public class ObjectIdSerializer extends JsonSerializer<ObjectId> {
    @Override
    public void serialize(
            ObjectId value,
            JsonGenerator jgen,
            SerializerProvider provider) throws IOException, JsonProcessingException {
        jgen.writeString(value.toString());
    }
}

public class ObjectIdDeserializer extends JsonDeserializer<ObjectId>{
    @Override
    public ObjectId deserialize(
            JsonParser jp,
            DeserializationContext ctxt) throws IOException, JsonProcessingException {
        ObjectCodec oc = jp.getCodec();
        JsonNode node = oc.readTree(jp);
        return new ObjectId(node.getTextValue());
    }
}

@Document
public class MyDocument {

    @JsonDeserialize(using = ObjectIdDeserializer.class)
    @JsonSerialize(using = ObjectIdSerializer.class)
    private ObjectId id;

    private String name;

    ...
}