How to serialize Optional classes with Gson?

Question

I have an object with the following attributes.

private final String messageBundle;
private final List messageParams;
private final String acti         


        

Answer 1

After several hours of gooling and coding - there is my version:

public class OptionalTypeAdapter extends TypeAdapter> {

    public static final TypeAdapterFactory FACTORY = new TypeAdapterFactory() {
        @Override
        public  TypeAdapter create(Gson gson, TypeToken type) {
            Class rawType = (Class) type.getRawType();
            if (rawType != Optional.class) {
                return null;
            }
            final ParameterizedType parameterizedType = (ParameterizedType) type.getType();
            final Type actualType = parameterizedType.getActualTypeArguments()[0];
            final TypeAdapter adapter = gson.getAdapter(TypeToken.get(actualType));
            return new OptionalTypeAdapter(adapter);
        }
    };
    private final TypeAdapter adapter;

    public OptionalTypeAdapter(TypeAdapter adapter) {

        this.adapter = adapter;
    }

    @Override
    public void write(JsonWriter out, Optional value) throws IOException {
        if(value.isPresent()){
            adapter.write(out, value.get());
        } else {
            out.nullValue();
        }
    }

    @Override
    public Optional read(JsonReader in) throws IOException {
        final JsonToken peek = in.peek();
        if(peek != JsonToken.NULL){
            return Optional.ofNullable(adapter.read(in));
        }

        in.nextNull();
        return Optional.empty();
    }

}

You can simple registered it with GsonBuilder like this:

instance.registerTypeAdapterFactory(OptionalTypeAdapter.FACTORY)

Please keep attention that Gson does not set values to your class field if field does not present in json. So you need to set default value Optional.empty() in your entity.