okhttp / okhttp3 / Cache

Cache

class Cache :Closeable,Flushable

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

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.

Constructors

Name Summary
<init> Create a cache of at most maxSize bytes in directory.Cache(directory:File, maxSize:Long)

Properties

Name Summary
directory val directory:File
isClosed val isClosed:Boolean

Functions

Name Summary
close fun close():Unit
delete Closes the cache and deletes all of its stored values. This will delete all files in the cache directory including files that weren’t created by the cache.fun delete():Unit
evictAll Deletes all values stored in the cache. In-flight writes to the cache will complete normally, but the corresponding responses will not be stored.fun evictAll():Unit
flush fun flush():Unit
hitCount fun hitCount():Int
initialize Initialize the cache. This will include reading the journal files from the storage and building up the necessary in-memory cache information.fun initialize():Unit
maxSize Max size of the cache (in bytes).fun maxSize():Long
networkCount fun networkCount():Int
requestCount fun requestCount():Int
size fun size():Long
urls Returns an iterator over the URLs in this cache. This iterator doesn’t throw ConcurrentModificationException, but if new responses are added while iterating, their URLs will not be returned. If existing responses are evicted during iteration, they will be absent (unless they were already returned).fun urls():MutableIterator<String>
writeAbortCount fun writeAbortCount():Int
writeSuccessCount fun writeSuccessCount():Int

Companion Object Functions

Name Summary
hasVaryAll Returns true if a Vary header contains an asterisk. Such responses cannot be cached.funResponse.hasVaryAll():Boolean
key fun key(url:HttpUrl):String
varyHeaders Returns the subset of the headers in this’s request that impact the content of this’s body.funResponse.varyHeaders():Headers
varyMatches Returns true if none of the Vary headers have changed between cachedRequest and newRequest.fun varyMatches(cachedResponse:Response, cachedRequest:Headers, newRequest:Request):Boolean