HttpResponseCache
class HttpResponseCache : ResponseCache, Closeable
kotlin.Any | ||
↳ | java.net.ResponseCache | |
↳ | android.net.http.HttpResponseCache |
Caches HTTP and HTTPS responses to the filesystem so they may be reused, saving time and bandwidth. This class supports and javax.net.ssl.HttpsURLConnection
; there is no platform-provided cache for DefaultHttpClient
or AndroidHttpClient
. Installation and instances are thread safe.
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 theapplication-specific
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(); } }</code>
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 android.content.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: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.
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 theno-cache
directive:
<code>connection.addRequestProperty("Cache-Control", "no-cache"); </code>
max-age=0
instead:
<code>connection.addRequestProperty("Cache-Control", "max-age=0"); </code>
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 theonly-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 } </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); </code>
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) { }</code>
Summary
Public methods | |
---|---|
Unit |
close() Uninstalls the cache and releases any active resources. |
Unit |
delete() Uninstalls the cache and deletes all of its stored contents. |
Unit |
flush() Force buffered operations to the filesystem. |
CacheResponse! |
get(uri: URI!, requestMethod: String!, requestHeaders: MutableMap<String!, MutableList<String!>!>!) |
Int |
Returns the number of HTTP requests whose response was provided by the cache. |
static HttpResponseCache! |
Returns the currently-installed |
Int |
Returns the number of HTTP requests that required the network to either supply a response or validate a locally cached response. |
Int |
Returns the total number of HTTP requests that were made. |
static HttpResponseCache! |
Creates a new HTTP response cache and sets it as the system default cache. |
Long |
maxSize() Returns the maximum number of bytes that this cache should use to store its data. |
CacheRequest! |
put(uri: URI!, urlConnection: URLConnection!) |
Long |
size() Returns the number of bytes currently being used to store the values in this cache. |
Inherited functions | |
---|---|
Public methods
close
fun close(): Unit
Uninstalls the cache and releases any active resources. Stored contents will remain on the filesystem.
Exceptions | |
---|---|
java.lang.Exception |
if this resource cannot be closed |
java.io.IOException |
if an I/O error occurs |
delete
fun delete(): Unit
Uninstalls the cache and deletes all of its stored contents.
flush
fun flush(): Unit
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.
get
fun get(
uri: URI!,
requestMethod: String!,
requestHeaders: MutableMap<String!, MutableList<String!>!>!
): CacheResponse!
Parameters | |
---|---|
uri |
URI!: a URI used to reference the requested network resource |
rqstMethod |
a String representing the request method |
rqstHeaders |
- a Map from request header field names to lists of field values representing the current request headers |
Return | |
---|---|
CacheResponse! |
a CacheResponse instance if available from cache, or null otherwise |
Exceptions | |
---|---|
java.io.IOException |
if an I/O error occurs |
java.lang.IllegalArgumentException |
if any one of the arguments is null |
getHitCount
fun getHitCount(): Int
Returns the number of HTTP requests whose response was provided by the cache. This may include conditional GET
requests that were validated over the network.
getInstalled
static fun getInstalled(): HttpResponseCache!
Returns the currently-installed HttpResponseCache
, or null if there is no cache installed or it is not a HttpResponseCache
.
getNetworkCount
fun getNetworkCount(): Int
Returns the number of HTTP requests that required the network to either supply a response or validate a locally cached response.
getRequestCount
fun getRequestCount(): Int
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.
install
static fun install(
directory: File!,
maxSize: Long
): HttpResponseCache!
Creates a new HTTP response cache and sets it as the system default cache.
Parameters | |
---|---|
directory |
File!: the directory to hold cache data. |
maxSize |
Long: the maximum size of the cache in bytes. |
Return | |
---|---|
HttpResponseCache! |
the newly-installed cache |
Exceptions | |
---|---|
java.io.IOException |
if directory cannot be used for this cache. Most applications should respond to this exception by logging a warning. |
maxSize
fun maxSize(): Long
Returns the maximum number of bytes that this cache should use to store its data.
put
fun put(
uri: URI!,
urlConnection: URLConnection!
): CacheRequest!
Parameters | |
---|---|
uri |
URI!: a URI used to reference the requested network resource |
conn |
- a URLConnection instance that is used to fetch the response to be cached |
Return | |
---|---|
CacheRequest! |
a CacheRequest for recording the response to be cached. Null return indicates that the caller does not intend to cache the response. |
Exceptions | |
---|---|
java.io.IOException |
if an I/O error occurs |
java.lang.IllegalArgumentException |
if any one of the arguments is null |
size
fun size(): Long
Returns the number of bytes currently being used to store the values in this cache. This may be greater than the maxSize
if a background deletion is pending. -1
is returned if the size cannot be determined.