OptionalAwareDecoder in feign core? #294

/**
 * Decorates a Feign {@link Decoder} such that it returns {@link Optional#absent} when observing an HTTP 404 error code
 * for a method with {@link Type} {@link Optional}. Propagates the exception returned by the given {@link ErrorDecoder}
 * in case the response is 404 for a non-Optional method.
 */
public final class OptionalAwareDecoder implements Decoder {

    private final Decoder delegate;
    private final ErrorDecoder errorDecoder;

    public OptionalAwareDecoder(Decoder delegate, ErrorDecoder errorDecoder) {
        this.delegate = delegate;
        this.errorDecoder = errorDecoder;
    }

    @Override
    public Object decode(Response response, Type type) throws IOException, FeignException {
        if (response.status() == 404) {
            if (Types.getRawType(type).equals(Optional.class)) {
                return Optional.absent();
            } else {
                throw Throwables.propagate(errorDecoder.decode(null, response));
            }
        } else {
            return delegate.decode(response, type);
        }
    }
}