HttpResponseCache 网络缓存使用

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

Installing an HTTP response cache  添加于 API 级别 13

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 theapplication-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) {
       }}