Convert Guava HashMultimap to json

Question

I want to print HashMultiMap as json.

HashMultimap<String,Object> multimap = HashMultimap.create();
multimap.put("a",Obj1);
multimap.put("a",Obj3);
multimap.put("b",Obj2);

to

{ 
  "a":[Obj1,Obj3],
  "b":[Obj2]
}

Obj1 and other objects should again be in json(to keep it clean, I have shown it as objects)
I can iterate over the individual keys and convert set of Objects to json using libraries such as Gson.

But to get the entire snapshot of the HashMultimap, I want to convert it to json and inspect it.

Gson could not convert the entire map, but could do individual values(list of objects to json)

Answer 1

Call asMap() on the MultiMap first. This converts the MultiMap to a standard Map where each value is a Collection.

In your example, the type of the resulting Map is Map<String, Collection<Object>>. Gson should be able to serialise this correctly.

Answer 2

You need to write a JsonAdapter or both JsonDeserializer and JsonSerializer. It's rather terrible, but I wanted to try.

Basically, you delegate everything to a Map<String, Collection<V>>.

static class MultimapAdapter implements JsonDeserializer<Multimap<String, ?>>, JsonSerializer<Multimap<String, ?>> {
    @Override public Multimap<String, ?> deserialize(JsonElement json, Type type,
            JsonDeserializationContext context) throws JsonParseException {
        final HashMultimap<String, Object> result = HashMultimap.create();
        final Map<String, Collection<?>> map = context.deserialize(json, multimapTypeToMapType(type));
        for (final Map.Entry<String, ?> e : map.entrySet()) {
            final Collection<?> value = (Collection<?>) e.getValue();
            result.putAll(e.getKey(), value);
        }
        return result;
    }

    @Override public JsonElement serialize(Multimap<String, ?> src, Type type, JsonSerializationContext context) {
        final Map<?, ?> map = src.asMap();
        return context.serialize(map);
    }

    private <V> Type multimapTypeToMapType(Type type) {
        final Type[] typeArguments = ((ParameterizedType) type).getActualTypeArguments();
        assert typeArguments.length == 2;
        @SuppressWarnings("unchecked")
        final TypeToken<Map<String, Collection<V>>> mapTypeToken = new TypeToken<Map<String, Collection<V>>>() {}
        .where(new TypeParameter<V>() {}, (TypeToken<V>) TypeToken.of(typeArguments[1]));
        return mapTypeToken.getType();
    }
}

The full code including a test can be found here.