Android深刻四大組件（五）Content Provider的啓動過程

時間  2019-11-29
標籤 android 深刻 四大 組件 content provider 啓動 過程 欄目 Android 简体版
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前言

Content Provider作爲四大組件之一，一般狀況下並無其餘的組件使用頻繁，但這不能做爲咱們不去深刻學習它的理由。關於Content Provider一篇文章是寫不完的，這一篇文章先來介紹它的啓動過程。java

1.query方法到AMS的調用過程

在Android IPC機制（四）用ContentProvider進行進程間通訊這篇文章我舉了一個Content Provider使用的例子，在Activity中我是使用以下代碼調用Content Provider的：android

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);
     ...
    }
}複製代碼

要想調用Content Provider，首先須要使用註釋1處的getContentResolver方法，以下所示。
frameworks/base/core/Java/android/content/ContextWrapper.java緩存

@Override
public ContentResolver getContentResolver() {
    return mBase.getContentResolver();
}複製代碼

這裏mBase指的是ContextImpl，ContextImpl的getContentResolver方法以下所示。微信

frameworks/base/core/java/android/app/ContextImpl.javaapp

@Override
public ContentResolver getContentResolver() {
    return mContentResolver;
}複製代碼

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的構造方法中被建立。
當咱們調用ContentResolver的insert、query、update等方法時就會啓動Content Provider，這裏拿query方法來進行舉例。
query方法的實如今ApplicationContentResolver的父類ContentResolver中，代碼以下所示。
frameworks/base/core/java/android/content/ContentResolver.javaide

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) {
               ...
            }
    ...
   }複製代碼

在註釋1處經過acquireUnstableProvider方法返回IContentProvider類型的unstableProvider對象，在註釋2處調用unstableProvider的query方法。咱們查看acquireUnstableProvider方法作了什麼，以下所示。
frameworks/base/core/java/android/content/ContentResolver.javaoop

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;
    }複製代碼

註釋1處用來檢查Uri的scheme是否等於"content"，若是不是則返回null。註釋2處調用了acquireUnstableProvider方法，這是個抽象方法，它的實如今ContentResolver的子類ApplicationContentResolver中：
frameworks/base/core/java/android/app/ContextImpl.java學習

@Override
    protected IContentProvider acquireUnstableProvider(Context c, String auth) {
        return mMainThread.acquireProvider(c,
                ContentProvider.getAuthorityWithoutUserId(auth),
                resolveUserIdFromAuthority(auth), false);
    }複製代碼

return了ActivityThread類型的mMainThread對象的acquireProvider方法：
frameworks/base/core/java/android/app/ActivityThread.javaui

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;
    }複製代碼

註釋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，沒有就會在註釋2處調用AMP的getContentProvider方法，最終會調用AMS的getContentProvider方法。註釋3處的installProvider方法用來將註釋2處返回的ContentProvider相關的數據存儲在mProviderMap中，起到緩存的做用，這樣使用相同的Content Provider時，就不須要每次都要調用AMS的getContentProvider方法。使用咱們接着查看AMS的getContentProvider方法，代碼以下所示。
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);
    }複製代碼

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");
                          ...
                        }
             ...           

}複製代碼

getContentProviderImpl方法的代碼不少，這裏截取了關鍵的部分。註釋1處經過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，若是該應用進程已經啓動就會調用註釋2處的代碼，不然就會調用註釋3的startProcessLocked方法來啓動進程。這裏咱們假設ContentProvider的應用進程尚未啓動，關於應用進程啓動過程，我在Android應用程序進程啓動過程（前篇）已經講過，最終會調用ActivityThread的main方法，代碼以下所示。
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");
    }複製代碼

註釋1處經過prepareMainLooper方法在ThreadLocal中獲取Looper，並在註釋3處開啓消息循環。在註釋2處建立了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();
            }
  ...          
}複製代碼

註釋1處最終會獲得AMS，在註釋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，以下面時序圖所示（省略應用程序進程啓動過程）。

2.AMS啓動Content Provider的過程

咱們接着來查看AMS的attachApplication方法，以下所示。
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);
    }
}複製代碼

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());
...
}複製代碼

attachApplicationLocked方法中調用了thread的bindApplication方法，thread是IApplicationThread類型的，從類型名字就能夠看出來是用於進程間通訊，這裏實現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();
    }
...
}複製代碼

到目前爲止，上面的調用過程仍是在AMS進程中執行的，所以，須要經過IBinder類型的mRemote對象向新建立的應用程序進程（目標Content Provider所在的進程）發送BIND_APPLICATION_TRANSACTION類型的通訊請求。處理這個通訊請求的是在新建立的應用程序進程中執行的ApplicationThread的bindApplication方法，以下所示。
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);
        }複製代碼

調用sendMessage方法像H發送BIND_APPLICATION類型消息，H的handleMessage方法以下所示。
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;
  ...
  }
  ... 
}複製代碼

咱們接着查看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
          ... 
  }複製代碼

handleBindApplication方法的代碼很長，這裏截取了主要的部分。註釋1處建立了ContextImpl 。註釋2處經過反射建立Instrumentation並在註釋3處初始化Instrumentation。註釋4處建立Application而且在註釋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啓動完畢，在這以前，註釋5處調用installContentProviders方法來啓動Content Provider，代碼以下所示。
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();
    }
}複製代碼

註釋1處遍歷當前應用程序進程的ProviderInfo列表，獲得每一個Content Provider的ProviderInfo（存儲Content Provider的信息），並在註釋2處調用installProvider方法來啓動這些Content Provider。在註釋3處經過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，起到緩存的做用，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方法時如何啓動Content Provider的，installProvider方法以下所示。
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;
    }複製代碼

在註釋1處經過反射來建立ContentProvider類型的localProvider對象，並在註釋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();
        }
    }複製代碼

在attachInfo方法中調用了onCreate方法，它是一個抽象方法。這樣Content Provider就啓動完畢。
最後給出AMS啓動Content Provider的時序圖。

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