Android深入四大组件（五）Content Provider的启动过程

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Android深入理解四大组件系列

前言

Content Provider做为四大组件之一，通常情况下并没有其他的组件使用频繁，但这不能作为我们不去深入学习它的理由。关于Content Provider一篇文章是写不完的，这一篇文章先来介绍它的启动过程。

1.query方法到AMS的调用过程

在Android IPC机制（四）用ContentProvider进行进程间通信这篇文章我举了一个Content Provider使用的例子，在Activity中我是使用如下代码调用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", "大航海时代ol");        mContentValues.put("describe", "最好玩的航海网游");        getContentResolver().insert(uri, mContentValues);//1        Cursor gameCursor = getContentResolver().query(uri, new String[]{"name", "describe"}, null, null, null);     ...    }}

要想调用Content Provider，首先需要使用注释1处的getContentResolver方法，如下所示。
frameworks/base/core/Java/android/content/ContextWrapper.java

Overridepublic ContentResolver getContentResolver() {    return mBase.getContentResolver();}

这里mBase指的是ContextImpl，ContextImpl的getContentResolver方法如下所示。

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

@Overridepublic ContentResolver getContentResolver() {    return mContentResolver;}

上面的代码return了ApplicationContentResolver类型的mContentResolver对象，ApplicationContentResolver是ContextImpl中的静态内部类，继承自ContentResolver，它在ContextImpl的构造方法中被创建。
当我们调用ContentResolver的insert、query、update等方法时就会启动Content Provider，这里拿query方法来进行举例。
query方法的实现在ApplicationContentResolver的父类ContentResolver中，代码如下所示。
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) {               ...            } ...    }

在注释1处通过acquireUnstableProvider方法返回IContentProvider类型的unstableProvider对象，在注释2处调用unstableProvider的query方法。我们查看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;    }

注释1处用来检查Uri的scheme是否等于”content”，如果不是则返回null。注释2处调用了acquireUnstableProvider方法，这是个抽象方法，它的实现在ContentResolver的子类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类型的mMainThread对象的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;    }

注释1处检查ActivityThread中的ArrayMap类型的mProviderMap中是否有目标ContentProvider存在，有则返回，没有就会在注释2处调用AMP的getContentProvider方法，最终会调用AMS的getContentProvider方法。注释3处的installProvider方法用来将注释2处返回的ContentProvider相关的数据存储在mProviderMap中，起到缓存的作用，这样使用相同的Content Provider时，就不需要每次都要调用AMS的getContentProvider方法。使用我们接着查看AMS的getContentProvider方法，代码如下所示。
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方法的代码很多，这里截取了关键的部分。注释1处通过getProcessRecordLocked方法来获取目标ContentProvider的应用程序进程信息，这些信息用ProcessRecord类型的proc来表示，如果该应用进程已经启动就会调用注释2处的代码，否则就会调用注释3的startProcessLocked方法来启动进程。这里我们假设ContentProvider的应用进程还没有启动，关于应用进程启动过程，我在Android应用程序进程启动过程（前篇）已经讲过，最终会调用ActivityThread的main方法，代码如下所示。
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");    }

注释1处通过prepareMainLooper方法在ThreadLocal中获取Looper，并在注释3处开启消息循环。在注释2处创建了ActivityThread并调用了它的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();            }  ...          }

注释1处最终会得到AMS，在注释2处调用AMS的attachApplication方法，并将ApplicationThread类型的mAppThread对象传进去。
query方法到AMS的调用过程，如下面时序图所示（省略应用程序进程启动过程）。

2.AMS启动Content Provider的过程

我们接着来查看AMS的attachApplication方法，如下所示。
frameworks/base/services/core/java/com/android/server/am/ActivityManagerService.java

@Overridepublic final void attachApplication(IApplicationThread thread) {    synchronized (this) {        int callingPid = Binder.getCallingPid();        final long origId = Binder.clearCallingIdentity();        attachApplicationLocked(thread, callingPid);        Binder.restoreCallingIdentity(origId);    }}

attachApplication方法中又调用了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方法中调用了thread的bindApplication方法，thread是IApplicationThread类型的，从类型名字就可以看出来是用于进程间通信，这里实现bindApplication方法的是ApplicationThreadProxy类，它实现了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();    }...}

到目前为止，上面的调用过程还是在AMS进程中执行的，因此，需要通过IBinder类型的mRemote对象向新创建的应用程序进程（目标Content Provider所在的进程）发送BIND_APPLICATION_TRANSACTION类型的通信请求。处理这个通信请求的是在新创建的应用程序进程中执行的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);        }

调用sendMessage方法像H发送BIND_APPLICATION类型消息，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;  ...  }  ... }

我们接着查看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方法的代码很长，这里截取了主要的部分。注释1处创建了ContextImpl 。注释2处通过反射创建Instrumentation并在注释3处初始化Instrumentation。注释4处创建Application并且在注释6处调用Application的onCreate方法，这意味着Content Provider所在的应用程序进程已经启动完毕，在这之前，注释5处调用installContentProviders方法来启动Content Provider，代码如下所示。
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();    }}

注释1处遍历当前应用程序进程的ProviderInfo列表，得到每个Content Provider的ProviderInfo（存储Content Provider的信息），并在注释2处调用installProvider方法来启动这些Content Provider。在注释3处通过AMS的publishContentProviders方法将这些Content Provider存储在AMS的mProviderMap中，这个mProviderMap在前面提到过，起到缓存的作用，防止每次使用相同的Content Provider时都会调用AMS的getContentProvider方法。来查看installProvider方法时如何启动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;    }

在注释1处通过反射来创建ContentProvider类型的localProvider对象，并在注释2处调用了它的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方法中调用了onCreate方法，它是一个抽象方法。这样Content Provider就启动完毕。
最后给出AMS启动Content Provider的时序图。

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