Android 4.0 Http缓存机制

来源：互联网发布：手机能注册淘宝店吗编辑：程序博客网时间：2024/06/03 21:05

Android早已经在4.0版本添加了缓存支持，以下内容是对Android官方文档的一个总结。

(一) HttpResponseCache 类在Android 4.0 版本添加支持，支持 HttpURLConnection和 HttpsURLConnection，但不支持DefaultHttpClient和AndroidHttpClient。

HttpResponseCache的好处：

1.明显一点节约电，减少了网络请求。

2.开发者不用自己在去写cache机制了。

3.最根本的一点就是，如果开发者在开发中不是使用的HttpClient, HttpDefaultClient...,而是用 HttpURLConnection的话,你根本不用改本來的 Code。

（1）.在程序开启的时候设定需要缓存，并设定缓存目录和缓存文件大小，例如：（最好把缓存目录设定在外部存储）

 static volatile Object cache;private static final Object lock = new Object();private static final int MIN_DISK_CACHE_SIZE = 5 * 1024 * 1024; // 5MBprivate static final int MAX_DISK_CACHE_SIZE = 50 * 1024 * 1024; // 50MBif (Build.VERSION.SDK_INT >= Build.VERSION_CODES.ICE_CREAM_SANDWICH){installCacheIfNeeded(context);}private static void installCacheIfNeeded(Context context){// DCL + volatile should be safe after Java 5.if (cache == null){try{synchronized (lock){if (cache == null){cache = install(context);}}}catch (IOException ignored){}}}public void install(Context context){File cacheDir = createDefaultCacheDir(context);HttpResponseCache cache = HttpResponseCache.getInstalled();if (cache == null){long maxSize = calculateDiskCacheSize(cacheDir);cache = HttpResponseCache.install(cacheDir, maxSize);}}static File createDefaultCacheDir(Context context){File cache = new File(context.getApplicationContext().getCacheDir(), "Cache_Name");if (!cache.exists()){// noinspection ResultOfMethodCallIgnoredcache.mkdirs();}return cache;}static long calculateDiskCacheSize(File dir){long size = MIN_DISK_CACHE_SIZE;try{StatFs statFs = new StatFs(dir.getAbsolutePath());long available = ((long) statFs.getBlockCount()) * statFs.getBlockSize();// Target 2% of the total space.size = available / 50;}catch (IllegalArgumentException ignored){}// Bound inside min/max size for disk cache.return Math.max(Math.min(size, MAX_DISK_CACHE_SIZE), MIN_DISK_CACHE_SIZE);}

（2）.根据请求需要设置缓存

    a.例如点击“刷新”按钮，或者手动刷新，这时候要强制刷新，就需要添加“no-cache”指令

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

    b.如果只需要强制缓存的响应由服务器进行验证，使用更高效的“max-age=0”指令来代替：

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

c. 有时候，你只想加载缓存数据，你需要设定“ only-if-cached”指令

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     } }

d.你还可以设定缓存的有效时间

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

个人理解会将http请求放在本地，如果有相同的请求，就不会再次调用网络请求，直接得到本地请求响应的值

[java] view plaincopy

public class HttpCacheApplication extends Application {
@Override
public void onCreate() {
super.onCreate();
new Thread() {
@Override
public void run() {
enableHttpResponseCache();
}
}.start();
}
private void enableHttpResponseCache() {
try {
long httpCacheSize = 10 * 1024 * 1024;// 10M
File httpCacheDir = new File(getCacheDir(), "http");
Class.forName("android.net.http.HttpResponseCache")
.getMethod("install", File.class, long.class)
.invoke(null, httpCacheDir, httpCacheSize);
} catch (Exception e) {
Log.e("===>", e.getMessage(), e);
}
}
}

接下来我们来看看HttpUrlConnection是怎么处理的，怎么缓存的。

[java] view plaincopy

public class MainActivity extends Activity {
private final String TAG = getClass().getSimpleName();
ImageView img;
Button msg;
TextView tv;
@Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
img = (ImageView) findViewById(R.id.imageView1);
tv = (TextView)findViewById(R.id.textView1);
msg = (Button) findViewById(R.id.button1);
msg.setOnClickListener(new OnClickListener() {
@Override
public void onClick(View v) {
new InternetTask().execute();
}
});
findViewById(R.id.button2).setOnClickListener(new OnClickListener() {
@Override
public void onClick(View v) {
MainActivity.this.finish();
}
});
}
class InternetTask extends AsyncTask<String, String, Boolean> {
Bitmap bitmap;
String jsonStr;
@Override
protected void onPostExecute(Boolean result) {
super.onPostExecute(result);
img.setImageBitmap(bitmap);
tv.setText(jsonStr);
}
@Override
protected Boolean doInBackground(String... params) {
// Test download image
try {
URL url = new URL("http://news.baidu.com/resource/img/logo_news_137_46.png");
HttpURLConnection conn = (HttpURLConnection) (url
.openConnection());
conn.connect();
InputStream is = conn.getInputStream();
BitmapFactory.Options ops = new BitmapFactory.Options();
bitmap = BitmapFactory.decodeStream(is, null, ops);
is.close();
conn.disconnect();
} catch (Exception e) {
Log.e(TAG, e.getMessage(), e);
}
// Test download JSON data
try {
URL url = new URL("http://www.baidu.com/");
HttpURLConnection conn = (HttpURLConnection) (url
.openConnection());
conn.connect();
BufferedReader reader = new BufferedReader(
new InputStreamReader(conn.getInputStream(), "UTF-8"));
jsonStr = reader.readLine();
InputStream is = conn.getInputStream();
is.close();
conn.disconnect();
} catch (Exception e) {
Log.e(TAG, e.getMessage(), e);
}
return true;
}
}
}

我们看下效果：

看下缓存文件，每个文件会产生两个文件，一个是数据文件，一个是http header 信息

0 0