Using Gson with Interface Types

Polymorphic mapping of the type described is not available in Gson without some level of custom coding. There is an extension type adapter available as an extra that provides a bulk of the functionality you are looking for, with the caveat that the polymorphic sub-types need to be declared to the adapter ahead of time. Here is an example of its use:

public interface Response {}

public interface Request {
    public Response process();
}

public class LoginRequest implements Request {
    private String userName;
    private String password;

    // Constructors, getters/setters, overrides
}

public class PingRequest implements Request {
    private String host;
    private Integer attempts;

    // Constructors, getters/setters, overrides
}

public class RequestTest {

    @Test
    public void testPolymorphicSerializeDeserializeWithGSON() throws Exception {
        final TypeToken<List<Request>> requestListTypeToken = new TypeToken<List<Request>>() {
        };

        final RuntimeTypeAdapterFactory<Request> typeFactory = RuntimeTypeAdapterFactory
                .of(Request.class, "type")
                .registerSubtype(LoginRequest.class)
                .registerSubtype(PingRequest.class);

        final Gson gson = new GsonBuilder().registerTypeAdapterFactory(
                typeFactory).create();

        final List<Request> requestList = Arrays.asList(new LoginRequest(
                "bob.villa", "passw0rd"), new LoginRequest("nantucket.jones",
                "crabdip"), new PingRequest("example.com", 5));

        final String serialized = gson.toJson(requestList,
                requestListTypeToken.getType());
        System.out.println("Original List: " + requestList);
        System.out.println("Serialized JSON: " + serialized);

        final List<Request> deserializedRequestList = gson.fromJson(serialized,
                requestListTypeToken.getType());

        System.out.println("Deserialized list: " + deserializedRequestList);
    }
}

Note that you don’t actually need to define the type property on the individual Java objects – it exists only in the JSON.