Gson serialize null for specific class or field

Question

I want to serialize nulls for a specific field or class.

In GSON, the option serializeNulls() applies to the whole JSON.

Example:

Answer 1

Create subclass of com.google.gson.TypeAdapter and register it for required field using annotation com.google.gson.annotations.JsonAdapter. Or register it using GsonBuilder.registerTypeAdapter. In that adapter write (and read) should be implemented. For example:

public class JsonTestNullableAdapter extends TypeAdapter {

    @Override
    public void write(JsonWriter out, Test value) throws IOException {
        out.beginObject();
        out.name("name");
        out.value(value.name);
        out.name("value");
        if (value.value == null) {
            out.setSerializeNulls(true);
            out.nullValue();
            out.setSerializeNulls(false);
        } else {
            out.value(value.value);
        }
        out.endObject();
    }

    @Override
    public Test read(JsonReader in) throws IOException {
        in.beginObject();
        Test result = new Test();
        in.nextName();
        if (in.peek() != NULL) {
            result.name = in.nextString();
        } else {
            in.nextNull();
        }
        in.nextName();
        if (in.peek() != NULL) {
            result.value = in.nextString();
        } else {
            in.nextNull();
        }
        in.endObject();
        return result;
    }

}

in MainClass add JsonAdapter annotation with the adapter to Test class field:

public static class MClass {
    public String id;
    public String name;
    @JsonAdapter(JsonTestNullableAdapter.class)
    public Test test;
}

the output of System.out.println(new Gson.toJson(mainClass)) is:

{
    "id": "101",
    "test": {
        "name": "testName",
        "value": null
    }
}