Cache

Caches HTTP and HTTPS responses to the filesystem so they may be reused, saving time and bandwidth.

The Cache instance must have exclusive access to the directory, since the internal data structures may cause corruption or runtime errors if not. It may however be shared amongst multiple OkHttpClient instances.

Cache Optimization

To measure cache effectiveness, this class tracks three statistics:

• Request Count: the number of HTTP requests issued since this cache was created.

• Network Count: the number of those requests that required network use.

• Hit Count: the number of those requests whose responses were served by the cache.

Sometimes a request will result in a conditional cache hit. If the cache contains a stale copy of the response, the client will issue a conditional GET. The server will then send either the updated response if it has changed, or a short 'not modified' response if the client's copy is still valid. Such responses increment both the network count and hit count.

The best way to improve the cache hit rate is by configuring the web server to return cacheable responses. Although this client honors all HTTP/1.1 (RFC 7234) cache headers, it doesn't cache partial responses.

Force a Network Response

In some situations, such as after a user clicks a 'refresh' button, it may be necessary to skip the cache, and fetch data directly from the server. To force a full refresh, add the no-cache directive:

Request request = new Request.Builder()
    .cacheControl(new CacheControl.Builder().noCache().build())
    .url("http://publicobject.com/helloworld.txt")
    .build();

If it is only necessary to force a cached response to be validated by the server, use the more efficient max-age=0 directive instead:

Request request = new Request.Builder()
    .cacheControl(new CacheControl.Builder()
        .maxAge(0, TimeUnit.SECONDS)
        .build())
    .url("http://publicobject.com/helloworld.txt")
    .build();

Force a Cache Response

Sometimes you'll want to show resources if they are available immediately, but not otherwise. This can be used so your application can show something while waiting for the latest data to be downloaded. To restrict a request to locally-cached resources, add the only-if-cached directive:

Request request = new Request.Builder()
    .cacheControl(new CacheControl.Builder()
        .onlyIfCached()
        .build())
    .url("http://publicobject.com/helloworld.txt")
    .build();
Response forceCacheResponse = client.newCall(request).execute();
if (forceCacheResponse.code() != 504) {
  // The resource was cached! Show it.
} else {
  // The resource was not cached.
}

This technique works even better in situations where a stale response is better than no response. To permit stale cached responses, use the max-stale directive with the maximum staleness in seconds:

Request request = new Request.Builder()
    .cacheControl(new CacheControl.Builder()
        .maxStale(365, TimeUnit.DAYS)
        .build())
    .url("http://publicobject.com/helloworld.txt")
    .build();

The CacheControl class can configure request caching directives and parse response caching directives. It even offers convenient constants CacheControl.FORCE_NETWORK and CacheControl.FORCE_CACHE that address the use cases above.