Java & JSON : How to serialize NULL

So how do you serialize NULL ?
NULL would typically mean that the attribute is omitted from the json, but what if you WANT the NULL to be there, to symbolize an attribute that should be REMOVED.

    public static class Product {
        public String model;
        public String color;
        public String shirtSize;

    void howToSerializeNULL() {
        Product bossShirt = new Product("super slim", "red", "xl");
        System.out.println("Product : " + JSONUtils.stringify(bossShirt));
        System.out.println("Product : " + JSONUtils.stringify(bossShirt));
        Map<String, Object> obj = new HashMap<>();
        obj.put("model", "super slim");
        obj.put("color", "red");
        obj.put("shirtSize", JSONObject.NULL);
        System.out.println("Product : " + JSONUtils.stringify(obj));
        GsonBuilder builder = new GsonBuilder();
        Gson gson = builder.create();
        System.out.println("Product : " + gson.toJson(obj));
        System.out.println("Product : " + gson.toJson(bossShirt));

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