How do you serialize polymorphic object lists with Gson using RuntimeTypeAdapterFactory?

Gson Serialization of Polymorphic Object Lists

Gson provides a solution to serialize polymorphic objects using RuntimeTypeAdapterFactory. This class automatically handles the serialization of inherited members, eliminating the need for writing custom serializers.

Implementation

To use RuntimeTypeAdapterFactory, follow these steps:

1. Create an instance of RuntimeTypeAdapterFactory for the base class ObixBaseObj.
2. Register all subclasses of ObixBaseObj using registerSubtype().
3. Provide the RuntimeTypeAdapterFactory to the GsonBuilder using registerTypeAdapterFactory().

Example

<code class="java">ObixBaseObj lobbyObj = new ObixBaseObj();
lobbyObj.setIs("obix:Lobby");

ObixOp batchOp = new ObixOp();
batchOp.setName("batch");
batchOp.setIn("obix:BatchIn");
batchOp.setOut("obix:BatchOut");

lobbyObj.addChild(batchOp);

RuntimeTypeAdapterFactory<ObixBaseObj> adapter = 
                    RuntimeTypeAdapterFactory
                   .of(ObixBaseObj.class)
                   .registerSubtype(ObixBaseObj.class)
                   .registerSubtype(ObixOp.class);

Gson gson2=new GsonBuilder().setPrettyPrinting().registerTypeAdapterFactory(adapter).create();
System.out.println(gson2.toJson(lobbyObj));</code>

Output

<code class="json">{
  "type": "ObixBaseObj",
  "obix": "obj",
  "is": "obix:Lobby",
  "children": [
    {
      "type": "ObixOp",
      "in": "obix:BatchIn",
      "out": "obix:BatchOut",
      "obix": "op",
      "name": "batch",
      "children": []
    }
  ]
}</code>

Advanced Use Case

To handle a large number of subclasses, create a utility class like GsonUtils to manage registration and provide a centralized Gson instance.

<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();
    }
}

public class ObixBaseObj {

    private static final RuntimeTypeAdapterFactory<ObixBaseObj> adapter = 
            RuntimeTypeAdapterFactory.of(ObixBaseObj.class);

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

    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";
    }
}

public class ObixOp extends ObixBaseObj {

    private String in;
    private String out;

    public ObixOp() {
        super();
        obix = "op";
    }

    public ObixOp(String in, String out) {
        super();
        obix = "op";
        this.in = in;
        this.out = out;
    }
}</code>

With this approach, all inherited members of polymorphic objects will be automatically serialized and deserialized, providing a convenient solution for handling complex inheritance hierarchies.

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