ContentProvider分析

ContentProvider何时创建呢？这是一个值得深思的问题？
据我这两天的了解是在你要用到的时候才会调用ContentProvider的onCreate函数进行创建。你就会什么时候叫要用到的时候呢？ 比如你要查询或删除修改数据库的时候通过ContentResolver的quire或delete来操纵数据时就会调用ContentProvider 的onCreate函数，若已经创建了数据库就不会再次创建。可以通过ContentResolver的源码来理解

1. public final Cursor query(Uri uri, String[] projection,
   2.             String selection, String[] selectionArgs, String sortOrder) {
   3.         IContentProvider provider = acquireProvider(uri);
   4.         if (provider == null) {
   5.             return null;
   6.         }
   7.         try {
   8.             Cursor qCursor = provider.query(uri, projection, selection, selectionArgs, sortOrder);
   9.             if(qCursor == null) {
  10.                 releaseProvider(provider);
  11.                 return null;
  12.             }
  13.             //Wrap the cursor object into CursorWrapperInner object
  14.             return new CursorWrapperInner(qCursor, provider);
  15.         } catch (RemoteException e) {
  16.             releaseProvider(provider);
  17.             return null;
  18.         } catch(RuntimeException e) {
  19.             releaseProvider(provider);
  20.             throw e;
  21.         }
  22.     }

ContentResolver中的acquireProvider(uri)来获得对应的Provider.通过uri中的Authority 的字段知道是要用哪个provider.也就是为什么要在Provider的menifester.xml中.


下面对android的几个已有的Provider的进行说明一下：
1,SettingsProvider

1. <!-- Permission to write Gservices in SettingsProvider -->
   2. <permission android:name="android.permission.WRITE_GSERVICES"
   3.     android:label="@string/permlab_writeGservices"
   4.     android:description="@string/permdesc_writeGservices"
   5.     android:protectionLevel="signature" />
   6.
   7. <application android:allowClearUserData="false"
   8.              android:label="Settings Storage"
   9.              android:icon="@drawable/ic_launcher_settings">
  10.
  11.     <provider android:name="SettingsProvider" android:authorities="settings"
  12.               android:process="system" android:multiprocess="false"
  13.               android:writePermission="android.permission.WRITE_SETTINGS"
  14.               android:initOrder="100" />
  15. </application>
  16. /manifest>

若你仔细查看SettingsProvider就会发现在它的manifester.xml中在android:process="system" 和android:sharedUserId="android.uid.system
        package="com.android.providers.settings"
        android:sharedUserId="android.uid.system">,我猜测它就是多了这两条语句使得系统加载完就加载它。若你注意开机的logcat就会发现04-29 02:32:20.612: INFO/ActivityThread(79): Publishing provider settings: com.android.providers.settings.SettingsProvider通过ActivityThread开始往上追你就会发现                                        
从SystemServer.java中的

Log.i(TAG, "Starting System Content Providers.");
 ActivityManagerService.installSystemProviders();

Log.i(TAG, "Starting System Content Providers."); ActivityManagerService.installSystemProviders();到ActivityManagerService.java中的

1. public static final void installSystemProviders() {
   2.         [color=blue]ProcessRecord app = mSelf.mProcessNames.get("system", Process.SYSTEM_UID);[/color]
   3.         List providers = mSelf.generateApplicationProvidersLocked(app);
   4.         mSystemThread.installSystemProviders(providers);
   5.     }

                  
再到ActivityThread.java：

1. public final void installSystemProviders(List providers) {
   2.         if (providers != null) {
   3.             installContentProviders(mInitialApplication,
   4.                                     (List<ProviderInfo>)providers);
   5.         }
   6.     }

1. private final void installContentProviders(
   2.             Context context, List<ProviderInfo> providers) {
   3.         final ArrayList<IActivityManager.ContentProviderHolder> results =
   4.             new ArrayList<IActivityManager.ContentProviderHolder>();
   5.
   6.         Iterator<ProviderInfo> i = providers.iterator();
   7.         while (i.hasNext()) {
   8.             ProviderInfo cpi = i.next();
   9.             StringBuilder buf = new StringBuilder(128);
  10.             buf.append("Publishing provider ");
  11.             buf.append(cpi.authority);
  12.             buf.append(": ");
  13.             buf.append(cpi.name);
  14.             Log.i(TAG, buf.toString());
  15.             IContentProvider cp = installProvider(context, null, cpi, false);
  16.             if (cp != null) {
  17.                 IActivityManager.ContentProviderHolder cph =
  18.                     new IActivityManager.ContentProviderHolder(cpi);
  19.                 cph.provider = cp;
  20.                 results.add(cph);
  21.                 // Don't ever unload this provider from the process.
  22.                 synchronized(mProviderMap) {
  23.                     mProviderRefCountMap.put(cp.asBinder(), new ProviderRefCount(10000));
  24.                 }
  25.             }
  26.         }

2,CalendarProvider
Manifester.xml

1. <receiver android:name="CalendarReceiver">
   2.             <intent-filter>
   3.                 <action android:name="android.intent.action.BOOT_COMPLETED" />
   4.             </intent-filter>
   5.         </receiver> <provider android:name="CalendarProvider" android:authorities="calendar"
   6.                 android:syncable="true" android:multiprocess="false"
   7.                 android:readPermission="android.permission.READ_CALENDAR"
   8.                 android:writePermission="android.permission.WRITE_CALENDAR" />

并不采用了SettingsProvider的方式而是通过了android.intent.action.BOOT_COMPLETED的方式：
CalendarReceiver.java

1. public class CalendarReceiver extends BroadcastReceiver {
   2.
   3.     static final String SCHEDULE = "com.android.providers.calendar.SCHEDULE_ALARM";
   4.
   5.     @Override
   6.     public void onReceive(Context context, Intent intent) {
   7.         String action = intent.getAction();
   8.         ContentResolver cr = context.getContentResolver();
   9.         CalendarProvider provider;
  10.         IContentProvider icp = cr.acquireProvider("calendar");
  11.         provider = (CalendarProvider) ContentProvider.
  12.                 coerceToLocalContentProvider(icp);
  13.         if (action.equals(SCHEDULE)) {
  14.             provider.scheduleNextAlarm(false /* do not remove alarms */);
  15.         } else if (action.equals(Intent.ACTION_BOOT_COMPLETED)) {
  16.             provider.bootCompleted();
  17.         }
  18.         cr.releaseProvider(icp);
  19.     }
  20. }