Trying to serialize/deserialize polymorphic objects

Question

I am trying to serialize/deserialize polymorphic objects using gson library. I have tried like this:

A.java

package testing;

public abstract class A {
    private final String id;
    private final String name;

    protected A(String id, String name) {
        this.id = id;
        this.name = name;
    }
}

B.java

package testing;

public class B extends A {
    public B(String id, String name) {
        super(id, name);
    }
}

C.java

package testing;

public class C extends A {
    private String c;

    public C(String id, String name, String cname) {
        super(id, name);
        c = cname;
    }

    public String getC() {
        return c;
    }

    public void setC(String c) {
        this.c = c;
    }
}

Container.java

package testing;

import java.util.ArrayList;
import java.util.List;

public class Container {
    private List<A> list;

    public List<A> getList() {
        return list;
    }


    public Container() {
        list = new ArrayList<>();
        list.add(new B("b_id1", "b_name1"));
        list.add(new C("c_id1", "c_name1", "c_name2"));
    }
}

AListContainer.java

package testing;

import com.google.gson.Gson;
import com.google.gson.GsonBuilder;
import com.google.gson.JsonArray;
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.JsonSerializationContext;
import com.google.gson.JsonSerializer;

import java.lang.reflect.Type;
import java.util.ArrayList;
import java.util.List;

public class AListAdapter implements JsonSerializer<List<A>>, JsonDeserializer<List<A>>
{
    private static final String CLASS_NAME = "CLASSNAME";
    private static final String INSTANCE = "INSTANCE";
    private Gson gson = new GsonBuilder()
            .registerTypeAdapter(B.class, new BAdapter())
            .registerTypeAdapter(C.class, new CAdapter())
            .create();

    @Override
    public JsonElement serialize(List<A> listA, Type typeOfSrc,
                                 JsonSerializationContext context) {
        JsonArray array = new JsonArray();
        for (A a : listA) {
            JsonObject obj = new JsonObject();
            String className = a.getClass().getCanonicalName();
            obj.add(CLASS_NAME, context.serialize(className));
            JsonElement elem = gson.toJsonTree(a);
            obj.add(INSTANCE, elem);
            array.add(obj);
        }
        return array;
    }

    @Override
    public List<A> deserialize(JsonElement json,
                                             Type typeOfT,
                                             JsonDeserializationContext context) throws JsonParseException {
        List<A> retVal = new ArrayList();
        JsonArray elems = json.getAsJsonArray();
        for (int i = 0; i < elems.size(); i++) {
            JsonElement oneElem = elems.get(i);
            JsonObject oneObj = oneElem.getAsJsonObject();
            JsonElement elem = oneObj.get(CLASS_NAME);
            String className = elem.getAsString();
            JsonElement metricViolation = oneObj.get(INSTANCE);
            try {
                Class t = Class.forName(className);
                A violation = (A) gson.fromJson(metricViolation, t);
                retVal.add(violation);
            } catch (ClassNotFoundException ex) {
                throw new JsonParseException(ex);
            }
        }
        return retVal;
    }

}

Main.java

package testing;

import com.google.gson.Gson;
import com.google.gson.GsonBuilder;
import com.google.gson.reflect.TypeToken;

import java.util.List;

public class Main {

    public static void main(String [] args) {

        Container d = new Container();
        List<A> listA = d.getList();

        Gson gson =
            new GsonBuilder()
                    .registerTypeAdapter((new TypeToken<List<A>>() {}).getType(), new AListAdapter())
                    .serializeSpecialFloatingPointValues()
                    .create();

        String json = gson.toJson(listA);
        List<A> resultA = gson.fromJson(json, List.class);
        System.out.println(resultA);
    }
}

gson is 2.8.5 version:

<dependency>
    <groupId>com.google.code.gson</groupId>
    <artifactId>gson</artifactId>
    <version>2.8.5</version>
</dependency>

It seems that testing.AListAdapter#serialize and testing.AListAdapter#deserialize are not called. Can you help to understand why?