改造：缓存每个请求

Question

我是Android的新手，我想缓存每个服务器的响应。我正在使用改造，而我正在使用“ ServiceGenerator”。我有此错误信息： “ ....'java.lang.String okhttp3.ResponseBody.string（）'在空对象上引用”当我的设备脱机时。而且我还没有一个HTTP缓存目录。请帮忙！这是我的代码：

private static Retrofit.Builder builder =
        new Retrofit.Builder()
                .baseUrl(BASE_URL);

private static Retrofit retrofit = builder.build();

private static OkHttpClient.Builder httpClient =
        new OkHttpClient().newBuilder();

public static <S> S createService(Class<S> serviceClass, final Context context) {
    // adding cache
    Cache cache = new Cache(context.getCacheDir(), cacheSize);
    httpClient.cache(cache);
    // adding interceptor
    httpClient.addInterceptor(provideInterceptor(context));
    builder.client(httpClient.build());
    retrofit = builder.build();
    builder.client(httpClient.build());
    return retrofit.create(serviceClass);
}

// add caching logic to retrofit
static Interceptor provideInterceptor(final Context context){
    return new Interceptor() {
        @Override
        public Response intercept(Chain chain) throws IOException {
            Request request = chain.request();
            if(isThereInternet(context)){
                // if there is internet connection get the data just when it is younger than 12 Hours
                request = request.newBuilder().header("Cache-Control", "public, max-age=" + (12 * 60 *60)).build();
                Log.e("Interceptor " , "there is internet");
            } else {
                // load from cache not older than 7 days
                request = request.newBuilder().header("Cache-Control", "public, only-if-cached, max-stale=" + 60 * 60 * 24 * 7).build();
            }
            return chain.proceed(request);
        }
    };

}