First class support for polymorphism (subclasses!)

[GoogleCodeExporter]

GSON always operates on the static type of an object. It has no mechanism to 
operate on the object's runtime type.

It would be handy if GsonBuilder permitted a way to specify the known 
subclasses of a type. Then at serialization/deserialization these fields could 
be included:

  public void testPolymorphism() {
    Rectangle r = new Rectangle();
    r.width = 5;
    r.height = 7;
    Circle c = new Circle();
    c.radius = 3;

    List<Shape> shapes = new ArrayList<Shape>();
    shapes.add(r);
    shapes.add(c);

    Gson gson = new GsonBuilder()
        .create();

    String json = gson.toJson(shapes, new TypeToken<List<Shape>>() {}.getType());
    assertEquals("[{\"width\":5,\"height\":7},{\"radius\":3}]", json);
  }

  static class Shape {}

  static class Rectangle extends Shape {
    int width;
    int height;
  }

  static class Circle extends Shape {
    int radius;
  }

It would be extra awesome if it could use the set of fields in a stream to 
infer the type to instantiate. In the example above, it could use that the 
'radius' field as evidence that the runtime type should be a Circle. Or perhaps 
this could be user-configured too, such as this:
  new GsonBuilder()
    .runtimeType(Shape.class, Circle.class, "radius")
    .runtimeType(Shape.class, Rectangle.class, "width", "height")
    .create();

Original issue reported on code.google.com by limpbizkit on 28 Aug 2010 at 6:00

[kiptoomm]

Was this ever released? I'm using Gson 2.3.1 and don't see it..

[Cy4n1d3]

I don't think so.
Guess I'll either have to write some generic converters or switch back to Jackson, as I rely on loads of Collections filled with abstract classes for a project...

[JonasVautherin]

GsonFire does something that could help (see "Type Selectors" here)

[Cy4n1d3]

Didn't know about that, thanks for the heads up!
Might come in handy some day 👍

[subha-sam]

Hey guys ,
I have a similar usecase as this
Class A{
// fields
}

Class B extends A{
// fields
}

Class C extends A{
// fields
}

Class D extends A{
// fields
}

Class Test extends A{
public List contents = new ArrayList(); // it may contain A, B, C etc.
private boolean isHeader = false;
private String myTestString = "test";
}
I am trying to serialize "contents " collection using gson libarary.

can someone tell me whats the solution for this. I found using RuntimeTypeAdaptorFactory would solve it but, I using gson 2..5, in that version I dont see this class.

Struck with this issues for a past three days. writing custom serializations code for my usecase runs for pages.kindly help. whats the work around that the current gson library is having.

[inder123]

Copy over RuntimeTypeAdapterFactory code. It is meant to be used as copied source-code.

[mgerhardy]

Add pull request #1055 that shows a few issues regarding Collection. I would be glad about feedback.

My problem is that I need to deserialize Collections at runtime - thus I can't use the new TypeToken<List>() {}.getType() syntax. I only have a Class instance. Do I have to register some special type of Collection adapter or JsonDeserializer? I tried to add a deserializer but this wasn't possible because I lack needed types for fromJson calls.

	private class RunTimeTypeCollectionAdapter implements JsonDeserializer<Collection> {
		@Override
		@SuppressWarnings({ "rawtypes", "unchecked" })
		public Collection deserialize(final JsonElement json, final Type typeOfT, final JsonDeserializationContext context)
				throws JsonParseException {
			if (json == null) {
				return null;
			}
			final TypeToken<?> typeToken = TypeToken.get(typeOfT);
			try {
				Class<?> collectionClass = typeToken.getRawType();
				if (collectionClass == List.class) {
					collectionClass = ArrayList.class;
				} else if (collectionClass == Set.class) {
					collectionClass = HashSet.class;
				} else if (collectionClass == Map.class) {
					collectionClass = HashMap.class;
				}
				final Collection collection = (Collection) Class.forName(collectionClass.getTypeName()).newInstance();
				final Class<?> rawType = typeToken.getRawType();
				if (json.isJsonArray()) {
					final JsonArray array = json.getAsJsonArray();
					for (int i = 0; i < array.size(); ++i) {
						final JsonElement element = array.get(i);
						final Object deserialize = context.deserialize(element, rawType);
						collection.add(deserialize);
					}
				} else {
					// final Object deserialize = context.deserialize(json,
					// rawType);
					// collection.add(deserialize);
				}
				return collection;
			} catch (final Exception e) {
				logger.error(e.getMessage(), e);
			}
			return null;
		}
	}

As you can see in this example, I am getting to a point where the deserializer doesn't get a json array - but MyClass (from Collection - this is a little bit weird for me - because I get those elements one by one, not via array.