Proper way to map ObjectId to String in DTO in Spring

Question

I am working with Spring WebSocket. I have this DTO of the Message

@AllArgsConstructor
@Data
public class MessageDto {

    private ObjectId messageId;
    private ObjectId chatId;
}

When I send this DTO as response, ObjectId fields in DTO maps like this:

How can I map ObjectId field into String like 62790c02513ad11442eec6d7?

Answer 1

Spring uses Jackson to map objects to JSON, so the proper way to map ObjectId field in a object is to configure Jackson serializers.

Create a serializer class:

import com.fasterxml.jackson.core.JsonGenerator;
import com.fasterxml.jackson.databind.JsonSerializer;
import com.fasterxml.jackson.databind.SerializerProvider;
import org.bson.types.ObjectId;

import java.io.IOException;

public class ObjectIdSerializer extends JsonSerializer<ObjectId> {

    @Override
    public void serialize(ObjectId id, JsonGenerator jsonGenerator, SerializerProvider serializerProvider)
            throws IOException {
        jsonGenerator.writeObject(id == null ? null : id.toString());
    }
}

And then specify the serializer in a DTO class:

@AllArgsConstructor
@Data
public class MessageDto {

    @JsonSerialize(using = ObjectIdSerializer.class)
    private ObjectId messageId;
    @JsonSerialize(using = ObjectIdSerializer.class)
    private ObjectId chatId;
}

In conclusion, the ObjectId field of the MessageDto object will be serialized like a string in JSON

Of course, there is a problem to deserialize JSON to an object. There are JsonDeserializer<> and @JsonDeserialize to solve this problem. But there (mapping ObjectId class field) deserializer is useless, because ObjectId already have a constructor, that receives a String parameter, so Jackson will use it.