Using Gson and Abstract Classes

abstract-class, gson, java, object

Solution

I finally solved it!

    // GSON

    GsonBuilder gsonBilder = new GsonBuilder();
    gsonBilder.registerTypeAdapter(Conteudo.class, new InterfaceAdapter<Conteudo>());
    gsonBilder.setPrettyPrinting();

    Gson gson =gsonBilder.create();

    String str2send = gson.toJson(message);

    Mensagem msg_recv = gson.fromJson(str2send,Mensagem.class);

Note that: "registerTypeAdapter(AbstractClass.class, new InterfaceAdapter());"

by AbstractClass.class i mean the class that you are implementing in my case it was Conteúdo that could be ConteudoTweet or ConteudoUserSystem and so on...

The implementation of InterfaceAdapter is :

import java.lang.reflect.Type;

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;

public class InterfaceAdapter<T>
    implements JsonSerializer<T>, JsonDeserializer<T> {

    @Override
    public final JsonElement serialize(final T object, final Type interfaceType, final JsonSerializationContext context) 
    {
        final JsonObject member = new JsonObject();

        member.addProperty("type", object.getClass().getName());

        member.add("data", context.serialize(object));

        return member;
    }

    @Override
    public final T deserialize(final JsonElement elem, final Type interfaceType, final JsonDeserializationContext context) 
            throws JsonParseException 
    {
        final JsonObject member = (JsonObject) elem;
        final JsonElement typeString = get(member, "type");
        final JsonElement data = get(member, "data");
        final Type actualType = typeForName(typeString);

        return context.deserialize(data, actualType);
    }

    private Type typeForName(final JsonElement typeElem) 
    {
        try 
        {
            return Class.forName(typeElem.getAsString());
        } 
        catch (ClassNotFoundException e) 
        {
            throw new JsonParseException(e);
        }
    }

    private JsonElement get(final JsonObject wrapper, final String memberName) 
    {
        final JsonElement elem = wrapper.get(memberName);

        if (elem == null) 
        {
            throw new JsonParseException(
                "no '" + memberName + "' member found in json file.");
        }
        return elem;
    }

}

And this InterfaceAdapter is generic so it should work in general...

That's it!

Problem

I'm trying to exchange messages between a client and a server using GSON. The problem is the following: I have this structure: ``` public class Message { private TypeOfContent type; // It's a enum private Content content; .... } ``` Then the object content can be a various set of Classes. I found 2 tutorials here and here, but none of them solves the problem. Edit1: The class Message is this one: ``` public class Mensagem { private TipoMensagem type; private Conteudo conteudo; private Cliente autor; private Cliente destino; // null -> to all(broadcast) } ``` And Content is this one: ``` public class Conteudo { protected TipoConteudo typeConteudo; protected String texto; protected Posicao posicao; public Conteudo(TipoConteudo typeConteudo, String texto, Posicao posicao) { this.texto = texto; this.posicao = posicao; this.typeConteudo = typeConteudo; } } ``` And an example of a extend class from conteudo is this one: ``` public class ConteudoTweet extends Conteudo { protected String pathImagem; public ConteudoTweet(TipoConteudo typeConteudo, String tweet, Posicao location, String picturePath) { super(typeConteudo,tweet, location); this.pathImagem = picturePath; } } ``` Finally what I do is like : "String strObject = new Gson().toJson(mensage);" which works but on deserialization it doesn't because it assumes always that it is from Content class

Original source

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