使用Gson反序列化ImmutableList

Question

I'm using quite a few immutable collections and I'm curious how to deserialize them using Gson. As nobody answered and I've found the solution myself, I'm simplifying the question and presenting my own answer.

I had two problems:

• How to write a single Deserializer working for all ImmutableList<XXX> ?
• How to register it for all ImmutableList<XXX> ?

Answer 1

Update: There's https://github.com/acebaggins/gson-serializers which covers many guava collections:

• ImmutableList
• ImmutableSet
• ImmutableSortedSet
• ImmutableMap
• ImmutableSortedMap

---

How to write a single Deserializer working for all ImmutableList?

The idea is simple, transform the passed Type representing an ImmutableList<T> into a Type representing List<T> , use the build-in Gson 's capability to create a List and convert it to an ImmutableList .

class MyJsonDeserializer implements JsonDeserializer<ImmutableList<?>> {
    @Override
    public ImmutableList<?> deserialize(JsonElement json, Type type, JsonDeserializationContext context) throws JsonParseException {
        final Type type2 = ParameterizedTypeImpl.make(List.class, ((ParameterizedType) type).getActualTypeArguments(), null);
        final List<?> list = context.deserialize(json, type2);
        return ImmutableList.copyOf(list);
    }
}

There are multiple ParameterizedTypeImpl classes in Java libraries I use, but none of them intended for public usage. I tested it with sun.reflect.generics.reflectiveObjects.ParameterizedTypeImpl .

How to register it for all ImmutableList?

That part is trivial, the first argument to register is java.lang.reflect.Type which mislead me to using ParameterizedType , where simply using Class does the job:

final Gson gson = new GsonBuilder()
    .registerTypeAdapter(ImmutableList.class, myJsonDeserializer)
    .create();

Answer 2

One more implementation without ParameterizedTypeImpl

@Override
public ImmutableList<?> deserialize(final JsonElement json, final Type type, final JsonDeserializationContext context) throws JsonParseException {
    @SuppressWarnings("unchecked")
    final TypeToken<ImmutableList<?>> immutableListToken = (TypeToken<ImmutableList<?>>) TypeToken.of(type);
    final TypeToken<? super ImmutableList<?>> listToken = immutableListToken.getSupertype(List.class);
    final List<?> list = context.deserialize(json, listToken.getType());
    return ImmutableList.copyOf(list);
}

Answer 3

@maaartinus already covered the second question, so I'll post a complementary Guava-based solution to the first question which doesn't require ParametrizedTypeImpl

public final class ImmutableListDeserializer implements JsonDeserializer<ImmutableList<?>> {
  @Override
  public ImmutableList<?> deserialize(final JsonElement json, final Type type,
                                      final JsonDeserializationContext context)
      throws JsonParseException {
    final Type[] typeArguments = ((ParameterizedType) type).getActualTypeArguments();
    final Type parameterizedType = listOf(typeArguments[0]).getType();
    final List<?> list = context.deserialize(json, parameterizedType);
    return ImmutableList.copyOf(list);
  }

  private static <E> TypeToken<List<E>> listOf(final Type arg) {
    return new TypeToken<List<E>>() {}
        .where(new TypeParameter<E>() {}, (TypeToken<E>) TypeToken.of(arg));   
  }
}