ContentProvider是一种内容共享型组件，它通过Binder向其他组件乃至其他应用提供数据。当ContentProvider所在的进程启动时，ContentProvider会同事启动并被发布到AMS中。需要注意的是，这个时候ContentProvider的onCreate要先于Application的onCreate而执行，这是四大组件中一个少有的现象。

当一个应用启动时，入口方法为ActivityThread的main方法，main方法是一个静态方法，在main方法中会创建ActivityThread的实例并创建主线程的消息队列，然后再ActivityThread的attach方法中会远程调用AMS的attachApplication方法并将ApplicationThread对象提供给AMS。ApplicationThread是一个Binder对象，它的Binder接口是IApplicationThread，主要用于ActivityThread和AMS之间的通信。在AMS的attachApplication方法中，会调用ApplicationThread的bindApplication方法，bindApplicaiton的逻辑会经过ActivityThread中的Handler切换到ActivityThread中去执行，具体的方法是handleBindApplication，在这个方法中ActivityThread会创建Applicaiton对象并加载ContentProvider。需要注意的是ActivityThread会先加载ContentProvider，然后再调用Applicaiton的onCreate方法。
ContentProvider启动后，外界就可以通过它所提供的增删改查四个接口来操作数据源，即insert、delete、update、query。这四个方法都是通过Binder来调用的，外界无法直接访问ContentProvider，它只能通过AMS根据Uri来获取对应的ContentProvider的Binder接口IContentProvider，然后在通过IContentProvider来访问ContentProvider中的数据源。
一般来说ContentProvider都应该是单实例的。具体是由android:multiprocess属性来决定的，当android:multiprocess为false时，ContentProvider为单实例，这也是默认值，当multiprocess为true时，为多实例，这个时候在每个调用者的进程中都存在一个ContentProvider对象。由于在实际开发中，并未发现多实例的ContentProvider的具体使用场景，因此我们可以简单认为ContentProvider都是单实例的。

启动流程

访问ContentProvider需要通过ContentResolver，ContentResolver是一个抽象类，通过Context的getContentResolver方法获取的实际上是ApplicationContentResolver对象。ApplicationContentResolver类继承了ContentResolver并实现了ContentResolver中的抽象方法。当ContentProvider所在的进程未启动时，第一次访问它时就会触发ContentProvider的创建。通过ContentProvider的四个方法中的任何一个都可以触发ContentProvider的启动过程。这里选择query方法。

ContentResolver.java

@Overridepublic final @Nullable Cursor query(final @RequiresPermission.Read @NonNull Uri uri,@Nullable String[] projection, @Nullable Bundle queryArgs,@Nullable CancellationSignal cancellationSignal) {Objects.requireNonNull(uri, "uri");try {if (mWrapped != null) {return mWrapped.query(uri, projection, queryArgs, cancellationSignal);}} catch (RemoteException e) {return null;}IContentProvider unstableProvider = acquireUnstableProvider(uri);if (unstableProvider == null) {return null;}IContentProvider stableProvider = null;Cursor qCursor = null;try {long startTime = SystemClock.uptimeMillis();ICancellationSignal remoteCancellationSignal = null;if (cancellationSignal != null) {cancellationSignal.throwIfCanceled();remoteCancellationSignal = unstableProvider.createCancellationSignal();cancellationSignal.setRemote(remoteCancellationSignal);}try {qCursor = unstableProvider.query(mPackageName, mAttributionTag, uri, projection,queryArgs, remoteCancellationSignal);} catch (DeadObjectException e) {// The remote process has died...  but we only hold an unstable// reference though, so we might recover!!!  Let's try!!!!// This is exciting!!1!!1!!!!1unstableProviderDied(unstableProvider);stableProvider = acquireProvider(uri);if (stableProvider == null) {return null;}qCursor = stableProvider.query(mPackageName, mAttributionTag, uri, projection,queryArgs, remoteCancellationSignal);}if (qCursor == null) {return null;}// Force query execution.  Might fail and throw a runtime exception here.qCursor.getCount();long durationMillis = SystemClock.uptimeMillis() - startTime;maybeLogQueryToEventLog(durationMillis, uri, projection, queryArgs);// Wrap the cursor object into CursorWrapperInner object.final IContentProvider provider = (stableProvider != null) ? stableProvider: acquireProvider(uri);final CursorWrapperInner wrapper = new CursorWrapperInner(qCursor, provider);stableProvider = null;qCursor = null;return wrapper;} catch (RemoteException e) {// Arbitrary and not worth documenting, as Activity// Manager will kill this process shortly anyway.return null;} finally {if (qCursor != null) {qCursor.close();}if (cancellationSignal != null) {cancellationSignal.setRemote(null);}if (unstableProvider != null) {releaseUnstableProvider(unstableProvider);}if (stableProvider != null) {releaseProvider(stableProvider);}}}

public final IContentProvider acquireUnstableProvider(Uri uri) {if (!SCHEME_CONTENT.equals(uri.getScheme())) {return null;}String auth = uri.getAuthority();if (auth != null) {return acquireUnstableProvider(mContext, uri.getAuthority());}return null;
}

首先会通过acquireUnstableProvider——>acquireProvider方法获取ContentProvider。该方法是个抽象方法，具体实现在ApplicationContentResolver类中，是ContextImpl的一个静态内部类。

 private static final class ApplicationContentResolver extends ContentResolver {...@Override@UnsupportedAppUsageprotected IContentProvider acquireProvider(Context context, String auth) {return mMainThread.acquireProvider(context,ContentProvider.getAuthorityWithoutUserId(auth),resolveUserIdFromAuthority(auth), true);}...

ApplicationContentResolver的acquireProvider方法并没有处理任何逻辑，它直接调用了ActivityThread的acquireProvider方法。

ActivityThread.java

  public final IContentProvider acquireProvider(Context c, String auth, int userId, boolean stable) {//从缓存中获取ContentProvider实例对象final IContentProvider provider = acquireExistingProvider(c, auth, userId, stable);if (provider != null) {return provider;}// There is a possible race here.  Another thread may try to acquire// the same provider at the same time.  When this happens, we want to ensure// that the first one wins.// Note that we cannot hold the lock while acquiring and installing the// provider since it might take a long time to run and it could also potentially// be re-entrant in the case where the provider is in the same process.ContentProviderHolder holder = null;try {//当缓存中没有ContentProvider实例时吗，需要通过AMS来创建一个ContentProvider实例synchronized (getGetProviderLock(auth, userId)) {holder = ActivityManager.getService().getContentProvider(getApplicationThread(), c.getOpPackageName(), auth, userId, stable);}} catch (RemoteException ex) {throw ex.rethrowFromSystemServer();}if (holder == null) {if (UserManager.get(c).isUserUnlocked(userId)) {Slog.e(TAG, "Failed to find provider info for " + auth);} else {Slog.w(TAG, "Failed to find provider info for " + auth + " (user not unlocked)");}return null;}// Install provider will increment the reference count for us, and break// any ties in the race.holder = installProvider(c, holder, holder.info,true /*noisy*/, holder.noReleaseNeeded, stable);return holder.provider;}

上面的代码先会从缓存中查找是否已经存在目标ContentProvider，如果存在就直接返回。ActivityThread通过mProviderMap来存储已经启动的ContentProvider对象。如果ContentProvider没有启动，那么就发送一个进程间请求给AMS让其启动目标ContentProvider，最后再通过installProvider方法来修改引用计数。
ContentProvider被启动时会伴随着进程的启动，在AMS中，首先会启动ContentProvider所在的进程，然后在启动ContentProvider。启动进程是由AMS的startProcessLocked方法来完成的，其内部主要是通过Process的start方法来向Zygote申请fork一个新进程的启动，新进程启动后会反射调用ActivityThread的main方法。

public static void main(String[] args) {Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "ActivityThreadMain");// Install selective syscall interceptionAndroidOs.install();// CloseGuard defaults to true and can be quite spammy.  We// disable it here, but selectively enable it later (via// StrictMode) on debug builds, but using DropBox, not logs.CloseGuard.setEnabled(false);Environment.initForCurrentUser();// Make sure TrustedCertificateStore looks in the right place for CA certificatesfinal File configDir = Environment.getUserConfigDirectory(UserHandle.myUserId());TrustedCertificateStore.setDefaultUserDirectory(configDir);// Call per-process mainline module initialization.initializeMainlineModules();Process.setArgV0("<pre-initialized>");Looper.prepareMainLooper();// Find the value for {@link #PROC_START_SEQ_IDENT} if provided on the command line.// It will be in the format "seq=114"long startSeq = 0;if (args != null) {for (int i = args.length - 1; i >= 0; --i) {if (args[i] != null && args[i].startsWith(PROC_START_SEQ_IDENT)) {startSeq = Long.parseLong(args[i].substring(PROC_START_SEQ_IDENT.length()));}}}ActivityThread thread = new ActivityThread();thread.attach(false, startSeq);if (sMainThreadHandler == null) {sMainThreadHandler = thread.getHandler();}if (false) {Looper.myLooper().setMessageLogging(newLogPrinter(Log.DEBUG, "ActivityThread"));}// End of event ActivityThreadMain.Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);Looper.loop();throw new RuntimeException("Main thread loop unexpectedly exited");}

main方法是个静态方法，首先会创建ActivityThread实例并调用attch来进行一系列的初始化，接着就开始进行消息循环了。ActivityThread的attach方法会将ApplicationThread对象通过AMS的attachApplication方法跨进程传递给AMS，最终AMS会完成ContentProvider的创建。

ActivityThread.java

@UnsupportedAppUsage
private void attach(boolean system, long startSeq) {sCurrentActivityThread = this;mSystemThread = system;if (!system) {android.ddm.DdmHandleAppName.setAppName("<pre-initialized>",UserHandle.myUserId());RuntimeInit.setApplicationObject(mAppThread.asBinder());final IActivityManager mgr = ActivityManager.getService();try {//在AMS中注册ApplicationThreadmgr.attachApplication(mAppThread, startSeq);} catch (RemoteException ex) {throw ex.rethrowFromSystemServer();}// Watch for getting close to heap limit.BinderInternal.addGcWatcher(new Runnable() {@Override public void run() {if (!mSomeActivitiesChanged) {return;}Runtime runtime = Runtime.getRuntime();long dalvikMax = runtime.maxMemory();long dalvikUsed = runtime.totalMemory() - runtime.freeMemory();if (dalvikUsed > ((3*dalvikMax)/4)) {if (DEBUG_MEMORY_TRIM) Slog.d(TAG, "Dalvik max=" + (dalvikMax/1024)+ " total=" + (runtime.totalMemory()/1024)+ " used=" + (dalvikUsed/1024));mSomeActivitiesChanged = false;try {ActivityTaskManager.getService().releaseSomeActivities(mAppThread);} catch (RemoteException e) {throw e.rethrowFromSystemServer();}}}});} else {// Don't set application object here -- if the system crashes,// we can't display an alert, we just want to die die die.android.ddm.DdmHandleAppName.setAppName("system_process",UserHandle.myUserId());try {mInstrumentation = new Instrumentation();mInstrumentation.basicInit(this);ContextImpl context = ContextImpl.createAppContext(this, getSystemContext().mPackageInfo);mInitialApplication = context.mPackageInfo.makeApplication(true, null);mInitialApplication.onCreate();} catch (Exception e) {throw new RuntimeException("Unable to instantiate Application():" + e.toString(), e);}}ViewRootImpl.ConfigChangedCallback configChangedCallback= (Configuration globalConfig) -> {synchronized (mResourcesManager) {// TODO (b/135719017): Temporary log for debugging IME service.if (Build.IS_DEBUGGABLE && mHasImeComponent) {Log.d(TAG, "ViewRootImpl.ConfigChangedCallback for IME, "+ "config=" + globalConfig);}// We need to apply this change to the resources immediately, because upon returning// the view hierarchy will be informed about it.if (mResourcesManager.applyConfigurationToResourcesLocked(globalConfig,null /* compat */)) {updateLocaleListFromAppContext(mInitialApplication.getApplicationContext(),mResourcesManager.getConfiguration().getLocales());// This actually changed the resources! Tell everyone about it.if (mPendingConfiguration == null|| mPendingConfiguration.isOtherSeqNewer(globalConfig)) {mPendingConfiguration = globalConfig;sendMessage(H.CONFIGURATION_CHANGED, globalConfig);}}}};ViewRootImpl.addConfigCallback(configChangedCallback);
}

AMS的attachApplication方法调用了attachApplicationLocked方法，attachApplicationLocked中又调用了ApplicationThread的bindApplication。

thread.bindApplication(processName, appInfo, providerList,instr2.mClass,profilerInfo, instr2.mArguments,instr2.mWatcher,instr2.mUiAutomationConnection, testMode,mBinderTransactionTrackingEnabled, enableTrackAllocation,isRestrictedBackupMode || !normalMode, app.isPersistent(),new Configuration(app.getWindowProcessController().getConfiguration()),app.compat, getCommonServicesLocked(app.isolated),mCoreSettingsObserver.getCoreSettingsLocked(),buildSerial, autofillOptions, contentCaptureOptions,app.mDisabledCompatChanges);

@Override
public final void bindApplication(String processName, ApplicationInfo appInfo,ProviderInfoList providerList, 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 services, Bundle coreSettings,String buildSerial, AutofillOptions autofillOptions,ContentCaptureOptions contentCaptureOptions, long[] disabledCompatChanges) {if (services != null) {if (false) {// Test code to make sure the app could see the passed-in services.for (Object oname : services.keySet()) {if (services.get(oname) == null) {continue; // AM just passed in a null service.}String name = (String) oname;// See b/79378449 about the following exemption.switch (name) {case "package":case Context.WINDOW_SERVICE:continue;}if (ServiceManager.getService(name) == null) {Log.wtf(TAG, "Service " + name + " should be accessible by this app");}}}// Setup the service cache in the ServiceManagerServiceManager.initServiceCache(services);}setCoreSettings(coreSettings);AppBindData data = new AppBindData();data.processName = processName;data.appInfo = appInfo;data.providers = providerList.getList();data.instrumentationName = instrumentationName;data.instrumentationArgs = instrumentationArgs;data.instrumentationWatcher = instrumentationWatcher;data.instrumentationUiAutomationConnection = instrumentationUiConnection;data.debugMode = debugMode;data.enableBinderTracking = enableBinderTracking;data.trackAllocation = trackAllocation;data.restrictedBackupMode = isRestrictedBackupMode;data.persistent = persistent;data.config = config;data.compatInfo = compatInfo;data.initProfilerInfo = profilerInfo;data.buildSerial = buildSerial;data.autofillOptions = autofillOptions;data.contentCaptureOptions = contentCaptureOptions;data.disabledCompatChanges = disabledCompatChanges;sendMessage(H.BIND_APPLICATION, data);
}

ActivityThread的bindApplicaiton会发送一个BIND_APPLICATION类型的消息给handler H，它收到消息后会调用ActivityThread的handleBindActivity方法，完成了Application的创建以及ContentProvider的创建。

创建ContextImpl和Instrumentation
创建Application对象
启动当前进程的ContentProvider并调用其onCreate方法
调用Applicaiton的onCreate方法

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