I am using the FasterXML Jackson library with JAXB annotations how can I bind the participants fields to two different Participant objects participantHome and participantAway using the side property of AWAY and HOME to bind the fields.

Using the following object won't work obviously because there are duplicate fields:

import javax.xml.bind.annotation.XmlElement;
import javax.xml.bind.annotation.XmlRootElement;

@XmlRootElement(name = "participants")
public class Participants {

    @XmlElement(name = "participant")
    Participant participantHome;

    @XmlElement(name = "participant")
    Participant participantAway;
}

How can I dynamically bind those elements using JAXB annotations or a custom JAXB implementation?

Answer 1

You need to write custom deserialiser because there is no annotation which allow to bind list item to given property in object. If you already use Jackson try to implement custom JsonDeserializer instead of custom XmlAdapter. We can simplify our custom deserialiser by deserialising internal Participant objects to Map. Simple example:

import com.fasterxml.jackson.core.JsonParser;
import com.fasterxml.jackson.core.JsonToken;
import com.fasterxml.jackson.databind.DeserializationContext;
import com.fasterxml.jackson.databind.JsonDeserializer;
import com.fasterxml.jackson.databind.annotation.JsonDeserialize;
import com.fasterxml.jackson.databind.type.MapType;
import com.fasterxml.jackson.dataformat.xml.XmlMapper;

import java.io.File;
import java.io.IOException;
import java.util.ArrayList;
import java.util.List;
import java.util.Map;

public class XmlMapperApp {

    public static void main(String[] args) throws Exception {
        File xmlFile = new File("./resource/test.xml").getAbsoluteFile();

        XmlMapper xmlMapper = new XmlMapper();

        Participants result = xmlMapper.readValue(xmlFile, Participants.class);
        System.out.println(result);
    }
}

class ParticipantsXmlAdapter extends JsonDeserializer<Participants> {

    @Override
    public Participants deserialize(JsonParser p, DeserializationContext ctxt) throws IOException {
        List<Map<String, Object>> participants = readParticipantsMap(p, ctxt);

        Participants result = new Participants();
        for (Map<String, Object> participantMap : participants) {
            Object side = participantMap.get("side").toString();
            if ("AWAY".equals(side)) {
                result.setParticipantAway(convert((Map<String, Object>) participantMap.get("team")));
            } else if ("HOME".equals(side)) {
                result.setParticipantHome(convert((Map<String, Object>) participantMap.get("team")));
            }
        }

        return result;
    }

    private List<Map<String, Object>> readParticipantsMap(JsonParser p, DeserializationContext ctxt) throws IOException {
        MapType mapType = ctxt.getTypeFactory().constructMapType(Map.class, String.class, Object.class);
        JsonDeserializer<Object> mapDeserializer = ctxt.findRootValueDeserializer(mapType);
        List<Map<String, Object>> participants = new ArrayList<>();
        p.nextToken(); // skip Start of Participants object
        while (p.currentToken() == JsonToken.FIELD_NAME) {
            p.nextToken(); // skip start of Participant
            Object participant = mapDeserializer.deserialize(p, ctxt);
            participants.add((Map<String, Object>) participant);
            p.nextToken(); // skip end of Participant
        }

        return participants;
    }

    private Participant convert(Map<String, Object> map) {
        Participant participant = new Participant();
        participant.setId(Integer.parseInt(map.get("id").toString()));
        participant.setName(map.get("name").toString());

        return participant;
    }
}

@JsonDeserialize(using = ParticipantsXmlAdapter.class)
class Participants {

    private Participant participantHome;
    private Participant participantAway;

    // getters, setters, toString
}

class Participant {
    private int id;
    private String name;

    // getters, setters, toString
}

prints:

Participants{participantHome=Participant{id=18594, name='Memphis Express'}, participantAway=Participant{id=18591, name='Orlando Apollos'}}