Android深入四大組件（九）Content Provider的啟動過程

前言

Content Provider做為四大組件之一，通常情況下並沒有其他的組件使用頻繁，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的調用過程

在Android IPC機制（四）用ContentProvider進行進程間通信這篇文章我舉了一個Content Provider使用的例子，在Activity中我是使用如下代碼調用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", "最好玩的航海網游");
        getContentResolver().insert(uri, mContentValues);//1
        Cursor gameCursor = getContentResolver().query(uri, new String[]{"name", "describe"}, null, null, null);
     ...
    }
}

要想調用Content Provider，首先需要使用注釋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();
}

這里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;
}

上面的代碼return了ApplicationContentResolver類型的mContentResolver對象，ApplicationContentResolver是ContextImpl中的靜態內部類，繼承自ContentResolver，它在ContextImpl的構造方法中被創建。
當我們調用ContentResolver的insert、query、update等方法時就會啟動Content Provider，這里拿query方法來進行舉例。
query方法的實現在ApplicationContentResolver的父類ContentResolver中，代碼如下所示。
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) {
               ...
            }
    ...
   }

在注釋1處通過acquireUnstableProvider方法返回IContentProvider類型的unstableProvider對象，在注釋2處調用unstableProvider的query方法。我們查看acquireUnstableProvider方法做了什么，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;
 }

注釋1處用來檢查Uri的scheme是否等於”content”，如果不是則返回null。注釋2處調用了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);
}

return了ActivityThread類型的mMainThread對象的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;
 }

注釋1處檢查ActivityThread中的ArrayMap類型的mProviderMap中是否有目標ContentProvider存在，有則返回，沒有就會在注釋2處調用AMP的getContentProvider方法，最終會調用AMS的getContentProvider方法。注釋3處的installProvider方法用來將注釋2處返回的ContentProvider相關的數據存儲在mProviderMap中，起到緩存的作用，這樣使用相同的Content Provider時，就不需要每次都要調用AMS的getContentProvider方法。使用我們接着查看AMS的getContentProvider方法，代碼如下所示。
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);
}

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");
                          ...
                        }
             ...           
                        
}

getContentProviderImpl方法的代碼很多，這里截取了關鍵的部分。注釋1處通過getProcessRecordLocked方法來獲取目標ContentProvider的應用程序進程信息，這些信息用ProcessRecord類型的proc來表示，如果該應用進程已經啟動就會調用注釋2處的代碼，否則就會調用注釋3的startProcessLocked方法來啟動進程。這里我們假設ContentProvider的應用進程還沒有啟動，關於應用進程啟動過程，我在Android應用程序進程啟動過程（前篇）已經講過，最終會調用ActivityThread的main方法，代碼如下所示。
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");
   }

注釋1處通過prepareMainLooper方法在ThreadLocal中獲取Looper，並在注釋3處開啟消息循環。在注釋2處創建了ActivityThread並調用了它的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();
            }
  ...          
}

注釋1處最終會得到AMS，在注釋2處調用AMS的attachApplication方法，並將ApplicationThread類型的mAppThread對象傳進去。
query方法到AMS的調用過程，如下面時序圖所示（省略應用程序進程啟動過程）。

2.AMS啟動Content Provider的過程

我們接着來查看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);
    }
}

attachApplication方法中又調用了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());
...
}

attachApplicationLocked方法中調用了thread的bindApplication方法，thread是IApplicationThread類型的，從類型名字就可以看出來是用於進程間通信，這里實現bindApplication方法的是ApplicationThreadProxy類，它實現了IApplicationThread接口。
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();
    }
...
}

到目前為止，上面的調用過程還是在AMS進程中執行的，因此，需要通過IBinder類型的mRemote對象向新創建的應用程序進程（目標Content Provider所在的進程）發送BIND_APPLICATION_TRANSACTION類型的通信請求。處理這個通信請求的是在新創建的應用程序進程中執行的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);
       }

調用sendMessage方法像H發送BIND_APPLICATION類型消息，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;
  ...
  }
  ... 
}

我們接着查看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
        ... 
}

handleBindApplication方法的代碼很長，這里截取了主要的部分。注釋1處創建了ContextImpl 。注釋2處通過反射創建Instrumentation並在注釋3處初始化Instrumentation。注釋4處創建Application並且在注釋6處調用Application的onCreate方法，這意味着Content Provider所在的應用程序進程已經啟動完畢，在這之前，注釋5處調用installContentProviders方法來啟動Content Provider，代碼如下所示。
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();
    }

注釋1處遍歷當前應用程序進程的ProviderInfo列表，得到每個Content Provider的ProviderInfo（存儲Content Provider的信息），並在注釋2處調用installProvider方法來啟動這些Content Provider。在注釋3處通過AMS的publishContentProviders方法將這些Content Provider存儲在AMS的mProviderMap中，這個mProviderMap在前面提到過，起到緩存的作用，防止每次使用相同的Content Provider時都會調用AMS的getContentProvider方法。來查看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;
   }

在注釋1處通過反射來創建ContentProvider類型的localProvider對象，並在注釋2處調用了它的attachInfo方法：
frameworks/base/core/java/android/content/ContentProvider.java

private void attachInfo(Context context, ProviderInfo info, boolean testing) {
     ...
          ContentProvider.this.onCreate();
      }
  }

在attachInfo方法中調用了onCreate方法，它是一個抽象方法。這樣Content Provider就啟動完畢。
最后給出AMS啟動Content Provider的時序圖。