GSON deserialization ignore empty objects in list

Question

A client has decided to start sending empty JSON objects at the end of arrays (to help their caching facepalm) but this has caused a whole bunch of unexpected behaviour in my app.

For example this is the data I am being sent...

[{object}, {object}, {}, {object}]

I currently use GSON to deserialize data sent from the server. I have been looking into type adapters to filter out these empty objects, however I am not fully sure how to implement the filtering. Any ideas on how to skip empty objects using GSON?

Are you having problems with the actual deserialization, or are you having problems after the parsing is complete and objects have been created? — Doug Stevenson, Mar 09 '16 at 03:09
The parsing was correct ... I just was wondering if there was a way to easily ignore empty objects.... — fergdev, Mar 09 '16 at 03:18
Can you simply iterate the resulting object items and remove the ones that have missing data? — Doug Stevenson, Mar 09 '16 at 03:19
Yeah that is essentially what I ended up doing ... was kind of hoping to do it through the deserialization process but just did it post deserialization. — fergdev, Mar 09 '16 at 03:23

score 2 · Answer 1 · answered Mar 09 '16 at 03:17

I solved this issue ... I had to make a TypeAdapterFactory that set empty objects to null then filtered out the nulls from the resulting list.

Here is my TypeAdapterFactory

private static class EmptyCheckTypeAdapterFactory implements TypeAdapterFactory {

    @Override
    public <T> TypeAdapter<T> create(final Gson gson, final TypeToken<T> type) {

        // We filter out the EmptyCheckTypeAdapter as we need to check this for emptiness!
        if (Story.class.isAssignableFrom(type.getRawType())) {
            final TypeAdapter<T> delegate = gson.getDelegateAdapter(this, type);
            final TypeAdapter<JsonElement> elementAdapter = gson.getAdapter(JsonElement.class);
            return new EmptyCheckTypeAdapter<>(delegate, elementAdapter).nullSafe();
        }
        return null;
    }

    public class EmptyCheckTypeAdapter<T> extends TypeAdapter<T> {

        private final TypeAdapter<T> delegate;
        private final TypeAdapter<JsonElement> elementAdapter;

        public EmptyCheckTypeAdapter(final TypeAdapter<T> delegate,
                                     final TypeAdapter<JsonElement> elementAdapter) {
            this.delegate = delegate;
            this.elementAdapter = elementAdapter;
        }

        @Override
        public void write(final JsonWriter out, final T value) throws IOException {
            this.delegate.write(out, value);
        }

        @Override
        public T read(final JsonReader in) throws IOException {
            final JsonObject asJsonObject = elementAdapter.read(in).getAsJsonObject();
            if (asJsonObject.entrySet().isEmpty()) {
                return null;
            }
            return this.delegate.fromJsonTree(asJsonObject);
        }
    }
}

Finally, filtered out the nulls using the following code

myDto.stories.removeAll(Collections.singleton(null));

Not optimal to filter the list after the fact, is it possible to tell GSON to not add the null element to the list in the first place? — lostintranslation, Dec 03 '18 at 03:51

score 1 · Answer 2 · edited May 23 '17 at 11:46

1

You can try this solution from here

class CollectionAdapter implements JsonSerializer<Collection<?>> {

     @Override
     public JsonElement serialize(Collection<?> src, Type typeOfSrc, JsonSerializationContext context) {
     if (src == null || src.isEmpty()) // exclusion is made here
        return null;

     JsonArray array = new JsonArray();

     for (Object child : src) {
         JsonElement element = context.serialize(child);
         array.add(element);
     }

     return array;
   }
}

Then register it

Gson gson = new GsonBuilder().registerTypeHierarchyAdapter(Collection.class, new CollectionAdapter()).create();

edited May 23 '17 at 11:46

Community

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answered Mar 09 '16 at 01:16

Inducesmile

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I do not need to serialize data ... can I do something similar for deserialization? – fergdev Mar 09 '16 at 01:19
1

Yup, this is not at all deserialization. – lostintranslation Dec 03 '18 at 03:50

GSON deserialization ignore empty objects in list

2 Answers2