I'm trying to setup an HTTP cache using Retrofit (2.1.0) and OkHttp (3.3.1). I have seen many posts related to this topic, but none of them helped.
I wrote some unit tests to see how the cache works. It works just fine, but once integrated in my app, the magic ends. I will first show you my implementation and then explain some of my investigation.
First, here is my Retrofit instantiation :
OkHttpClient.Builder httpBuilder = new OkHttpClient.Builder();
HttpLoggingInterceptor loggingInterceptor = new HttpLoggingInterceptor();
interceptor.setLevel(HttpLoggingInterceptor.Level.HEADERS);
OkHttpClient client = httpBuilder
.addNetworkInterceptor(INTERCEPTOR_RESPONSE_SET_CACHE)
.addNetworkInterceptor(INTERCEPTOR_REQUEST_ADD_CHECKSUM)
.addInterceptor(loggingInterceptor)
.cache(cacheHttpClient).build();
Retrofit retrofit = new Retrofit.Builder()
.addConverterFactory(GsonConverterFactory.create())
.client(client)
.baseUrl(BASE_URL)
.build();
Here is the interceptor adding a header to set cache control:
private final Interceptor INTERCEPTOR_RESPONSE_SET_CACHE = new Interceptor() {
@Override
public Response intercept(Chain chain) throws IOException {
Response response = chain.proceed(chain.request());
response = response.newBuilder()
.header("Cache-Control", "max-age=600") //+ Integer.toString(3600 * 5)
.build();
return response;
}
};
The last interceptor adds 2 URL parameters:
private static final Interceptor INTERCEPTOR_REQUEST_ADD_CHECKSUM = new Interceptor() {
@Override
public Response intercept(Interceptor.Chain chain) throws IOException {
HttpUrl url = chain.request().url();
url = url.newBuilder().addQueryParameter("rd", "random1").addQueryParameter("chk","check1").build();
Request request = chain.request().newBuilder().url(url).build();
return chain.proceed(request);
}
};
Finally, the single method of my service :
@Headers("Cache-Control: public, max-stale=500")
@GET("/get_data")
Call<DataResponse> getData(@Query("year") int year, @Query("month") int month, @Query("day") int day);
About my investigation, I setup an interceptor logger (app side, not network) to see what is happening. I can see lines such as "Cache-Control: public, max-stale=500" in my logs. This means (at least to me) that the header should give an opportunity to the OkHttp client to check the cache.
The cache itself seems to be correctly initialised. When I create it, I force the initialisation and log all the urls present in the cache. Here is how it is implemented:
File httpCacheDirectory = new File(getCacheDir(), "responses");
httpCacheDirectory.getParentFile().mkdirs();
int cacheSize = 10 * 1024 * 1024; // 10 MiB
Cache cache = new Cache(httpCacheDirectory, cacheSize);
try {
cache.initialize();
Iterator<String> iterator = cache.urls();
Log.i(TAG, "URLs in cacheHttpClient : ");
while (iterator.hasNext()) {
Log.i(TAG, iterator.next());
}
} catch (IOException e) {
e.printStackTrace();
Log.i(TAG, "CACHE NOT INIT");
}
When I launch my app with Wifi available, I get the expected responses. Then I kill my app, disable Wifi and relaunch the app. I expect the cache to serve data at this moment. But it fails and I can only see OkHttp printed lines in logs :
HTTP FAILED: java.net.UnknownHostException: Unable to resolve host "my-domain.com": No address associated with hostname
Last thing, in RFC 2616, one can read :
max-stale : Indicates that the client is willing to accept a response that has exceeded its expiration time. If max-stale is assigned a value, then the client is willing to accept a response that has exceeded its expiration time by no more than the specified number of seconds. If no value is assigned to max-stale, then the client is willing to accept a stale response of any age.
When I don't specify an value, it actually works (I get a response even when the Wifi is down). For now this is the only way I found to make it "work". So maybe I just misunderstand the cache-control directive !?
At this point I'm really confused. I really would like to be able to use OkHttp cache system, but somehow I'm missing something.
Thank you for reading all that text !
