How to serialise field with empty collection of object?

Question

Here is my POJO:

public class Target {

    @JsonProperty(value="sizes",required=true)
    private Set<Size> Sizes;

    @JsonProperty(value="domains",required=true)
    private Set<String> Domains;

    // getters and setters

}

I initialize this POJO like this:

Target target = new Target(new HashSet<>(), new HashSet<>());

How to get Json from this object with empty fields? I mean smth. like that:

{"sizes":[], "domains":[]}

I tried use ObjectMapper with different parameters, but it was no useful..