源码基于Android 11 API 30

BroadcastReceiver是一种消息型组件，用于在不同的组件或者是不同的应用之间传递信息。同样的BroadcastReceiver也不能被用户感知，BroadcastReceiver也叫广播，广播的注册有两种方式：静态注册和动态注册。静态注册是指在AndroidManifest中注册广播，这种广播在应用安装时会被系统解析，此种形式的广播不需要应用启动就可以收到相应的广播。动态注册广播需要通过Context.registerReceiver()来实现，并且在不需要的时候要通过Context.unRegisterReceiver()来解除广播，此种形态的广播必须要应用启动才能注册并接收广播，因为应用不启动就无法注册广播，无法注册广播就无法收到相应的广播。在实际开发中通过Context的一系列send方法来发送广播，被发送的广播会被系统发送给感兴趣的广播接收者，发送和接收过程的匹配是通过广播接收者的<intent-filter>来描述的。可以发现，BroadcastReceiver组件可以用来实现低耦合的观察者模式，观察者和被观察者之间可以没有任何耦合。由于BroadcastReceiver的特性，它不适合用来执行耗时操作。BroadcastReceiver组件一般来说不需要停止，它也没有停止的概念。
下面从广播的注册、发送与接收几个过程展开介绍。

1. 广播的注册

广播的注册分为动态注册与静态注册两种。静态注册的注册过程是在应用安装时由PackageManager-Service完成注册的。我们这里主要介绍动态注册的过程。下面是一个典型的动态注册、发送于接受广播的代码：

...
    //通常需要自定义一个BroadcastReceiver，这里只是示例避免代码过长
    BroadcastReceiver broadcastReceiver = new BroadcastReceiver() {
        @Override
        public void onReceive(Context context, Intent intent) {
            //Do something
            abortBroadcast();
        }
    };
    IntentFilter intentFilter;
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_broadcast);
        intentFilter = new IntentFilter();
        intentFilter.addAction("com.example.myapplication.DEMOBROADCAST");
        registerReceiver(broadcastReceiver,intentFilter);

        Button button = findViewById(R.id.button);
        button.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                Intent intent = new Intent("com.example.myapplication.DEMOBROADCAST");
                sendBroadcast(intent);
            }
        });
    }
...

上面的代码可以明显的看到，广播的着册方法是registerReceiver，这个方法是在ContextWrapper中定义的，与Service的启动与绑定类似，registerReceiver方法有调用了mBase的方法,mBase的实现是ContextImpl，ContextImpl的registerReceiver方法在嵌套调用后最后调用了registerReceiverInternal方法，具体看下代码：

//ContextWrapper.java
...
 @Override
    public Intent registerReceiver(
        BroadcastReceiver receiver, IntentFilter filter) {
        return mBase.registerReceiver(receiver, filter);
    }
...

//ContextImpl.java
...
private Intent registerReceiverInternal(BroadcastReceiver receiver, int userId,
            IntentFilter filter, String broadcastPermission,
            Handler scheduler, Context context, int flags) {
        IIntentReceiver rd = null;
        if (receiver != null) {//判断receiver是否存在
            
             //判断mPackageInfo（LoadedApk类型）与Context是否都存在
            if (mPackageInfo != null && context != null) {
                if (scheduler == null) {
                    scheduler = mMainThread.getHandler();
                }
              //获取IIntentReceiver
                rd = mPackageInfo.getReceiverDispatcher(
                    receiver, context, scheduler,
                    mMainThread.getInstrumentation(), true);
            } else {
            //mPackageInfo不存在

                if (scheduler == null) {
                    scheduler = mMainThread.getHandler();
                }
              //创建IIntentReceiver
                rd = new LoadedApk.ReceiverDispatcher(
                        receiver, context, scheduler, null, true).getIIntentReceiver();
            }
        }
        try {
            //获取AMS调用registerReceiverWithFeature方法
            final Intent intent = ActivityManager.getService().registerReceiverWithFeature(
                    mMainThread.getApplicationThread(), mBasePackageName, getAttributionTag(), rd,
                    filter, broadcastPermission, userId, flags);
            if (intent != null) {
                intent.setExtrasClassLoader(getClassLoader());
                intent.prepareToEnterProcess();
            }
            return intent;
        } catch (RemoteException e) {
            throw e.rethrowFromSystemServer();
        }
    }
...

最后可以看到调用了AMS的registerReceiverWithFeature，上面代码中有一个重要变量rd，IIntentReceiver类型，这显然是一个Binder接口，具体实现在LoadedApk.ReceiverDispatcher.InnerReceiver：

 static final class ReceiverDispatcher {

        final static class InnerReceiver extends IIntentReceiver.Stub {
            final WeakReference<LoadedApk.ReceiverDispatcher> mDispatcher;
            final LoadedApk.ReceiverDispatcher mStrongRef;

            InnerReceiver(LoadedApk.ReceiverDispatcher rd, boolean strong) {
                mDispatcher = new WeakReference<LoadedApk.ReceiverDispatcher>(rd);
                mStrongRef = strong ? rd : null;
            }
...
}

回到AMS的registerReceiverWithFeature方法，方法比较长，核心处理添加了注释：

public Intent registerReceiverWithFeature(IApplicationThread caller, String callerPackage,
            String callerFeatureId, IIntentReceiver receiver, IntentFilter filter,
            String permission, int userId, int flags) {
        enforceNotIsolatedCaller("registerReceiver");
        ArrayList<Intent> stickyIntents = null;
        ProcessRecord callerApp = null;
        final boolean visibleToInstantApps
                = (flags & Context.RECEIVER_VISIBLE_TO_INSTANT_APPS) != 0;
        int callingUid;
        int callingPid;
        boolean instantApp;
        synchronized(this) {
            if (caller != null) {
                //获取ProcessRecord，其用于描述注册的接收者Activity所在应用程序进程
                callerApp = getRecordForAppLocked(caller);
               ...
                callingUid = callerApp.info.uid;
                callingPid = callerApp.pid;
            } else {
                callerPackage = null;
                callingUid = Binder.getCallingUid();
                callingPid = Binder.getCallingPid();
            }

             ...
            //根据传入的IntentFilter获取Action列表
            Iterator<String> actions = filter.actionsIterator();
            if (actions == null) {
                ArrayList<String> noAction = new ArrayList<String>(1);
                noAction.add(null);
                actions = noAction.iterator();
            }

            // userIds表示应用程序的uid，对于Android系统来说，用户就是一个个的应用程序。
            int[] userIds = { UserHandle.USER_ALL, UserHandle.getUserId(callingUid) };
            while (actions.hasNext()) {
                String action = actions.next();
                for (int id : userIds) {
                    //根据Action列表与应用程序列表（userIds）获取所有粘性广播（StickyBroadcast）
                    ArrayMap<String, ArrayList<Intent>> stickies = mStickyBroadcasts.get(id);
                    if (stickies != null) {
                    //获取所有粘性广播的intent
                        ArrayList<Intent> intents = stickies.get(action);
                        if (intents != null) {
                            if (stickyIntents == null) {
                                stickyIntents = new ArrayList<Intent>();
                            }
                            //保存粘性广播的intent
                            stickyIntents.addAll(intents);
                        }
                    }
                }
            }
        }

        ArrayList<Intent> allSticky = null;
        if (stickyIntents != null) {
            final ContentResolver resolver = mContext.getContentResolver();
            // 查找匹配的粘性广播
            for (int i = 0, N = stickyIntents.size(); i < N; i++) {
                Intent intent = stickyIntents.get(i);
                // Don't provided intents that aren't available to instant apps. 
                if (instantApp &&
                        (intent.getFlags() & Intent.FLAG_RECEIVER_VISIBLE_TO_INSTANT_APPS) == 0) {
                    continue;
                }
              ...
            }
        }

      ...

        synchronized (this) {
           ...
            //获取ReceiverList
            ReceiverList rl = mRegisteredReceivers.get(receiver.asBinder());
            if (rl == null) {
                //ReceiverList为空新建
                rl = new ReceiverList(this, callerApp, callingPid, callingUid,
                        userId, receiver);
                if (rl.app != null) {
                  //判断当前receiver是否超过允许的最的值
                    final int totalReceiversForApp = rl.app.receivers.size();
                    if (totalReceiversForApp >= MAX_RECEIVERS_ALLOWED_PER_APP) {
                        throw new IllegalStateException("Too many receivers, total of "
                                + totalReceiversForApp + ", registered for pid: "
                                + rl.pid + ", callerPackage: " + callerPackage);
                    }
                    rl.app.receivers.add(rl);
                } else {
                   ...
                }
            }
            //创建BroadcastFilter用来记录已注册的广播接收者信息
            BroadcastFilter bf = new BroadcastFilter(filter, rl, callerPackage, callerFeatureId,
                    permission, callingUid, userId, instantApp, visibleToInstantApps);
            //判断接收者列表ReceiverList中是否存在对应接收者，不存在则添加
            if (rl.containsFilter(filter)) {
                Slog.w(TAG, "Receiver with filter " + filter
                        + " already registered for pid " + rl.pid
                        + ", callerPackage is " + callerPackage);
            } else {
                rl.add(bf);
                if (!bf.debugCheck()) {
                    Slog.w(TAG, "==> For Dynamic broadcast");
                }
                //记录接收者，AMS全局变量，
                //当AMS接收到广播时会从mReceiverResolver找到所有的接收者，实现注册的效果。
                mReceiverResolver.addFilter(bf);
            }

            // 广播队列中入队所有将符合filter的粘性广播
            if (allSticky != null) {
                ArrayList receivers = new ArrayList();
                receivers.add(bf);

                final int stickyCount = allSticky.size();
                for (int i = 0; i < stickyCount; i++) {
                    Intent intent = allSticky.get(i);
                    BroadcastQueue queue = broadcastQueueForIntent(intent);
                    BroadcastRecord r = new BroadcastRecord(queue, intent, null,
                            null, null, -1, -1, false, null, null, OP_NONE, null, receivers,
                            null, 0, null, null, false, true, true, -1, false,
                            false /* only PRE_BOOT_COMPLETED should be exempt, no stickies */);
                    queue.enqueueParallelBroadcastLocked(r);
                    queue.scheduleBroadcastsLocked();
                }
            }

            return sticky;
        }
    }

以上就是广播注册的过程，具体的步骤在注释中一有所体现。

2. 广播的发送与接收

2.1 ContextImp 调用AMS

我们知道广播有标准广播（无序广播）、有序广播和粘性广播。下面还是以开始那个例子（标准广播）展开介绍。与广播的注册类似，首先是调用了ContextWraper的sendBroadcast：

...
 @Override
    public void sendBroadcast(Intent intent) {
        mBase.sendBroadcast(intent);
    }
...

sendBroadcast调用了ContextImp的sendBroadcast方法：

...
@Override
    public void sendBroadcast(Intent intent) {
        warnIfCallingFromSystemProcess();
        String resolvedType = intent.resolveTypeIfNeeded(getContentResolver());
        try {
            intent.prepareToLeaveProcess(this);
            ActivityManager.getService().broadcastIntentWithFeature(
                    mMainThread.getApplicationThread(), getAttributionTag(), intent, resolvedType,
                    null, Activity.RESULT_OK, null, null, null, AppOpsManager.OP_NONE, null, false,
                    false, getUserId());
        } catch (RemoteException e) {
            throw e.rethrowFromSystemServer();
        }
    }
...

上面的代码，很明显调用了AMS的broadcastIntentWithFeature方法：

public final int broadcastIntentWithFeature(IApplicationThread caller, String callingFeatureId,
            Intent intent, String resolvedType, IIntentReceiver resultTo,
            int resultCode, String resultData, Bundle resultExtras,
            String[] requiredPermissions, int appOp, Bundle bOptions,
            boolean serialized, boolean sticky, int userId) {
        enforceNotIsolatedCaller("broadcastIntent");
        synchronized(this) {
            //验证intent
            intent = verifyBroadcastLocked(intent);
            
            //获取应用程序进程信息
            final ProcessRecord callerApp = getRecordForAppLocked(caller);
            final int callingPid = Binder.getCallingPid();
            final int callingUid = Binder.getCallingUid();

            final long origId = Binder.clearCallingIdentity();
            try {
                return broadcastIntentLocked(callerApp,
                        callerApp != null ? callerApp.info.packageName : null, callingFeatureId,
                        intent, resolvedType, resultTo, resultCode, resultData, resultExtras,
                        requiredPermissions, appOp, bOptions, serialized, sticky,
                        callingPid, callingUid, callingUid, callingPid, userId);
            } finally {
                Binder.restoreCallingIdentity(origId);
            }
        }
    }

最后调用了AMS的broadcastIntentLocked方法，方法特别的长，这里这给出部分源码：

 final int broadcastIntentLocked(ProcessRecord callerApp, String callerPackage,
            @Nullable String callerFeatureId, Intent intent, String resolvedType,
            IIntentReceiver resultTo, int resultCode, String resultData,
            Bundle resultExtras, String[] requiredPermissions, int appOp, Bundle bOptions,
            boolean ordered, boolean sticky, int callingPid, int callingUid, int realCallingUid,
            int realCallingPid, int userId, boolean allowBackgroundActivityStarts,
            @Nullable int[] broadcastWhitelist) {
            ...
            //获取注册的粘性广播
            ArrayMap<String, ArrayList<Intent>> stickies = mStickyBroadcasts.get(userId);
            if (stickies == null) {
                stickies = new ArrayMap<>();
                mStickyBroadcasts.put(userId, stickies);
            }
            //找到符合当前Action的广播
            ArrayList<Intent> list = stickies.get(intent.getAction());
            if (list == null) {
                list = new ArrayList<>();
                stickies.put(intent.getAction(), list);
            }
            ...
            //接收者存在
            if ((receivers != null && receivers.size() > 0)
                || resultTo != null) {
            //获取广播队列，获取BroadcastRecord信息
            BroadcastQueue queue = broadcastQueueForIntent(intent);
            BroadcastRecord r = new BroadcastRecord(queue, intent, callerApp, callerPackage,
                    callerFeatureId, callingPid, callingUid, callerInstantApp, resolvedType,
                    requiredPermissions, appOp, brOptions, receivers, resultTo, resultCode,
                    resultData, resultExtras, ordered, sticky, false, userId,
                    allowBackgroundActivityStarts, timeoutExempt);
              ...
             queue.enqueueOrderedBroadcastLocked(r);
            //执行广播schedule
             queue.scheduleBroadcastsLocked();
              }
...
             return ActivityManager.BROADCAST_SUCCESS;
}

从上面的代码可以看出，最后AMS调用了BroadcastQueue的scheduleBroadcastsLocked方法。

2.2 ContextImp 调用AMS调用BroadcastReceiver

接上一节的代码继续看，BroadcastQueue的scheduleBroadcastsLocked方法：

public void scheduleBroadcastsLocked() {
        if (DEBUG_BROADCAST) Slog.v(TAG_BROADCAST, "Schedule broadcasts ["
                + mQueueName + "]: current="
                + mBroadcastsScheduled);

        if (mBroadcastsScheduled) {
            return;
        }
        mHandler.sendMessage(mHandler.obtainMessage(BROADCAST_INTENT_MSG, this));
        mBroadcastsScheduled = true;
    }

scheduleBroadcastsLocked方法内部，通过BroadcastHandler发送了一个消息：BROADCAST_INTENT_MSG，这肯定交由对应的handleMessage方法处理，我们再看下BroadcastHandler的handleMessage方法：

 @Override
        public void handleMessage(Message msg) {
            switch (msg.what) {
                case BROADCAST_INTENT_MSG: {
                    if (DEBUG_BROADCAST) Slog.v(
                            TAG_BROADCAST, "Received BROADCAST_INTENT_MSG ["
                            + mQueueName + "]");
                    processNextBroadcast(true);
                } break;
                case BROADCAST_TIMEOUT_MSG: {
                    synchronized (mService) {
                        broadcastTimeoutLocked(true);
                    }
                } break;
            }
        }

上面代码很明显，处理BROADCAST_INTENT_MSG类型消息后，调用了processNextBroadcast方法，然后又调用了processNextBroadcastLocked方法，方法比较长，特殊处理也很多，这里只关注下标准广播的处理：

final void processNextBroadcast(boolean fromMsg) {
        synchronized (mService) {
            processNextBroadcastLocked(fromMsg, false);
        }
    }
...
 final void processNextBroadcastLocked(boolean fromMsg, boolean skipOomAdj) {
  ...   
   if (fromMsg) {//已经处理了BROADCAST_INTENT_MSG类型的消息
            mBroadcastsScheduled = false;
        }

        // 处理无序广播，也就是标准的广播
        while (mParallelBroadcasts.size() > 0) {
          //获取广播
            r = mParallelBroadcasts.remove(0);
           ...
            for (int i=0; i<N; i++) {
                Object target = r.receivers.get(i);
                if (DEBUG_BROADCAST)  Slog.v(TAG_BROADCAST,
                        "Delivering non-ordered on [" + mQueueName + "] to registered "
                        + target + ": " + r);
              //广播发送给相应的接收者
                deliverToRegisteredReceiverLocked(r, (BroadcastFilter)target, false, i);
            }
            addBroadcastToHistoryLocked(r);
            if (DEBUG_BROADCAST_LIGHT) Slog.v(TAG_BROADCAST, "Done with parallel broadcast ["
                    + mQueueName + "] " + r);
        }
...
}

下面继续看下deliverToRegisteredReceiverLocked方法：

private void deliverToRegisteredReceiverLocked(BroadcastRecord r,
            BroadcastFilter filter, boolean ordered, int index) {

            ....//省略一堆验证
        
        // 有序广播的处理.
        if (ordered) {
            r.receiver = filter.receiverList.receiver.asBinder();
            r.curFilter = filter;
            filter.receiverList.curBroadcast = r;
            r.state = BroadcastRecord.CALL_IN_RECEIVE;
            if (filter.receiverList.app != null) {
                r.curApp = filter.receiverList.app;
                filter.receiverList.app.curReceivers.add(r);
                mService.updateOomAdjLocked(r.curApp, true,
                        OomAdjuster.OOM_ADJ_REASON_START_RECEIVER);
            }
        }
        try {
            if (DEBUG_BROADCAST_LIGHT) Slog.i(TAG_BROADCAST,
                    "Delivering to " + filter + " : " + r);
            if (filter.receiverList.app != null && filter.receiverList.app.inFullBackup) {
                //如果是有序广播，继续传给下一个接收者.
                if (ordered) {
                    skipReceiverLocked(r);
                }
            } else {
                r.receiverTime = SystemClock.uptimeMillis();
                maybeAddAllowBackgroundActivityStartsToken(filter.receiverList.app, r);
                //各种检查及验证后最终调用的方法
                performReceiveLocked(filter.receiverList.app, filter.receiverList.receiver,
                        new Intent(r.intent), r.resultCode, r.resultData,
                        r.resultExtras, r.ordered, r.initialSticky, r.userId);
               
                if (r.allowBackgroundActivityStarts && !r.ordered) {
                    postActivityStartTokenRemoval(filter.receiverList.app, r);
                }
            }
            if (ordered) {
                r.state = BroadcastRecord.CALL_DONE_RECEIVE;
            }
        } catch (RemoteException e) {
            ...
        }
}

继续看performReceiveLocked方法：

void performReceiveLocked(ProcessRecord app, IIntentReceiver receiver,
            Intent intent, int resultCode, String data, Bundle extras,
            boolean ordered, boolean sticky, int sendingUser)
            throws RemoteException {
        // Send the intent to the receiver asynchronously using one-way binder calls.
        if (app != null) {//验证应用程序进程是否存在
            if (app.thread != null) {//验证应用程序进程是否已运行
                // If we have an app thread, do the call through that so it is
                // 应用程序进程存在，执行一个单向请求
                try {
                    app.thread.scheduleRegisteredReceiver(receiver, intent, resultCode,
                            data, extras, ordered, sticky, sendingUser, app.getReportedProcState());
                } catch (RemoteException ex) {
                   ...
                }
            } else {
                throw new RemoteException("app.thread must not be null");
            }
        } else {
            receiver.performReceive(intent, resultCode, data, extras, ordered,
                    sticky, sendingUser);
        }
    }

上面的应用程序进程app自然是ApplicationThread，继续看其方法scheduleRegisteredReceiver：

        //这个方法是为了保证所有接收者的分发是正确有序的
        public void scheduleRegisteredReceiver(IIntentReceiver receiver, Intent intent,
                int resultCode, String dataStr, Bundle extras, boolean ordered,
                boolean sticky, int sendingUser, int processState) throws RemoteException {
            updateProcessState(processState, false);
            receiver.performReceive(intent, resultCode, dataStr, extras, ordered,
                    sticky, sendingUser);
        }

上面代码很简洁，调用了IIntentReceiver的performReceive方法，IIntentReceiver是通过LoadedApk中的InnerReceiver实现的：

final static class InnerReceiver extends IIntentReceiver.Stub {
...
@Override
            public void performReceive(Intent intent, int resultCode, String data,
                    Bundle extras, boolean ordered, boolean sticky, int sendingUser) {
                final LoadedApk.ReceiverDispatcher rd;
                if (intent == null) {
                    Log.wtf(TAG, "Null intent received");
                    rd = null;
                } else {
                    rd = mDispatcher.get();
                }
              ...
                if (rd != null) {
                  //ReceiverDispatcher的调用
                    rd.performReceive(intent, resultCode, data, extras,
                            ordered, sticky, sendingUser);
                } else {
                ...
                }
            }
...
}

上面的IIntentReceiver通过AIDL实现进程间通信，最后调用了ReceiverDispatcher的performReceive方法：

public void performReceive(Intent intent, int resultCode, String data,
                Bundle extras, boolean ordered, boolean sticky, int sendingUser) {
            //封装广播的intent信息
            final Args args = new Args(intent, resultCode, data, extras, ordered,
                    sticky, sendingUser);
            if (intent == null) {
                Log.wtf(TAG, "Null intent received");
            } else {
               ...
            }
            if (intent == null || !mActivityThread.post(args.getRunnable())) {

                if (mRegistered && ordered) {
                    IActivityManager mgr = ActivityManager.getService();
                    if (ActivityThread.DEBUG_BROADCAST) Slog.i(ActivityThread.TAG,
                            "Finishing sync broadcast to " + mReceiver);
                    args.sendFinished(mgr);
                }
            }
        }

mActivityThread.post(args.getRunnable())的mActivityThread是个Handler，具体实现是H类，这里将args.getRunnable()作为参数传入Handler的post方法，看下Args的getRunnable方法：

public final Runnable getRunnable() {
                return () -> {
                    final BroadcastReceiver receiver = mReceiver;
                    final boolean ordered = mOrdered;

                   ...

                    //获取AMS
                    final IActivityManager mgr = ActivityManager.getService();
                    final Intent intent = mCurIntent;
                   ...
                    try {
                        //创建BroadcastReceiver
                        ClassLoader cl = mReceiver.getClass().getClassLoader();
                        intent.setExtrasClassLoader(cl);
                        intent.prepareToEnterProcess();
                        setExtrasClassLoader(cl);
                        receiver.setPendingResult(this);
                        //调用onReceive方法
                        receiver.onReceive(mContext, intent);
                    } catch (Exception e) {
                      ...
                    }
                 ...
                };
            }
        }

代码最后创建了一个BroadcastReceiver并调用了onReceive方法。

3.总结

3.1 广播注册 ContextWraper到启动AMS的过程

image.png

3.2 广播发送 ContextWraper到启动AMS的过程

image.png

3.3广播发送与接收 AMS到启动BroadcastReceiver的过程

image.png