Deserializing an abstract class in Gson

I have a tree object in JSON format I'm trying to deserialize with Gson. Each node contains its child nodes as fields of object type Node. Node is an interface, which has several concrete class implementations. During the deserialization process, how can I communicate to Gson which concrete class to implement when deserializing the node, if I do not know a priori which type the node belongs to? Each Node has a member field specifying the type. Is there a way to access the field when the object is in serialized form, and somehow communicate the type to Gson?

Thanks!

Solution 1:

I'd suggest adding a custom JsonDeserializer for Nodes:

Gson gson = new GsonBuilder()
    .registerTypeAdapter(Node.class, new NodeDeserializer())
    .create();

You will be able to access the JsonElement representing the node in the deserializer's method, convert that to a JsonObject, and retrieve the field that specifies the type. You can then create an instance of the correct type of Node based on that.

Solution 2:

You will need to register both JSONSerializer and JSONDeserializer. Also you can implement a generic adapter for all your interfaces in the following way:

• During Serialization : Add a META-info of the actual impl class type.
• During DeSerialization : Retrieve that meta info and call the JSONDeserailize of that class

Here is the implementation that I have used for myself and works fine.

public class PropertyBasedInterfaceMarshal implements
        JsonSerializer<Object>, JsonDeserializer<Object> {

    private static final String CLASS_META_KEY = "CLASS_META_KEY";

    @Override
    public Object deserialize(JsonElement jsonElement, Type type,
            JsonDeserializationContext jsonDeserializationContext)
            throws JsonParseException {
        JsonObject jsonObj = jsonElement.getAsJsonObject();
        String className = jsonObj.get(CLASS_META_KEY).getAsString();
        try {
            Class<?> clz = Class.forName(className);
            return jsonDeserializationContext.deserialize(jsonElement, clz);
        } catch (ClassNotFoundException e) {
            throw new JsonParseException(e);
        }
    }

    @Override
    public JsonElement serialize(Object object, Type type,
            JsonSerializationContext jsonSerializationContext) {
        JsonElement jsonEle = jsonSerializationContext.serialize(object, object.getClass());
        jsonEle.getAsJsonObject().addProperty(CLASS_META_KEY,
                object.getClass().getCanonicalName());
        return jsonEle;
    }

}

Then you could register this adapter for all your interfaces as follows

Gson gson = new GsonBuilder()
        .registerTypeAdapter(IInterfaceOne.class,
                new PropertyBasedInterfaceMarshal())
        .registerTypeAdapter(IInterfaceTwo.class,
                new PropertyBasedInterfaceMarshal()).create();