Gson deserialize interface to its Class implementation

Question

I am using Retrofit 2.1.0 with converter-gson:2.1.0 and separately gson:2.6.2 in order to customize the serialization/deserialization. The problem is that my POJOs should be hidden behind interfaces and I want to tell Gson which class should be the deserialized interface. And after the deserialization/ serialization Retrofit should be able to return the interface. It would be good if I can take advantage of Generics and easily create a way to tell Gson or Retrofit to serialize/deserialize FooInterface to FooClass.

Answer 1

I am assuming that you want to create a single deserializer for all of your interfaces and their respective implementations. Follow these steps please:

1. Create a base interface that will be extended by your other app interfaces. It is required to create a single deserializer for all of your interfaces and implementation classes.

public interface Convertable {
     String getClassName();
}

2. Create your feature interface and implementation class. As an example, lets name them FooInterface and FooClass. FooInterface should extend Convertable interface.

FooInterface

public interface FooInterface extends Convertable {

}

FooClass

public class FooClass implements FooInterface {

    // DISCRIMINATOR FIELD
    private final String className;

    private String field1;

    private String field2;

    public FooClass() {
        this.className = getClass().getName();
    }

    public String getClassName() {
        return className;
    }

    public String getField1() {
        return field1;
    }

    public void setField1(String field1) {
        this.field1 = field1;
    }

    public String getField2() {
        return field2;
    }

    public void setField2(String field2) {
        this.field2 = field2;
    }

}

Note that the value returned by getClassName() is used as discriminator field that will be used in Gson Deserializer (next step) to initialize returnable instance. I am assuming that your serializer and deserializer class will reside in the same package even if they are in different client and server applications. If not, then you will need to change getClassInstance() implementation, but would be pretty simple to do so.

3. Implement a custom Gson Serializer for all of your application

import com.google.gson.JsonDeserializationContext;
import com.google.gson.JsonDeserializer;
import com.google.gson.JsonElement;
import com.google.gson.JsonObject;
import com.google.gson.JsonParseException;
import com.google.gson.JsonPrimitive;

public class ConvertableDeserializer<T extends Convertable> implements JsonDeserializer<T> {

    private static final String CLASSNAME = "className";

    public T deserialize(final JsonElement jsonElement, final Type type,
                         final JsonDeserializationContext deserializationContext
                        ) throws JsonParseException {

        final JsonObject jsonObject = jsonElement.getAsJsonObject();
        final JsonPrimitive prim = (JsonPrimitive) jsonObject.get(CLASSNAME);
        final String className = prim.getAsString();
        final Class<T> clazz = getClassInstance(className);
        return deserializationContext.deserialize(jsonObject, clazz);
    }

    @SuppressWarnings("unchecked")
    public Class<T> getClassInstance(String className) {
        try {
            return (Class<T>) Class.forName(className);
        } catch (ClassNotFoundException cnfe) {
            throw new JsonParseException(cnfe.getMessage());
        }
    }

}

4. Register Deserializer with Gson and initialize retrofit

 private static GsonConverterFactory buildGsonConverter() {

        final GsonBuilder builder = new GsonBuilder();

        // Adding custom deserializers
        builder.registerTypeAdapter(FooInterface.class, 
                                    new ConvertableDeserializer<FooInterface>());
        final Gson gson = builder.create();

        return GsonConverterFactory.create(myGson);
    }


    public void initRetrofit() {
        Retrofit retrofit = new Retrofit.Builder()
                .baseUrl("REST_ENDPOINT")
                .addConverterFactory(buildGsonConverter())
                .client(httpClient)
                .build();
    }

You may register the adapter for all of your implementations if you want, using:

builder.registerTypeAdapter(Convertable.class, new ConvertableDeserializer<Convertable>());