前言
Content Provider做為四大元件之一，通常情況下並沒有其他的元件使用頻繁，但這不能作為我們不去深入學習它的理由。關於Content Provider一篇文章是寫不完的，這一篇文章先來介紹它的啟動過程。
1.query方法到AMS的呼叫過程
在Android IPC機制（四）用ContentProvider進行程式間通訊這篇文章我舉了一個Content Provider使用的例子，在Activity中我是使用如下程式碼呼叫Content Provider的：
public class ContentProviderActivity extends AppCompatActivity {
private final static String TAG = "ContentProviderActivity";
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_content_provider);
Uri uri = Uri.parse("content://com.example.liuwangshu.mooncontentprovide.GameProvider");
ContentValues mContentValues = new ContentValues();
mContentValues.put("_id", 2);
mContentValues.put("name", "大航海時代ol");
mContentValues.put("describe", "最好玩的航海網遊");
getContentResolver().insert(uri, mContentValues);//1
Cursor gameCursor = getContentResolver().query(uri, new String[]{"name", "describe"}, null, null, null);
...
}
}複製程式碼要想呼叫Content Provider，首先需要使用註釋1處的getContentResolver方法，如下所示。
frameworks/base/core/Java/android/content/ContextWrapper.java
@Override
public ContentResolver getContentResolver() {
return mBase.getContentResolver();
}複製程式碼這裡mBase指的是ContextImpl，ContextImpl的getContentResolver方法如下所示。
frameworks/base/core/java/android/app/ContextImpl.java
@Override
public ContentResolver getContentResolver() {
return mContentResolver;
}複製程式碼上面的程式碼return了ApplicationContentResolver型別的mContentResolver物件，ApplicationContentResolver是ContextImpl中的靜態內部類，繼承自ContentResolver，它在ContextImpl的構造方法中被建立。
當我們呼叫ContentResolver的insert、query、update等方法時就會啟動Content Provider，這裡拿query方法來進行舉例。
query方法的實現在ApplicationContentResolver的父類ContentResolver中，程式碼如下所示。
frameworks/base/core/java/android/content/ContentResolver.java
public final @Nullable Cursor query(final @RequiresPermission.Read @NonNull Uri uri,
@Nullable String[] projection, @Nullable String selection,
@Nullable String[] selectionArgs, @Nullable String sortOrder,
@Nullable CancellationSignal cancellationSignal) {
Preconditions.checkNotNull(uri, "uri");
IContentProvider unstableProvider = acquireUnstableProvider(uri);//1
...
try {
...
try {
qCursor = unstableProvider.query(mPackageName, uri, projection,
selection, selectionArgs, sortOrder, remoteCancellationSignal);//2
} catch (DeadObjectException e) {
...
}
...
}複製程式碼在註釋1處通過acquireUnstableProvider方法返回IContentProvider型別的unstableProvider物件，在註釋2處呼叫unstableProvider的query方法。我們檢視acquireUnstableProvider方法做了什麼，如下所示。
frameworks/base/core/java/android/content/ContentResolver.java
public final IContentProvider acquireUnstableProvider(Uri uri) {
if (!SCHEME_CONTENT.equals(uri.getScheme())) {//1
return null;
}
String auth = uri.getAuthority();
if (auth != null) {
return acquireUnstableProvider(mContext, uri.getAuthority());//2
}
return null;
}複製程式碼註釋1處用來檢查Uri的scheme是否等於"content"，如果不是則返回null。註釋2處呼叫了acquireUnstableProvider方法，這是個抽象方法，它的實現在ContentResolver的子類ApplicationContentResolver中：
frameworks/base/core/java/android/app/ContextImpl.java
@Override
protected IContentProvider acquireUnstableProvider(Context c, String auth) {
return mMainThread.acquireProvider(c,
ContentProvider.getAuthorityWithoutUserId(auth),
resolveUserIdFromAuthority(auth), false);
}複製程式碼return了ActivityThread型別的mMainThread物件的acquireProvider方法：
frameworks/base/core/java/android/app/ActivityThread.java
public final IContentProvider acquireProvider(
Context c, String auth, int userId, boolean stable) {
final IContentProvider provider = acquireExistingProvider(c, auth, userId, stable);//1
if (provider != null) {
return provider;
}
IActivityManager.ContentProviderHolder holder = null;
try {
holder = ActivityManagerNative.getDefault().getContentProvider(
getApplicationThread(), auth, userId, stable);//2
} catch (RemoteException ex) {
throw ex.rethrowFromSystemServer();
}
if (holder == null) {
Slog.e(TAG, "Failed to find provider info for " + auth);
return null;
}
holder = installProvider(c, holder, holder.info,
true /*noisy*/, holder.noReleaseNeeded, stable);//3
return holder.provider;
}複製程式碼註釋1處檢查ActivityThread中的ArrayMap型別的mProviderMap中是否有目標ContentProvider存在，有則返回，沒有就會在註釋2處呼叫AMP的getContentProvider方法，最終會呼叫AMS的getContentProvider方法。註釋3處的installProvider方法用來將註釋2處返回的ContentProvider相關的資料儲存在mProviderMap中，起到快取的作用，這樣使用相同的Content Provider時，就不需要每次都要呼叫AMS的getContentProvider方法。使用我們接著檢視AMS的getContentProvider方法，程式碼如下所示。
frameworks/base/services/core/java/com/android/server/am/ActivityManagerService.java
@Override
public final ContentProviderHolder getContentProvider(
IApplicationThread caller, String name, int userId, boolean stable) {
...
return getContentProviderImpl(caller, name, null, stable, userId);
}複製程式碼getContentProvider方法return了getContentProviderImpl方法：
frameworks/base/services/core/java/com/android/server/am/ActivityManagerService.java
private ContentProviderHolder getContentProviderImpl(IApplicationThread caller,
String name, IBinder token, boolean stable, int userId) {
...
ProcessRecord proc = getProcessRecordLocked(
cpi.processName, cpr.appInfo.uid, false);//1
if (proc != null && proc.thread != null && !proc.killed) {
...
if (!proc.pubProviders.containsKey(cpi.name)) {
checkTime(startTime, "getContentProviderImpl: scheduling install");
proc.pubProviders.put(cpi.name, cpr);
try {
proc.thread.scheduleInstallProvider(cpi);//2
} catch (RemoteException e) {
}
}
} else {
checkTime(startTime, "getContentProviderImpl: before start process");
proc = startProcessLocked(cpi.processName,
cpr.appInfo, false, 0, "content provider",
new ComponentName(cpi.applicationInfo.packageName,
cpi.name), false, false, false);//3
checkTime(startTime, "getContentProviderImpl: after start process");
...
}
...
}複製程式碼getContentProviderImpl方法的程式碼很多，這裡擷取了關鍵的部分。註釋1處通過getProcessRecordLocked方法來獲取目標ContentProvider的應用程式程式資訊，這些資訊用ProcessRecord型別的proc來表示，如果該應用程式已經啟動就會呼叫註釋2處的程式碼，否則就會呼叫註釋3的startProcessLocked方法來啟動程式。這裡我們假設ContentProvider的應用程式還沒有啟動，關於應用程式啟動過程，我在Android應用程式程式啟動過程（前篇）已經講過，最終會呼叫ActivityThread的main方法，程式碼如下所示。
frameworks/base/core/java/android/app/ActivityThread.java
public static void main(String[] args) {
...
Looper.prepareMainLooper();//1
ActivityThread thread = new ActivityThread();//2
thread.attach(false);
if (sMainThreadHandler == null) {
sMainThreadHandler = thread.getHandler();
}
if (false) {
Looper.myLooper().setMessageLogging(new
LogPrinter(Log.DEBUG, "ActivityThread"));
}
// End of event ActivityThreadMain.
Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);
Looper.loop();//3
throw new RuntimeException("Main thread loop unexpectedly exited");
}複製程式碼註釋1處通過prepareMainLooper方法在ThreadLocal中獲取Looper，並在註釋3處開啟訊息迴圈。在註釋2處建立了ActivityThread並呼叫了它的attach方法：
frameworks/base/core/java/android/app/ActivityThread.java
private void attach(boolean system) {
...
final IActivityManager mgr = ActivityManagerNative.getDefault();//1
try {
mgr.attachApplication(mAppThread);//2
} catch (RemoteException ex) {
throw ex.rethrowFromSystemServer();
}
...
}複製程式碼註釋1處最終會得到AMS，在註釋2處呼叫AMS的attachApplication方法，並將ApplicationThread型別的mAppThread物件傳進去。
query方法到AMS的呼叫過程，如下面時序圖所示（省略應用程式程式啟動過程）。
2.AMS啟動Content Provider的過程
我們接著來檢視AMS的attachApplication方法，如下所示。
frameworks/base/services/core/java/com/android/server/am/ActivityManagerService.java
@Override
public final void attachApplication(IApplicationThread thread) {
synchronized (this) {
int callingPid = Binder.getCallingPid();
final long origId = Binder.clearCallingIdentity();
attachApplicationLocked(thread, callingPid);
Binder.restoreCallingIdentity(origId);
}
}複製程式碼attachApplication方法中又呼叫了attachApplicationLocked方法：
frameworks/base/services/core/java/com/android/server/am/ActivityManagerService.java
private final boolean attachApplicationLocked(IApplicationThread thread,
int pid) {
...
thread.bindApplication(processName, appInfo, providers, app.instrumentationClass,
profilerInfo, app.instrumentationArguments, app.instrumentationWatcher,
app.instrumentationUiAutomationConnection, testMode,
mBinderTransactionTrackingEnabled, enableTrackAllocation,
isRestrictedBackupMode || !normalMode, app.persistent,
new Configuration(mConfiguration), app.compat,
getCommonServicesLocked(app.isolated),
mCoreSettingsObserver.getCoreSettingsLocked());
...
}複製程式碼attachApplicationLocked方法中呼叫了thread的bindApplication方法，thread是IApplicationThread型別的，從型別名字就可以看出來是用於程式間通訊，這裡實現bindApplication方法的是ApplicationThreadProxy類，它實現了IApplicationThread介面。
frameworks/base/core/java/android/app/ApplicationThreadNative.java
class ApplicationThreadProxy implements IApplicationThread {
...
@Override
public final void bindApplication(String packageName, ApplicationInfo info,
List<ProviderInfo> providers, ComponentName testName, ProfilerInfo profilerInfo,
Bundle testArgs, IInstrumentationWatcher testWatcher,
IUiAutomationConnection uiAutomationConnection, int debugMode,
boolean enableBinderTracking, boolean trackAllocation, boolean restrictedBackupMode,
boolean persistent, Configuration config, CompatibilityInfo compatInfo,
Map<String, IBinder> services, Bundle coreSettings) throws RemoteException {
...
mRemote.transact(BIND_APPLICATION_TRANSACTION, data, null,
IBinder.FLAG_ONEWAY);
data.recycle();
}
...
}複製程式碼到目前為止，上面的呼叫過程還是在AMS程式中執行的，因此，需要通過IBinder型別的mRemote物件向新建立的應用程式程式（目標Content Provider所在的程式）傳送BIND_APPLICATION_TRANSACTION型別的通訊請求。處理這個通訊請求的是在新建立的應用程式程式中執行的ApplicationThread的bindApplication方法，如下所示。
frameworks/base/core/java/android/app/ActivityThread.java
public final void bindApplication(String processName, ApplicationInfo appInfo,
List<ProviderInfo> providers, ComponentName instrumentationName,
ProfilerInfo profilerInfo, Bundle instrumentationArgs,
IInstrumentationWatcher instrumentationWatcher,
IUiAutomationConnection instrumentationUiConnection, int debugMode,
boolean enableBinderTracking, boolean trackAllocation,
boolean isRestrictedBackupMode, boolean persistent, Configuration config,
CompatibilityInfo compatInfo, Map<String, IBinder> services, Bundle coreSettings) {
...
sendMessage(H.BIND_APPLICATION, data);
}複製程式碼呼叫sendMessage方法像H傳送BIND_APPLICATION型別訊息，H的handleMessage方法如下所示。
frameworks/base/core/java/android/app/ActivityThread.java
public void handleMessage(Message msg) {
if (DEBUG_MESSAGES) Slog.v(TAG, ">>> handling: " + codeToString(msg.what));
switch (msg.what) {
...
case BIND_APPLICATION:
Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "bindApplication");
AppBindData data = (AppBindData)msg.obj;
handleBindApplication(data);
Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);
break;
...
}
...
}複製程式碼我們接著檢視handleBindApplication方法：
frameworks/base/core/java/android/app/ActivityThread.java
private void handleBindApplication(AppBindData data) {
...
final ContextImpl appContext = ContextImpl.createAppContext(this, data.info);//1
try {
final ClassLoader cl = instrContext.getClassLoader();
mInstrumentation = (Instrumentation)
cl.loadClass(data.instrumentationName.getClassName()).newInstance();//2
} catch (Exception e) {
...
}
final ComponentName component = new ComponentName(ii.packageName, ii.name);
mInstrumentation.init(this, instrContext, appContext, component,
data.instrumentationWatcher, data.instrumentationUiAutomationConnection);//3
...
Application app = data.info.makeApplication(data.restrictedBackupMode, null);//4
mInitialApplication = app;
if (!data.restrictedBackupMode) {
if (!ArrayUtils.isEmpty(data.providers)) {
installContentProviders(app, data.providers);//5
mH.sendEmptyMessageDelayed(H.ENABLE_JIT, 10*1000);
}
}
...
mInstrumentation.callApplicationOnCreate(app);//6
...
}複製程式碼handleBindApplication方法的程式碼很長，這裡擷取了主要的部分。註釋1處建立了ContextImpl 。註釋2處通過反射建立Instrumentation並在註釋3處初始化Instrumentation。註釋4處建立Application並且在註釋6處呼叫Application的onCreate方法，這意味著Content Provider所在的應用程式程式已經啟動完畢，在這之前，註釋5處呼叫installContentProviders方法來啟動Content Provider，程式碼如下所示。
frameworks/base/core/java/android/app/ActivityThread.java
private void installContentProviders(
Context context, List<ProviderInfo> providers) {
final ArrayList<IActivityManager.ContentProviderHolder> results =
new ArrayList<IActivityManager.ContentProviderHolder>();
for (ProviderInfo cpi : providers) {//1
...
IActivityManager.ContentProviderHolder cph = installProvider(context, null, cpi,
false /*noisy*/, true /*noReleaseNeeded*/, true /*stable*/);//2
...
}
try {
ActivityManagerNative.getDefault().publishContentProviders(
getApplicationThread(), results);//3
} catch (RemoteException ex) {
throw ex.rethrowFromSystemServer();
}
}複製程式碼註釋1處遍歷當前應用程式程式的ProviderInfo列表，得到每個Content Provider的ProviderInfo（儲存Content Provider的資訊），並在註釋2處呼叫installProvider方法來啟動這些Content Provider。在註釋3處通過AMS的publishContentProviders方法將這些Content Provider儲存在AMS的mProviderMap中，這個mProviderMap在前面提到過，起到快取的作用，防止每次使用相同的Content Provider時都會呼叫AMS的getContentProvider方法。來檢視installProvider方法時如何啟動Content Provider的，installProvider方法如下所示。
frameworks/base/core/java/android/app/ActivityThread.java
private IActivityManager.ContentProviderHolder installProvider(Context context,
IActivityManager.ContentProviderHolder holder, ProviderInfo info,
boolean noisy, boolean noReleaseNeeded, boolean stable) {
ContentProvider localProvider = null;
...
final java.lang.ClassLoader cl = c.getClassLoader();
localProvider = (ContentProvider)cl.
loadClass(info.name).newInstance();//1
provider = localProvider.getIContentProvider();
if (provider == null) {
...
return null;
}
if (DEBUG_PROVIDER) Slog.v(
TAG, "Instantiating local provider " + info.name);
localProvider.attachInfo(c, info);//2
} catch (java.lang.Exception e) {
...
}
return null;
}
}
...
return retHolder;
}複製程式碼在註釋1處通過反射來建立ContentProvider型別的localProvider物件，並在註釋2處呼叫了它的attachInfo方法：
frameworks/base/core/java/android/content/ContentProvider.java
private void attachInfo(Context context, ProviderInfo info, boolean testing) {
...
ContentProvider.this.onCreate();
}
}複製程式碼在attachInfo方法中呼叫了onCreate方法，它是一個抽象方法。這樣Content Provider就啟動完畢。
最後給出AMS啟動Content Provider的時序圖。
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