gson: Treat null as empty String

In the actual version of gson you can do that:

    Object instance = c.getConstructor().newInstance();
    GsonBuilder gb = new GsonBuilder(); 
    gb.serializeNulls();
    Gson gson = gb.create(); 
    String stringJson = gson.toJson(instance);

The above answer works okay for serialisation, but on deserialisation, if there is a field with null value, Gson will skip it and won't enter the deserialize method of the type adapter, therefore you need to register a TypeAdapterFactory and return type adapter in it.

Gson gson = GsonBuilder().registerTypeAdapterFactory(new NullStringToEmptyAdapterFactory()).create();


public static class NullStringToEmptyAdapterFactory<T> implements TypeAdapterFactory {
    public <T> TypeAdapter<T> create(Gson gson, TypeToken<T> type) {

        Class<T> rawType = (Class<T>) type.getRawType();
        if (rawType != String.class) {
            return null;
        }
        return (TypeAdapter<T>) new StringAdapter();
    }
}

public static class StringAdapter extends TypeAdapter<String> {
    public String read(JsonReader reader) throws IOException {
        if (reader.peek() == JsonToken.NULL) {
            reader.nextNull();
            return "";
        }
        return reader.nextString();
    }
    public void write(JsonWriter writer, String value) throws IOException {
        if (value == null) {
            writer.nullValue();
            return;
        }
        writer.value(value);
    }
}