HttpResponseCache

Class Overview

Caches HTTP and HTTPS responses to the filesystem so they may be reused, saving time and bandwidth. This class supports HttpURLConnection and HttpsURLConnection; there is no platform-provided cache for DefaultHttpClient or AndroidHttpClient.

Installing an HTTP response cache

Enable caching of all of your application's HTTP requests by installing the cache at application startup. For example, this code installs a 10 MiB cache in the application-specific cache directory of the filesystem}:

   protected void onCreate(Bundle savedInstanceState) {
       ...

       try {
           File httpCacheDir = new File(context.getCacheDir(), "http");
           long httpCacheSize = 10 * 1024 * 1024; // 10 MiB
           HttpResponseCache.install(httpCacheDir, httpCacheSize);
        catch (IOException e) {
           Log.i(TAG, "HTTP response cache installation failed:" + e);
       }
   }

   protected void onStop() {
       ...

       HttpResponseCache cache = HttpResponseCache.getInstalled();
       if (cache != null) {
           cache.flush();
       }
   }}

This cache will evict entries as necessary to keep its size from exceeding 10 MiB. The best cache size is application specific and depends on the size and frequency of the files being downloaded. Increasing the limit may improve the hit rate, but it may also just waste filesystem space!

For some applications it may be preferable to create the cache in the external storage directory. There are no access controls on the external storage directory so it should not be used for caches that could contain private data. Although it often has more free space, external storage is optional and—even if available—can disappear during use. Retrieve the external cache directory using getExternalCacheDir(). If this method returns null, your application should fall back to either not caching or caching on non-external storage. If the external storage is removed during use, the cache hit rate will drop to zero and ongoing cache reads will fail.

Flushing the cache forces its data to the filesystem. This ensures that all responses written to the cache will be readable the next time the activity starts.

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 2068) 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:

   connection.addRequestProperty("Cache-Control", "no-cache");
 

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

   connection.addRequestProperty("Cache-Control", "max-age=0");
 

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:

   try {
         connection.addRequestProperty("Cache-Control", "only-if-cached");
         InputStream cached = connection.getInputStream();
         // the resource was cached! show it
      catch (FileNotFoundException e) {
         // 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:

   int maxStale = 60 * 60 * 24 * 28; // tolerate 4-weeks stale
         connection.addRequestProperty("Cache-Control", "max-stale=" + maxStale);
 

Working With Earlier Releases

This class was added in Android 4.0 (Ice Cream Sandwich). Use reflection to enable the response cache without impacting earlier releases:

   try {
           File httpCacheDir = new File(context.getCacheDir(), "http");
           long httpCacheSize = 10 * 1024 * 1024; // 10 MiB
           Class.forName("android.net.http.HttpResponseCache")
                   .getMethod("install", File.class, long.class)
                   .invoke(null, httpCacheDir, httpCacheSize);
        catch (Exception httpResponseCacheNotAvailable) {
       }}

Summary

Public Methods
Example ? Examples	void	close() Uninstalls the cache and releases any active resources.
Example ? Examples	void	delete() Uninstalls the cache and deletes all of its stored contents.
Example ? Examples	void	flush() Force buffered operations to the filesystem.
Example ? Examples	CacheResponse	get( URI uri, String requestMethod, Map< String,  List< String>> requestHeaders) Returns the cached response corresponding to the given request.
Example ? Examples	int	getHitCount() Returns the number of HTTP requests whose response was provided by the cache.
Example ? Examples	static HttpResponseCache	getInstalled() Returns the currently-installed HttpResponseCache, or null if there is no cache installed or it is not a HttpResponseCache.
Example ? Examples	int	getNetworkCount() Returns the number of HTTP requests that required the network to either supply a response or validate a locally cached response.
Example ? Examples	int	getRequestCount() Returns the total number of HTTP requests that were made.
Example ? Examples	static HttpResponseCache	install( File directory, long maxSize) Creates a new HTTP response cache and sets it as the system default cache.
Example ? Examples	long	maxSize() Returns the maximum number of bytes that this cache should use to store its data.
Example ? Examples	CacheRequest	put( URI uri, URLConnection urlConnection) Allows the protocol handler to cache data after retrieving resources.
Example ? Examples	long	size() Returns the number of bytes currently being used to store the values in this cache.

[Expand] Inherited Methods
From class java.net.ResponseCache abstract CacheResponse get( URI uri, String requestMethod, Map< String,  List< String>> requestHeaders) Returns the cached response corresponding to the given request. static ResponseCache getDefault() Returns the system's default response cache, or null. abstract CacheRequest put( URI uri, URLConnection connection) Allows the protocol handler to cache data after retrieving resources. static void setDefault( ResponseCache responseCache) Sets the system's default response cache.
From class java.lang.Object Object clone() Creates and returns a copy of this Object. boolean equals( Object o) Compares this instance with the specified object and indicates if they are equal. void finalize() Invoked when the garbage collector has detected that this instance is no longer reachable. final Class<?> getClass() Returns the unique instance of Class that represents this object's class. int hashCode() Returns an integer hash code for this object. final void notify() Causes a thread which is waiting on this object's monitor (by means of calling one of the wait() methods) to be woken up. final void notifyAll() Causes all threads which are waiting on this object's monitor (by means of calling one of the wait() methods) to be woken up. String toString() Returns a string containing a concise, human-readable description of this object. final void wait() Causes the calling thread to wait until another thread calls the notify() or notifyAll() method of this object. final void wait(long millis, int nanos) Causes the calling thread to wait until another thread calls the notify() or notifyAll() method of this object or until the specified timeout expires. final void wait(long millis) Causes the calling thread to wait until another thread calls the notify() or notifyAll() method of this object or until the specified timeout expires.
From interface java.io.Closeable abstract void close() Closes the object and release any system resources it holds.