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HttpResponseCache.javaAPI DocAndroid 5.1 API12001Thu Mar 12 22:22:10 GMT 2015android.net.http

HttpResponseCache

public final class HttpResponseCache extends ResponseCache implements Closeable
Caches HTTP and HTTPS responses to the filesystem so they may be reused, saving time and bandwidth. This class supports {@link HttpURLConnection} and {@link HttpsURLConnection}; there is no platform-provided cache for {@link DefaultHttpClient} or {@link 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 {@link Context#getCacheDir() application-specific cache directory} of the filesystem}:
 {@code
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 {@link Context#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:
  • {@link #getRequestCount() Request Count:} the number of HTTP requests issued since this cache was created.
  • {@link #getNetworkCount() Network Count:} the number of those requests that required network use.
  • {@link #getHitCount() 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 {@code 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 {@code no-cache} directive:
 {@code
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 {@code max-age=0} instead:
 {@code
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 {@code only-if-cached} directive:
 {@code
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 {@code max-stale} directive with the maximum staleness in seconds:
 {@code
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:
 {@code
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) {
}}

Fields Summary
private final com.android.okhttp.HttpResponseCache
delegate
Constructors Summary
private HttpResponseCache(com.android.okhttp.HttpResponseCache delegate)

        this.delegate = delegate;
    
Methods Summary
public voidclose()
Uninstalls the cache and releases any active resources. Stored contents will remain on the filesystem.

        if (ResponseCache.getDefault() == this.delegate) {
            ResponseCache.setDefault(null);
        }
        delegate.close();
    
public voiddelete()
Uninstalls the cache and deletes all of its stored contents.

        if (ResponseCache.getDefault() == this.delegate) {
            ResponseCache.setDefault(null);
        }
        delegate.delete();
    
public voidflush()
Force buffered operations to the filesystem. This ensures that responses written to the cache will be available the next time the cache is opened, even if this process is killed.

        try {
            delegate.flush();
        } catch (IOException ignored) {
        }
    
public java.net.CacheResponseget(java.net.URI uri, java.lang.String requestMethod, java.util.Map requestHeaders)

        return delegate.get(uri, requestMethod, requestHeaders);
    
public intgetHitCount()
Returns the number of HTTP requests whose response was provided by the cache. This may include conditional {@code GET} requests that were validated over the network.

        return delegate.getHitCount();
    
public static android.net.http.HttpResponseCachegetInstalled()
Returns the currently-installed {@code HttpResponseCache}, or null if there is no cache installed or it is not a {@code HttpResponseCache}.

        ResponseCache installed = ResponseCache.getDefault();
        if (installed instanceof com.android.okhttp.HttpResponseCache) {
            return new HttpResponseCache(
                    (com.android.okhttp.HttpResponseCache) installed);
        }

        return null;
    
public intgetNetworkCount()
Returns the number of HTTP requests that required the network to either supply a response or validate a locally cached response.

        return delegate.getNetworkCount();
    
public intgetRequestCount()
Returns the total number of HTTP requests that were made. This includes both client requests and requests that were made on the client's behalf to handle a redirects and retries.

        return delegate.getRequestCount();
    
public static android.net.http.HttpResponseCacheinstall(java.io.File directory, long maxSize)
Creates a new HTTP response cache and {@link ResponseCache#setDefault sets it} as the system default cache.

param
directory the directory to hold cache data.
param
maxSize the maximum size of the cache in bytes.
return
the newly-installed cache
throws
IOException if {@code directory} cannot be used for this cache. Most applications should respond to this exception by logging a warning.

        ResponseCache installed = ResponseCache.getDefault();
        if (installed instanceof com.android.okhttp.HttpResponseCache) {
            com.android.okhttp.HttpResponseCache installedCache =
                    (com.android.okhttp.HttpResponseCache) installed;
            // don't close and reopen if an equivalent cache is already installed
            if (installedCache.getDirectory().equals(directory)
                    && installedCache.getMaxSize() == maxSize
                    && !installedCache.isClosed()) {
                return new HttpResponseCache(installedCache);
            } else {
                // The HttpResponseCache that owns this object is about to be replaced.
                installedCache.close();
            }
        }

        com.android.okhttp.HttpResponseCache responseCache =
                new com.android.okhttp.HttpResponseCache(directory, maxSize);
        ResponseCache.setDefault(responseCache);
        return new HttpResponseCache(responseCache);
    
public longmaxSize()
Returns the maximum number of bytes that this cache should use to store its data.

        return delegate.getMaxSize();
    
public java.net.CacheRequestput(java.net.URI uri, java.net.URLConnection urlConnection)

        return delegate.put(uri, urlConnection);
    
public longsize()
Returns the number of bytes currently being used to store the values in this cache. This may be greater than the {@link #maxSize} if a background deletion is pending.

        return delegate.getSize();