如何使用Gson序列化Optional <T>类？

Question

我有一个具有以下属性的对象.

private final String messageBundle;
private final List<String> messageParams;
private final String actionBundle;
private final Map<String, String> data;
private final Optional<Pair<Integer,TimeUnit>> ttl;
private final Optional<Integer> badgeNumber;
private final Optional<String> collapseKey;

该对象位于库中,我宁愿不仅仅为了序列化目的而修改它,并且希望避免创建另一个DTO的成本.

如何序列化/反序列化可选属性？可选没有默认构造函数(apache commons Pair),但是我不能使用InstanceCreator,并且不太了解如何创建一个简单地将序列化委托给底层Optional元素的TypeAdapter.

Answer 1

经过几个小时的gooling和编码 - 有我的版本:

public class OptionalTypeAdapter<E> extends TypeAdapter<Optional<E>> {

    public static final TypeAdapterFactory FACTORY = new TypeAdapterFactory() {
        @Override
        public <T> TypeAdapter<T> create(Gson gson, TypeToken<T> type) {
            Class<T> rawType = (Class<T>) 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<E> adapter;

    public OptionalTypeAdapter(TypeAdapter<E> adapter) {

        this.adapter = adapter;
    }

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

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

}

您可以GsonBuilder像这样简单地注册它:

instance.registerTypeAdapterFactory(OptionalTypeAdapter.FACTORY)

请注意,如果json中没有字段,Gson不会为您的类字段设置值.因此,您需要Optional.empty()在实体中设置默认值.

Answer 2

Ilya的解决方案忽略了类型参数,因此在一般情况下它无法真正起作用.我的解决方案相当复杂,因为需要区分null和Optional.absent()- 否则你可以将封装剥离为列表.

public class GsonOptionalDeserializer<T>
implements JsonSerializer<Optional<T>>, JsonDeserializer<Optional<T>> {

    @Override
    public Optional<T> deserialize(JsonElement json, Type typeOfT, JsonDeserializationContext context)
            throws JsonParseException {
        final JsonArray asJsonArray = json.getAsJsonArray();
        final JsonElement jsonElement = asJsonArray.get(0);
        final T value = context.deserialize(jsonElement, ((ParameterizedType) typeOfT).getActualTypeArguments()[0]);
        return Optional.fromNullable(value);
    }

    @Override
    public JsonElement serialize(Optional<T> src, Type typeOfSrc, JsonSerializationContext context) {
        final JsonElement element = context.serialize(src.orNull());
        final JsonArray result = new JsonArray();
        result.add(element);
        return result;
    }
}