OKHTTP3离线缓存

Question

I want to implement cache even if there is no internet connection(Offline) but still had no success, already look many example but still no luck

//FeedInterceptor Class 
public static Interceptor getOfflineInterceptor(final Context context){
    Interceptor interceptor = new Interceptor() {
        @Override
        public okhttp3.Response intercept(Chain chain) throws IOException {
            Request request = chain.request();
            if (!isNetworkAvailable(context)) {
                request = request.newBuilder()
                        .removeHeader("Pragma")
                        .header("Cache-Control", "public, only-if-cached")
                        .build();
            }
            return chain.proceed(request);
        }
    };

    return interceptor;
}


//OnCreate Activity
client = new OkHttpClient.Builder()
            .addNetworkInterceptor(FeedInterceptor.getOnlineInterceptor(this))
            .addInterceptor(FeedInterceptor.getOfflineInterceptor(this))
            .cache(cache)
            .build();

//After build Request
Response response = client.newCall(request).execute();
return response.body().string();

if offline the return comes as empty string. Is there something I miss or wrong?

Answer 1

In order to be able to reuse a cached response, it is necessary that the response itself declares itself to be "cacheable", by providing some headers (eg, Cache-Control: public or Cache-Control: max-age=3600 ). Did you check if your response contains such headers?

Also, consider using the built-in constant CacheControl.FORCE_CACHE as a value for your cacheControl setter .

Finally, be aware that (quoting from OkHttp documentation ):

Be warned: if you use FORCE_CACHE and the response requires the network, OkHttp will return a 504 Unsatisfiable Request response

EDIT:

Here's a full example.

final CacheControl cacheControl = new CacheControl.Builder()
        .maxAge(60, TimeUnit.MINUTES)
        .build();

// Interceptor to ask for cached only responses
Interceptor cacheResponseInterceptor = chain -> {
    Response response = chain.proceed(chain.request());
    return response.newBuilder()
            .header("Cache-Control", cacheControl.toString())
            .build();
};

// Interceptor to cache responses
Interceptor cacheRequestInterceptor = chain -> {
    Request request = chain.request();
    request = request.newBuilder()
            .cacheControl(CacheControl.FORCE_CACHE)
            .build();

    return chain.proceed(request);
};

// Create a directory to cache responses. Max size = 10 MiB
Cache cache = new Cache(new File("okhttp.cache"), 10 * 1024 * 1024);

/*
 * Let's create the client. At the beginning the cache will be empty, so we will
 * add the interceptor for the request only after, for the 2nd call
 */
OkHttpClient client = new OkHttpClient.Builder()
        .cache(cache)
        .addNetworkInterceptor(cacheResponseInterceptor)
        .build();

// Let's do the call
Request request = new Request.Builder()
        .url("http://httpbin.org/get")
        .get()
        .build();
Response response = client.newCall(request).execute();
response.close();

// Let's add the interceptor for the request
client = client.newBuilder()
        .addInterceptor(cacheRequestInterceptor)
        .build();

// Let's do the same call
request = new Request.Builder()
        .url("http://httpbin.org/get")
        .get()
        .build();
response = client.newCall(request).execute();
response.close();

// Let's see if we had some issues with the cache
System.out.println("Is successful? " + response.isSuccessful());

Please note that since this is a "standalone" example, I had to do 2 calls:

1. this call uses the network to get a valid response to cache, otherwise my cache would be empty. In your case you don't need it, since normally you'd use the device data to do a "real" call; Note that here I use the network interceptor, the one that adds the header saying "ok, you can cache this response".
2. I added the interceptor that forces to use the cache. Here the network interceptor won't be used, since either the response is found in the cache, or a 504 response is returned.

Note also that you need to close the response , otherwise the cache won't be populated correctly.