Change default enum serialization & deserialization in gson

Question

I'm using Gson in a slightly "different" way and I'm wondering if the following is possible...

I'd like to change the default serialization/deserialization format for enums so that it uses fully-qualified class names, but maintain support for the @SerializedName annotation on said enums. Basically, given the following enum...

package com.example;
public class MyClass {
    public enum MyEnum {

        OPTION_ONE, 

        OPTION_TWO, 

        @SerializedName("someSpecialName")
        OPTION_THREE
    }
}

I'd like the following to be true...

gson.toJson(MyEnum.OPTION_ONE) == "com.example.MyClass.MyEnum.OPTION_ONE"
&&
gson.toJson(MyEnum.OPTION_TWO) == "com.example.MyClass.MyEnum.OPTION_TWO"
&&
gson.toJson(MyEnum.OPTION_THREE) == "someSpecialName"

and vice-versa.

(for those curious, I'm trying to build a small lib that allows me to treat android's intent's actions as enums, so that I can write switch statements instead of a bunch of ugly if-elses + string compares, and I want to support the annotation so that I can also include custom pre-existing action strings like Intent.ACTION_VIEW, etc in the same enum).

So would anyone know if it's possible to register a type adapter that can fall back if the @SerializedName field is present? Would I just have to check for that annotation myself in my own TypeAdapter?

Thanks in advance.

Answer 1

I created pretty nice solution for this issue :

package your.package.name
import com.google.gson.Gson;
import com.google.gson.TypeAdapter;
import com.google.gson.TypeAdapterFactory;
import com.google.gson.reflect.TypeToken;
import com.google.gson.stream.JsonReader;
import com.google.gson.stream.JsonWriter;
import java.io.IOException;
import java.lang.reflect.Field;

public class EnumAdapterFactory implements TypeAdapterFactory {

    @Override
    public <T> TypeAdapter<T> create(final Gson gson, final TypeToken<T> type) {
        Class<? super T> rawType = type.getRawType();
        if (rawType.isEnum()) {
            return new EnumTypeAdapter<T>();
        }
        return null;
    }

    public class EnumTypeAdapter<T> extends TypeAdapter<T> {

        public void write(JsonWriter out, T value) throws IOException {
            if (value == null) {
                out.nullValue();
                return;
            }
            Enum<?> realEnums = Enum.valueOf(value.getClass().asSubclass(Enum.class), value.toString());
            Field[] enumFields = realEnums.getClass().getDeclaredFields();
            out.beginObject();
            out.name("name");
            out.value(realEnums.name());
            for (Field enumField : enumFields) {
                if (enumField.isEnumConstant() || enumField.getName().equals("$VALUES")) {
                    continue;
                }
                enumField.setAccessible(true);
                try {
                    out.name(enumField.getName());
                    out.value(enumField.get(realEnums).toString());
                } catch (Throwable th) {
                    out.value("");
                }
            }
            out.endObject();
        }

        public T read(JsonReader in) throws IOException {
            return null;
        }
    }
}

and of course :

new GsonBuilder().registerTypeAdapterFactory(new EnumAdapterFactory()).create();

Hope this helps !

Answer 2

Did some google-ing and found the source for Gson's EnumTypeAdapter and the related AdapterFactory here: https://code.google.com/p/google-gson/source/browse/trunk/gson/src/main/java/com/google/gson/internal/bind/TypeAdapters.java#717

From the looks of it, I would, in fact, have to check for the @SerializedName attribute manually, but it looks pretty simple to do. I'm planning on copying-over both the adapter and adapter factory (almost line-for-line) and modifying the default value of name (line 724) to include the full class name.

The resulting TypeAdapter would look something like this...

private static final class EnumTypeAdapter<T extends Enum<T>> extends TypeAdapter<T> {
    private final Map<String, T> nameToConstant = new HashMap<String, T>();
    private final Map<T, String> constantToName = new HashMap<T, String>();

    public EnumTypeAdapter(Class<T> classOfT) {
      try {
        String classPrefix = classOfT.getName() + ".";
        for (T constant : classOfT.getEnumConstants()) {
          String name = constant.name();
          SerializedName annotation = classOfT.getField(name).getAnnotation(SerializedName.class);
          if (annotation != null) {
            name = annotation.value();
          } else {
            name = classPrefix + name;
          }
          nameToConstant.put(name, constant);
          constantToName.put(constant, name);
        }
      } catch (NoSuchFieldException e) {
        throw new AssertionError();
      }
    }

    public T read(JsonReader in) throws IOException {
      if (in.peek() == JsonToken.NULL) {
        in.nextNull();
        return null;
      }
      return nameToConstant.get(in.nextString());
    }

    public void write(JsonWriter out, T value) throws IOException {
      out.value(value == null ? null : constantToName.get(value));
    }
}