Android源码解析四大组件系列（八）---广播几个问题的深入理解

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接上篇文章，这篇文章主要是总结前面知识，并且了解一些细节问题，加深对广播机制的理解，比如有播有序是怎么保证有序的？广播拦截机制是怎么实现的？广播发送超时了是怎么处理的？registerReceiver方法发返回值有什么用？粘性广播等等。

Android源码解析四大组件系列（五）—广播的注册过程

Android源码解析四大组件系列（六）—广播的处理过程

Android源码解析四大组件系列（七）—广播的发送过程

1、广播相关数据结构的再次理解

ReceiverDispatcher: 客户端广播分发者对象，第一篇讲的很清楚了，ReceiverDispatcher的内部类InnerReceiver为binder对象，用于与AMS的传递与通信。
ReceiverList: 继承自ArrayList，存放了Receiver的binder对象以及其注册的BroadcastFilter列表。AMS中定义了
final HashMap

２、有序广播是怎么保证有序的

上一篇文章中说了processNextBroadcast()只会处理一个BroadcastRecord的一个receiver，那怎么将广播传递给下一个receiver呢？广播接受者有“动态”和“静态”之分，广播消息也有“串行”和“并行”之分，或者叫“有序”和“无序”之分。广播的处理方式跟广播的接收者和广播消息类型有关系。有序广播是怎么保证有序的这个问题，得分情况讨论，对于动态注册的receiver，先回到最终onReceive回调的地方，分析如下：

 static final class ReceiverDispatcher {.....final class Args extends BroadcastReceiver.PendingResult implements Runnable {.....public Args(Intent intent, int resultCode, String resultData, Bundle resultExtras,boolean ordered, boolean sticky, int sendingUser) {//mRegistered传进来的是truesuper(resultCode, resultData, resultExtras,mRegistered ? TYPE_REGISTERED : TYPE_UNREGISTERED, ordered,sticky, mIIntentReceiver.asBinder(), sendingUser, intent.getFlags());mCurIntent = intent;mOrdered = ordered;}public void run() {.....try {ClassLoader cl =  mReceiver.getClass().getClassLoader();intent.setExtrasClassLoader(cl);intent.prepareToEnterProcess();setExtrasClassLoader(cl);receiver.setPendingResult(this);//广播的onReceive方法回调receiver.onReceive(mContext, intent);} catch (Exception e) {if (mRegistered && ordered) {if (ActivityThread.DEBUG_BROADCAST) Slog.i(ActivityThread.TAG,"Finishing failed broadcast to " + mReceiver);sendFinished(mgr);}if (mInstrumentation == null ||!mInstrumentation.onException(mReceiver, e)) {Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);throw new RuntimeException("Error receiving broadcast " + intent+ " in " + mReceiver, e);}}if (receiver.getPendingResult() != null) {finish();}Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);}}}

因为在调用onReceive之前，执行了 receiver.setPendingResult(this)，所以在下面receiver.getPendingResult()就不是null,则就进入BroadcastReceiver的内部类PendingResult的finish方法。

 public final void finish() {if (mType == TYPE_COMPONENT) {final IActivityManager mgr = ActivityManagerNative.getDefault();if (QueuedWork.hasPendingWork()) {......QueuedWork.singleThreadExecutor().execute( new Runnable() {@Override public void run() {if (ActivityThread.DEBUG_BROADCAST) Slog.i(ActivityThread.TAG,"Finishing broadcast after work to component " + mToken);sendFinished(mgr);}});} else {if (ActivityThread.DEBUG_BROADCAST) Slog.i(ActivityThread.TAG,"Finishing broadcast to component " + mToken);sendFinished(mgr);}} else if (mOrderedHint && mType != TYPE_UNREGISTERED) {if (ActivityThread.DEBUG_BROADCAST) Slog.i(ActivityThread.TAG,"Finishing broadcast to " + mToken);final IActivityManager mgr = ActivityManagerNative.getDefault();sendFinished(mgr);}}

finish方法中根据mType的值有两个分支。mType是PendingResult的成员变量，在PendingResult的构造函数中进行赋值的。

     public PendingResult(int resultCode, String resultData, Bundle resultExtras, int type,boolean ordered, boolean sticky, IBinder token, int userId, int flags) {mResultCode = resultCode;mResultData = resultData;mResultExtras = resultExtras;mType = type;mOrderedHint = ordered;mInitialStickyHint = sticky;mToken = token;mSendingUser = userId;mFlags = flags;}

这个构造方法是在BroadcastReceiver.PendingResult的子类Args中调用的

 final class Args extends BroadcastReceiver.PendingResult implements Runnable {private Intent mCurIntent;private final boolean mOrdered;private boolean mDispatched;public Args(Intent intent, int resultCode, String resultData, Bundle resultExtras,boolean ordered, boolean sticky, int sendingUser) {super(resultCode, resultData, resultExtras,mRegistered ? TYPE_REGISTERED : TYPE_UNREGISTERED, ordered,sticky, mIIntentReceiver.asBinder(), sendingUser, intent.getFlags());mCurIntent = intent;mOrdered = ordered;}
}

由于mRegistered是动态注册广播接收者传进来的，值是true,所以上面mType的值是TYPE_REGISTERED，由于是有序广播ordered值是true，那么mOrderedHint为true,所以要走第二个分支：

   if (ActivityThread.DEBUG_BROADCAST) Slog.i(ActivityThread.TAG,"Finishing broadcast to " + mToken);final IActivityManager mgr = ActivityManagerNative.getDefault();sendFinished(mgr);

BroadcastReceiver的sendFinished方法如下：

 public void sendFinished(IActivityManager am) {synchronized (this) {if (mFinished) {throw new IllegalStateException("Broadcast already finished");}mFinished = true;try {if (mResultExtras != null) {mResultExtras.setAllowFds(false);}if (mOrderedHint) {am.finishReceiver(mToken, mResultCode, mResultData, mResultExtras,mAbortBroadcast, mFlags);} else {// This broadcast was sent to a component; it is not ordered,// but we still need to tell the activity manager we are done.am.finishReceiver(mToken, 0, null, null, false, mFlags);}} catch (RemoteException ex) {}}}

有序广播mOrderedHint值为true,所以进入到AMS的finishReceiver方法。

public void finishReceiver(IBinder who, int resultCode, String resultData,Bundle resultExtras, boolean resultAbort, int flags) {if (DEBUG_BROADCAST) Slog.v(TAG_BROADCAST, "Finish receiver: " + who);// Refuse possible leaked file descriptorsif (resultExtras != null && resultExtras.hasFileDescriptors()) {throw new IllegalArgumentException("File descriptors passed in Bundle");}final long origId = Binder.clearCallingIdentity();try {boolean doNext = false;BroadcastRecord r;synchronized(this) {BroadcastQueue queue = (flags & Intent.FLAG_RECEIVER_FOREGROUND) != 0? mFgBroadcastQueue : mBgBroadcastQueue;r = queue.getMatchingOrderedReceiver(who);if (r != null) {doNext = r.queue.finishReceiverLocked(r, resultCode,resultData, resultExtras, resultAbort, true);}}if (doNext) {//再次执行processNextBroadcast处理广播r.queue.processNextBroadcast(false);}trimApplications();} finally {Binder.restoreCallingIdentity(origId);}}

上面是分析了动态的广播接收者是怎么按照一个接着一个处理的。在看看静态注册的receiver，回到静态广播回调onReceive方法的地方。

private void handleReceiver(ReceiverData data) {....IActivityManager mgr = ActivityManagerNative.getDefault();BroadcastReceiver receiver;try {java.lang.ClassLoader cl = packageInfo.getClassLoader();data.intent.setExtrasClassLoader(cl);data.intent.prepareToEnterProcess();data.setExtrasClassLoader(cl);//反射出BroadcastReceiverreceiver = (BroadcastReceiver)cl.loadClass(component).newInstance();} catch (Exception e) {....}try {Application app = packageInfo.makeApplication(false, mInstrumentation);....ContextImpl context = (ContextImpl)app.getBaseContext();sCurrentBroadcastIntent.set(data.intent);receiver.setPendingResult(data);//回调广播的onReceive方法receiver.onReceive(context.getReceiverRestrictedContext(),data.intent);} catch (Exception e) {....}} finally {sCurrentBroadcastIntent.set(null);}if (receiver.getPendingResult() != null) {data.finish();}}

在回调onReceiver方法之前，执行了 receiver.setPendingResult(data)，所以下面receiver.getPendingResult() != null成立，走 data.finish()，data是ReceiverData对象，handleReceiver方法传进来的，在scheduleReceiver方法中初始化。

  public final void scheduleReceiver(Intent intent, ActivityInfo info,CompatibilityInfo compatInfo, int resultCode, String data, Bundle extras,boolean sync, int sendingUser, int processState) {updateProcessState(processState, false);ReceiverData r = new ReceiverData(intent, resultCode, data, extras,sync, false, mAppThread.asBinder(), sendingUser);r.info = info;r.compatInfo = compatInfo;sendMessage(H.RECEIVER, r);}

我们看 data.finish()方法

  public final void finish() {if (mType == TYPE_COMPONENT) {final IActivityManager mgr = ActivityManagerNative.getDefault();if (QueuedWork.hasPendingWork()) {QueuedWork.singleThreadExecutor().execute( new Runnable() {@Override public void run() {if (ActivityThread.DEBUG_BROADCAST) Slog.i(ActivityThread.TAG,"Finishing broadcast after work to component " + mToken);sendFinished(mgr);}});} else {if (ActivityThread.DEBUG_BROADCAST) Slog.i(ActivityThread.TAG,"Finishing broadcast to component " + mToken);sendFinished(mgr);}} else if (mOrderedHint && mType != TYPE_UNREGISTERED) {if (ActivityThread.DEBUG_BROADCAST) Slog.i(ActivityThread.TAG,"Finishing broadcast to " + mToken);final IActivityManager mgr = ActivityManagerNative.getDefault();sendFinished(mgr);}}

此时mType分析后值是TYPE_COMPONENT，同样会走sendFinished，后面AMS的处理逻辑是一样的，不赘述。

３、广播超时是怎么处理的?

AMS维护了两个广播队列BroadcastQueue，mFgBroadcastQueue，前台队列的超时时间是10秒，mBgBroadcastQueue，后台队列的超时时间是60秒，如果广播没有在规定的时间内处理完就会发生ANR,如果你想你的广播进入前台广播队列，那么在发送的时候，在intent中加入Intent.FLAG_RECEIVER_FOREGROUND标记，如果不加，系统默认是后台广播。mFgBroadcastQueue会有更高的权限，被优先处理。

在processNextBroadcast方法中有下面一段代码，与广播超时有关系，一旦超时就会出现ANR。

do {int numReceivers = (r.receivers != null) ? r.receivers.size() : 0;if (mService.mProcessesReady && r.dispatchTime > 0) {long now = SystemClock.uptimeMillis();//广播消息的第一个ANR监测机制if ((numReceivers > 0) &&(now > r.dispatchTime + (2*mTimeoutPeriod*numReceivers))) {Slog.w(TAG, "Hung broadcast ["+ mQueueName + "] discarded after timeout failure:"+ " now=" + now+ " dispatchTime=" + r.dispatchTime+ " startTime=" + r.receiverTime+ " intent=" + r.intent+ " numReceivers=" + numReceivers+ " nextReceiver=" + r.nextReceiver+ " state=" + r.state);broadcastTimeoutLocked(false); // 超时处理forceReceive = true;r.state = BroadcastRecord.IDLE;}}//判断广播有没有处理完毕if (r.receivers == null || r.nextReceiver >= numReceivers|| r.resultAbort || forceReceive) {// No more receivers for this broadcast!  Send the final// result if requested...if (r.resultTo != null) {try {     performReceiveLocked(r.callerApp, r.resultTo, new Intent(r.intent), r.resultCode r.resultData, r.resultExtras, false, false, r.userId);    r.resultTo = null;} catch (RemoteException e) {......}}} while (r == null);

广播的超时机制是针对有序广播来说的，无序广播一次性全部处理了，肯定不会超时，超时的这段逻辑都在broadcastTimeoutLocked中，首先判断是否超时，公式：r.dispatchTime + 2×mTimeoutPeriod×numReceivers，现在解释一下这几个时间：
- dispatchTime的意义是标记实际处理BroadcastRecord的起始时间，有序广播是一个接着一个进行处理的，第一次dispatchTime=0,并不会进入该条件判断

mTimeoutPeriod由当前BroadcastQueue的类型决定(mFgBroadcastQueue为10秒，mBgBroadcastQueue为60秒)

   // How long we allow a receiver to run before giving up on it.static final int BROADCAST_FG_TIMEOUT = 10*1000;static final int BROADCAST_BG_TIMEOUT = 60*1000;mFgBroadcastQueue = new BroadcastQueue(this, mHandler,  "foreground", BROADCAST_FG_TIMEOUT, false);mBgBroadcastQueue = new BroadcastQueue(this, mHandler,  "background", BROADCAST_BG_TIMEOUT, true);

所以上面公式翻译过来就是：实际处理BroadcastRecord的起始时间＋广播默认的超时时间*广播接收者的数量。话说回来，这个公式为什么要这么设计呢？如果一个前台的广播消息有两个接收者，那么在２0秒（２ x 10）之内搞定就可以了，也可能第一个消息执行了15秒，第二个消息执行４.99秒，即使第一消息超过了10秒的规定，也不会出现ＡＮＲ。但是系统任务繁忙，可能有其他活要干，我们要尽可能的减少ANR的发生，所以前面乘以２倍。

假设现在广播超时还没处理，满足if条件，就会进入，打印Hung broadcast [“+ mQueueName + “] discarded after timeout failure….的log，然后执行 broadcastTimeoutLocked(false)强制停止广播，broadcastTimeoutLocked相关代码代码如下：

    final void broadcastTimeoutLocked(boolean fromMsg) {.....long timeoutTime = r.receiverTime + mTimeoutPeriod;if (timeoutTime > now) {if (DEBUG_BROADCAST) Slog.v(TAG_BROADCAST,"Premature timeout ["+ mQueueName + "] @ " + now + ": resetting BROADCAST_TIMEOUT_MSG for "+ timeoutTime);setBroadcastTimeoutLocked(timeoutTime);return;}}.....}

内部调用setBroadcastTimeoutLocked()设置一个延迟消息

 final void setBroadcastTimeoutLocked(long timeoutTime) {if (! mPendingBroadcastTimeoutMessage) {Message msg = mHandler.obtainMessage(BROADCAST_TIMEOUT_MSG, this);mHandler.sendMessageAtTime(msg, timeoutTime);mPendingBroadcastTimeoutMessage = true;}}

如果广播消息能够处理完毕，就会执行cancelBroadcastTimeoutLocked，将超时的Message移除掉。

final void cancelBroadcastTimeoutLocked() {if (mPendingBroadcastTimeoutMessage) {mHandler.removeMessages(BROADCAST_TIMEOUT_MSG, this);mPendingBroadcastTimeoutMessage = false;}

如果广播消息没有在timeout时间内处理掉，下面BroadcastHandler发送的消息就会执行。

private final class BroadcastHandler extends Handler {.....@Overridepublic void handleMessage(Message msg) {switch (msg.what) {.....case BROADCAST_TIMEOUT_MSG: {synchronized (mService) {broadcastTimeoutLocked(true);}} break;.....}}}

再次进入broadcastTimeoutLocked方法里面

 final void broadcastTimeoutLocked(boolean fromMsg) {//传进来是ture if (fromMsg) {mPendingBroadcastTimeoutMessage = false;}//队列没有广播处理了，返回if (mOrderedBroadcasts.size() == 0) {return;}long now = SystemClock.uptimeMillis();BroadcastRecord r = mOrderedBroadcasts.get(0);if (fromMsg) {//正在执行dexopt,返回if (mService.mDidDexOpt) {// Delay timeouts until dexopt finishes.mService.mDidDexOpt = false;long timeoutTime = SystemClock.uptimeMillis() + mTimeoutPeriod;setBroadcastTimeoutLocked(timeoutTime);return;}//系统还没有进入ready状态if (!mService.mProcessesReady) {// Only process broadcast timeouts if the system is ready. That way// PRE_BOOT_COMPLETED broadcasts can't timeout as they are intended// to do heavy lifting for system up.return;}//如果当前正在执行的receiver没有超时，则重新设置广播超时long timeoutTime = r.receiverTime + mTimeoutPeriod;if (timeoutTime > now) {// We can observe premature timeouts because we do not cancel and reset the// broadcast timeout message after each receiver finishes.  Instead, we set up// an initial timeout then kick it down the road a little further as needed// when it expires.if (DEBUG_BROADCAST) Slog.v(TAG_BROADCAST,"Premature timeout ["+ mQueueName + "] @ " + now + ": resetting BROADCAST_TIMEOUT_MSG for "+ timeoutTime);setBroadcastTimeoutLocked(timeoutTime);return;}}//当前正在执行的receiver没有超时，则重新设置广播超时，处理下一条广播BroadcastRecord br = mOrderedBroadcasts.get(0);if (br.state == BroadcastRecord.WAITING_SERVICES) {// In this case the broadcast had already finished, but we had decided to wait// for started services to finish as well before going on.  So if we have actually// waited long enough time timeout the broadcast, let's give up on the whole thing// and just move on to the next.Slog.i(TAG, "Waited long enough for: " + (br.curComponent != null? br.curComponent.flattenToShortString() : "(null)"));br.curComponent = null;br.state = BroadcastRecord.IDLE;processNextBroadcast(false);return;}Slog.w(TAG, "Timeout of broadcast " + r + " - receiver=" + r. receiver+ ", started " + (now - r.receiverTime) + "ms ago");r.receiverTime = now;r.anrCount++;// Current receiver has passed its expiration date.if (r.nextReceiver <= 0) {Slog.w(TAG, "Timeout on receiver with nextReceiver <= 0");return;}ProcessRecord app = null;String anrMessage = null;Object curReceiver = r.receivers.get(r.nextReceiver-1);r.delivery[r.nextReceiver-1] = BroadcastRecord.DELIVERY_TIMEOUT;Slog.w(TAG, "Receiver during timeout: " + curReceiver);logBroadcastReceiverDiscardLocked(r);if (curReceiver instanceof BroadcastFilter) {BroadcastFilter bf = (BroadcastFilter)curReceiver;if (bf.receiverList.pid != 0&& bf.receiverList.pid != ActivityManagerService.MY_PID) {synchronized (mService.mPidsSelfLocked) {app = mService.mPidsSelfLocked.get(bf.receiverList.pid);}}} else {app = r.curApp;}//进程存在，anrMessage赋值if (app != null) {anrMessage = "Broadcast of " + r.intent.toString();}if (mPendingBroadcast == r) {mPendingBroadcast = null;}// Move on to the next receiver.finishReceiverLocked(r, r.resultCode, r.resultData,r.resultExtras, r.resultAbort, false);//处理下一条广播scheduleBroadcastsLocked();if (anrMessage != null) {// Post the ANR to the handler since we do not want to process ANRs while// potentially holding our lock.mHandler.post(new AppNotResponding(app, anrMessage));}}

所以当一个receiver超时后，系统会放弃继续处理它给出ANR提示，并再次调用scheduleBroadcastsLocked()，尝试处理下一个receiver，

private final class AppNotResponding implements Runnable {private final ProcessRecord mApp;private final String mAnnotation;public AppNotResponding(ProcessRecord app, String annotation) {mApp = app;mAnnotation = annotation;}@Overridepublic void run() {//内部创建ANR显示的DialogmService.mAppErrors.appNotResponding(mApp, null, null, false, mAnnotation);}}

４、广播拦截处理分析

广播消息可以有多个接收者，对于有序广播是一个接着一个处理的，优先级高的接收者可以优先执行，并且可以调用BroadcastReceiver的abortBroadcast()方法拦截广播，如果我们在receiver的onReceive()中调用这个方法，那么它后面的接收者就不会收到广播。

public abstract class BroadcastReceiver {private PendingResult mPendingResult;public final void abortBroadcast() {checkSynchronousHint();mPendingResult.mAbortBroadcast = true;}}

把BroadcastReceiver::PendingResult的成员变量mAbortBroadcast设置成true，

 final class Args extends BroadcastReceiver.PendingResult implements Runnable {private Intent mCurIntent;private final boolean mOrdered;private boolean mDispatched;public Args(Intent intent, int resultCode, String resultData, Bundle resultExtras,boolean ordered, boolean sticky, int sendingUser) {super(resultCode, resultData, resultExtras,mRegistered ? TYPE_REGISTERED : TYPE_UNREGISTERED, ordered,sticky, mIIntentReceiver.asBinder(), sendingUser, intent.getFlags());mCurIntent = intent;mOrdered = ordered;}public void run() {.....try {ClassLoader cl =  mReceiver.getClass().getClassLoader();intent.setExtrasClassLoader(cl);intent.prepareToEnterProcess();setExtrasClassLoader(cl);//设置PendingResult，这个PendingResult中mAbortBroadcast为truereceiver.setPendingResult(this);receiver.onReceive(mContext, intent);} catch (Exception e) {.....}if (receiver.getPendingResult() != null) {//告知AMS处理下一个广播finish();}}}

finish()会告知AMS处理下一个广播，在第一小节已经分析过，最终进入AMS的finishReceiver方法

  public void finishReceiver(IBinder who, int resultCode, String resultData,Bundle resultExtras, boolean resultAbort, int flags) {.....try {boolean doNext = false;BroadcastRecord r;synchronized(this) {BroadcastQueue queue = (flags & Intent.FLAG_RECEIVER_FOREGROUND) != 0? mFgBroadcastQueue : mBgBroadcastQueue;r = queue.getMatchingOrderedReceiver(who);if (r != null) {//resultAbort传进来是true，doNext = r.queue.finishReceiverLocked(r, resultCode,resultData, resultExtras, resultAbort, true);}}//调用processNextBroadcast处理广播if (doNext) {r.queue.processNextBroadcast(false);}trimApplications();} finally {Binder.restoreCallingIdentity(origId);}}```  
processNextBroadcast方法中有一个检查广播有没有发送完毕的逻辑。```  do {.....r = mOrderedBroadcasts.get(0);//检查广播有没有发送完，resultAbort为=tureif (r.receivers == null || r.nextReceiver >= numReceivers|| r.resultAbort || forceReceive) {.....//mOrderedBroadcasts里删除广播消息mOrderedBroadcasts.remove(0);r = null;looped = true;continue;}} while (r == null);

当resultAbort为=ture时候，广播消息从mOrderedBroadcasts删除，后面也就收不到广播了。

５、理解粘性广播

sticky广播通过Context.sendStickyBroadcast()函数来发送，用此函数发送的广播会一直滞留，当有匹配此广播的广播接收器被注册后，该广播接收器就会收到此条信息。使用此函数需要发送广播时，需要获得BROADCAST_STICKY权限。粘性广播可以使用广播接收器进行接收，但是正确的接收方式是调用registerReceiver能接受广播，信息将在调用registerReceiver的返回值中给出。对于粘性广播的发送，和普通广播的发送方式是一致的，例子来自与Android 粘性广播StickyBroadcast的使用

private void sendStickyBroadcast(){Intent i = new Intent(); i.setAction(StickyBroadcastReceiver.Action); i.putExtra("info", "sticky broadcast has been receiver"); sendStickyBroadcast(i);Log.i("Other","sticky broadcast send ok!"); 
}

可以使用BroadcastReceiver来接收

public class StickyBroadcastReceiver extends BroadcastReceiver {@Overridepublic void onReceive(Context context, Intent intent) {//收到广播}
}

<!--使用粘性广播发送权限-->
<uses-permission android:name="android.permission.BROADCAST_STICKY" />

IntentFilter intentFilter = new IntentFilter(StickyBroadcastReceiver.Action);Intent data = registerReceiver(null, intentFilter);if(data!=null&&StickyBroadcastReceiver.Action.equals(data.getAction()))  {Toast.makeText(this, data.getStringExtra("info"), Toast.LENGTH_SHORT).show();
}