How can I serialize polymorphic objects with Gson and ensure all fields are included in the JSON output?

Polymorphic Object Serialization with Gson

Gson provides comprehensive support for serializing polymorphic objects in a JSON format. Polymorphism arises when objects have a shared base class but different implementations. To effectively serialize such objects, Gson offers a feature known as RuntimeTypeAdapterFactory.

Consider the following scenario: you have a base class ObixBaseObj and multiple child classes inheriting from it, such as ObixOp. When serializing an object of type ObixBaseObj that contains a child object, Gson's default behavior is to only serialize the fields defined in the base class. This can result in the omission of specific fields from the child class.

To address this, you can utilize RuntimeTypeAdapterFactory. It enables you to register different subtypes under a single type adapter. By doing so, Gson can effectively serialize the fields specific to each subtype, providing a complete representation of the polymorphic object.

<code class="java">RuntimeTypeAdapterFactory<ObixBaseObj> adapter = 
                    RuntimeTypeAdapterFactory
                   .of(ObixBaseObj.class)
                   .registerSubtype(ObixBaseObj.class)
                   .registerSubtype(ObixOp.class);</code>

The above code registers both ObixBaseObj and ObixOp subtypes with the adapter. It can then be integrated into Gson's configuration to enable polymorphic serialization:

<code class="java">Gson gson2=new GsonBuilder().setPrettyPrinting().registerTypeAdapterFactory(adapter).create();</code>

By using this modified Gson instance, polymorphic objects can be effectively serialized and deserialized, accurately preserving the fields inherited from the base class and the specific child class.

Alternatively, you can implement a more robust and scalable solution by leveraging the GsonUtils class:

<code class="java">public class GsonUtils {

    private static final GsonBuilder gsonBuilder = new GsonBuilder()
            .setPrettyPrinting();

    public static void registerType(
            RuntimeTypeAdapterFactory<?> adapter) {
        gsonBuilder.registerTypeAdapterFactory(adapter);
    }

    public static Gson getGson() {
        return gsonBuilder.create();
    }
}</code>

Within ObixBaseObj and its child classes, you can utilize GsonUtils to ensure that subtype registration is performed automatically:

<code class="java">public class ObixBaseObj {
    private static final RuntimeTypeAdapterFactory<ObixBaseObj> adapter = 
            RuntimeTypeAdapterFactory.of(ObixBaseObj.class);

    private static final HashSet<Class<?>> registeredClasses= new HashSet<Class<?>>();

    static {
        GsonUtils.registerType(adapter);
    }

    private synchronized void registerClass() {
        if (!registeredClasses.contains(this.getClass())) {
            registeredClasses.add(this.getClass());
            adapter.registerSubtype(this.getClass());
        }
    }
    public ObixBaseObj() {
        registerClass();
        obix = "obj";
    }
}</code>

With this approach, whenever an instance of a base or child class is created, it will automatically register its subtype with the RuntimeTypeAdapterFactory, ensuring comprehensive serialization.

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