Jackson Mapper serialize empty object instead of null

Question

Say I have classes Foo

public class Foo {
    private Bar bar;
}

and Bar

public class Bar {
    private String fizz;
    pr         


        

Answer 1

I was also required to produce such a structure for legacy client compatibility, here is my solution (depends on Spring Boot since uses @JsonComponent annotation)

Create "special object" that will be treated as empty

public class EmptyObject {
}

Create property in your model

@JsonProperty("data")
private EmptyObject data = new EmptyObject();

public EmptyObject getData() {
    return data;
}

Create serializer that will process empty object above

import com.fasterxml.jackson.core.JsonGenerator;
import com.fasterxml.jackson.databind.SerializerProvider;
import com.fasterxml.jackson.databind.ser.std.StdSerializer;
import com.sevensenders.datahub.api.service.response.model.EmptyObject;
import org.springframework.boot.jackson.JsonComponent;

import java.io.IOException;

@JsonComponent
public class EmptyObjectSerializer extends StdSerializer {

    public EmptyObjectSerializer() {
        this(null);
    }

    public EmptyObjectSerializer(Class t) {
        super(t);
    }

    @Override
    public void serialize(EmptyObject value, JsonGenerator gen, SerializerProvider provider) throws IOException {
        // to maintain AF compatible format it is required to write {} instead of null
        gen.writeStartObject();
        gen.writeEndObject();
    }
}

Output:

{
  ...
  "data": {}
}