死磕Android_BroadcastReceiver 工作过程

今天药忘吃喽~ 2022-01-12 00:33 191阅读 0赞

建议阅读本文之前先阅读[死磕Android\_Service启动流程分析(一)][Android_Service],因为有些内容是一致的,方便融合.

早期的时候,广播的特性被各种流氓APP利用.好多好多流氓APP监听比如打电话,收发短信,有些流氓APP甚至直接拦截短信,,当然那个年代早已是过去式了,应该是4.4以前吧,反正以前那会儿挺乱的.现在好多了,想要读取短信内容,难上加难,更不能拦截短信.除非是将APP设置成了默认应用,可以收到短信通知,而且也不能拦截这条广播.以前那会儿即使进程被杀了,只要注册了静态广播,都是能收到的.某些APP收到广播后,就自启,自启后就干些流氓的事情....

特别是Android 8.0之后,大部分的静态广播差不多都失效了.所以我们还是尽量用动态广播比较好,需要的时候注册,不需要的时候解注册.如果只是应用内自己和自己通信,那是么可以使用LocalBroadcastManager的,通信安全.

## 1. 简单回顾使用方式 ##

定义一个简单的广播

class StaticReceiver : BroadcastReceiver() {
    
        override fun onReceive(context: Context, intent: Intent) {
            Log.e("xfhy", "我收到消息了")
        }
    }
    复制代码

下面是**静态注册**

需要到清单文件中进行注册

<receiver
            android:name=".StaticReceiver"
            android:enabled="true"
            android:exported="true">
        <intent-filter>
            <action android:name="com.xfhy.staticreceiver"/>
        </intent-filter>
    </receiver>
    复制代码

如果是动态注册,那么需要注册和解注册.

//注册
    val intentFilter = IntentFilter()
    registerReceiver(myBroadcast, intentFilter)
    
    //解注册
    unregisterReceiver(myBroadcast)
    复制代码

注册之后,就可以发广播了

val intent = Intent(applicationContext, StaticReceiver::class.java)
    intent.action = "com.xfhy.staticreceiver"
    sendBroadcast(intent)
    复制代码

## 2. 广播注册原理 ##

> 由于现在基本都用动态广播了(不知道是不是....因为Android 8.0的限制),所以直接讲解动态广播的注册方式.静态注册的方式是通过PMS在应用安装的时候就注册好了,其他3个组件也是.

事不宜迟,我们从registerReceiver方法开始深入,来到ContextWrapper的registerReceiver方法.我们知道Activity是继承了ContextWrapper的.

@Override
    public Intent registerReceiver(
        BroadcastReceiver receiver, IntentFilter filter) {
        return mBase.registerReceiver(receiver, filter);
    }
    复制代码

ContextWrapper几乎是不干实事的,全是交给mBase去做.而mBase是ContextImpl,在Service启动那篇文章中已讲解.

@Override
    public Intent registerReceiver(BroadcastReceiver receiver, IntentFilter filter) {
        return registerReceiver(receiver, filter, null, null);
    }
    @Override
    public Intent registerReceiver(BroadcastReceiver receiver, IntentFilter filter,
            String broadcastPermission, Handler scheduler) {
        return registerReceiverInternal(receiver, getUserId(),
                filter, broadcastPermission, scheduler, getOuterContext(), 0);
    }
    private Intent registerReceiverInternal(BroadcastReceiver receiver, int userId,
            IntentFilter filter, String broadcastPermission,
            Handler scheduler, Context context, int flags) {
        IIntentReceiver rd = null;
        if (receiver != null) {
            if (mPackageInfo != null && context != null) {
                if (scheduler == null) {
                    scheduler = mMainThread.getHandler();
                }
                rd = mPackageInfo.getReceiverDispatcher(
                    receiver, context, scheduler,
                    mMainThread.getInstrumentation(), true);
            } else {
                if (scheduler == null) {
                    scheduler = mMainThread.getHandler();
                }
                rd = new LoadedApk.ReceiverDispatcher(
                        receiver, context, scheduler, null, true).getIIntentReceiver();
            }
        }
        try {
            final Intent intent = ActivityManager.getService().registerReceiver(
                    mMainThread.getApplicationThread(), mBasePackageName, rd, filter,
                    broadcastPermission, userId, flags);
            if (intent != null) {
                intent.setExtrasClassLoader(getClassLoader());
                intent.prepareToEnterProcess();
            }
            return intent;
        } catch (RemoteException e) {
            throw e.rethrowFromSystemServer();
        }
    }
    复制代码

最终会调用registerReceiverInternal方法.而这里有一个IIntentReceiver对象,需要注意.由于在注册的时候是向AMS发起请求需要注册,而BroadcastReceiver是不能进行跨进程传输的,所以需要一个介质.而IIntentReceiver就是专门拿来与AMS通信的,它是一个Binder对象.这里的ReceiverDispatcher和[Service绑定流程][Service]那里的ServiceDispatcher简直一模一样.ReceiverDispatcher里面有BroadcastReceiver和IIntentReceiver对象,都是在ReceiverDispatcher的构造方法就初始化好了,当收到广播的时候直接调用BroadcastReceiver对象的onReceive方法,简单方便.

来来来,我们看下LoadedApk的getReceiverDispatcher方法

//缓存  
    private final ArrayMap<Context, ArrayMap<BroadcastReceiver, ReceiverDispatcher>> mReceivers
            = new ArrayMap<>();
    
    public IIntentReceiver getReceiverDispatcher(BroadcastReceiver r,
        Context context, Handler handler,
        Instrumentation instrumentation, boolean registered) {
        synchronized (mReceivers) {
            LoadedApk.ReceiverDispatcher rd = null;
            ArrayMap<BroadcastReceiver, LoadedApk.ReceiverDispatcher> map = null;
            
            //如果是已经注册  有缓存则用缓存
            if (registered) {
                map = mReceivers.get(context);
                if (map != null) {
                    rd = map.get(r);
                }
            }
            
            //第一次
            if (rd == null) {
                //ReceiverDispatcher初始化,这里会将BroadcastReceiver放进去,然后构造方法会将InnerReceiver(用来跨进程通信的)初始化
                rd = new ReceiverDispatcher(r, context, handler,
                        instrumentation, registered);
                if (registered) {
                    if (map == null) {
                        map = new ArrayMap<BroadcastReceiver, LoadedApk.ReceiverDispatcher>();
                        mReceivers.put(context, map);
                    }
                    map.put(r, rd);
                }
            } else {
                rd.validate(context, handler);
            }
            rd.mForgotten = false;
            return rd.getIIntentReceiver();
        }
    }
    复制代码

谷歌工程师写的代码就是牛批,原理几乎一毛一样.

好了,当IIntentReceiver创建好了之后,就可以和AMS进行通信了.上面的ActivityManager.getService()就是AMS哈,在之前的文章经常提到.来到AMS的registerReceiver方法

final HashMap<IBinder, ReceiverList> mRegisteredReceivers = new HashMap<>();
    
    public Intent registerReceiver(IApplicationThread caller, String callerPackage,
    IIntentReceiver receiver, IntentFilter filter, String permission, int userId,
    int flags) {
        ...... 
        mRegisteredReceivers.put(receiver.asBinder(), rl);
        BroadcastFilter bf = new BroadcastFilter(filter, rl, callerPackage,
                        permission, callingUid, userId, instantApp, visibleToInstantApps);
        mReceiverResolver.addFilter(bf);
    }
    复制代码

上面这个过程会把InnerReceiver(待会儿需要拿来跨进程通信的)和IntentFilter存起来,其实广播的注册就完成了.

## 3. 广播发送和接收原理 ##

通过sendBroadcast方法发送广播,其实就是在调用的父类ContextWrapper的sendBroadcast方法,之前我们分析过,ContextWrapper本身是不干啥实事的,它其实是调用的ContextImpl的sendBroadcast方法.

@Override
    public void sendBroadcast(Intent intent) {
        warnIfCallingFromSystemProcess();
        String resolvedType = intent.resolveTypeIfNeeded(getContentResolver());
        intent.prepareToLeaveProcess(this);
        ActivityManager.getService().broadcastIntent(
                mMainThread.getApplicationThread(), intent, resolvedType, null,
                Activity.RESULT_OK, null, null, null, AppOpsManager.OP_NONE, null, false, false,
                getUserId());
    }
    复制代码

ActivityManager.getService()是AMS哈,分析过很多次了.这里调用了AMS的broadcastIntent方法

public final int broadcastIntent(IApplicationThread caller,
            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) {
            intent = verifyBroadcastLocked(intent);
    
            final ProcessRecord callerApp = getRecordForAppLocked(caller);
            final int callingPid = Binder.getCallingPid();
            final int callingUid = Binder.getCallingUid();
            final long origId = Binder.clearCallingIdentity();
            int res = broadcastIntentLocked(callerApp,
                    callerApp != null ? callerApp.info.packageName : null,
                    intent, resolvedType, resultTo, resultCode, resultData, resultExtras,
                    requiredPermissions, appOp, bOptions, serialized, sticky,
                    callingPid, callingUid, userId);
            Binder.restoreCallingIdentity(origId);
            return res;
        }
    }
    复制代码

上面最核心的就是broadcastIntentLocked方法了,当我看到这个方法的行数时,我傻眼了,,,大家感受一下.600多行的代码,,,,

我们挑选核心的代码来分析

@GuardedBy("this")
    final int broadcastIntentLocked(ProcessRecord callerApp,
        String callerPackage, 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 userId) {
            
        // By default broadcasts do not go to stopped apps.
        //已经停止的APP 无法收到广播
        intent.addFlags(Intent.FLAG_EXCLUDE_STOPPED_PACKAGES);   
        
        final BroadcastQueue queue = broadcastQueueForIntent(intent);
        BroadcastRecord r = new BroadcastRecord(queue, intent, callerApp,
                callerPackage, callingPid, callingUid, callerInstantApp, resolvedType,
                requiredPermissions, appOp, brOptions, registeredReceivers, resultTo,
                resultCode, resultData, resultExtras, ordered, sticky, false, userId);
        if (DEBUG_BROADCAST) Slog.v(TAG_BROADCAST, "Enqueueing parallel broadcast " + r);
        final boolean replaced = replacePending
                && (queue.replaceParallelBroadcastLocked(r) != null);
        // Note: We assume resultTo is null for non-ordered broadcasts.
        if (!replaced) {
            //将符合条件的广播,添加到BroadcastQueue中,然后进行发送
            queue.enqueueParallelBroadcastLocked(r);
            queue.scheduleBroadcastsLocked();
        }
        
    }
    
    复制代码

我们在源码中可以看到,已经停止了的APP是收不到广播的.然后就是经过一系列的复杂的筛选过程,将符合条件的广播添加到 BroadcastQueue中,然后进行发送广播.

public void scheduleBroadcastsLocked() {    
        ......
        //发送了一个消息
        mHandler.sendMessage(mHandler.obtainMessage(BROADCAST_INTENT_MSG, this));
    }
    
    private final class BroadcastHandler extends Handler {
        @Override
        public void handleMessage(Message msg) {
            switch (msg.what) {
                case BROADCAST_INTENT_MSG: {
                    if (DEBUG_BROADCAST) Slog.v(
                            TAG_BROADCAST, "Received BROADCAST_INTENT_MSG");
                    processNextBroadcast(true);
                } break;
            }
        }
    }
    
    final void processNextBroadcast(boolean fromMsg) {
        synchronized (mService) {
            processNextBroadcastLocked(fromMsg, false);
        }
    }
    
    
     final ArrayList<BroadcastRecord> mParallelBroadcasts = new ArrayList<>();
    final void processNextBroadcastLocked(boolean fromMsg, boolean skipOomAdj) {
        ......
        while (mParallelBroadcasts.size() > 0) {
            r = mParallelBroadcasts.remove(0);
            r.dispatchTime = SystemClock.uptimeMillis();
            r.dispatchClockTime = System.currentTimeMillis();
    
            final int N = r.receivers.size();
    
            for (int i=0; i<N; i++) {
                Object target = r.receivers.get(i);
                //发送广播这里的r是BroadcastRecord,这个方法里面会调用performReceiveLocked方法
                deliverToRegisteredReceiverLocked(r, (BroadcastFilter)target, false, i);
            }
            addBroadcastToHistoryLocked(r);
        }
    }
    
    //具体的发送过程
    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) {
            if (app.thread != null) {
                try {
                    //调用ApplicationThread中的scheduleRegisteredReceiver方法
                    app.thread.scheduleRegisteredReceiver(receiver, intent, resultCode,
                            data, extras, ordered, sticky, sendingUser, app.repProcState);
                } catch (RemoteException ex) {
                    synchronized (mService) {
                        Slog.w(TAG, "Can't deliver broadcast to " + app.processName
                                + " (pid " + app.pid + "). Crashing it.");
                        app.scheduleCrash("can't deliver broadcast");
                    }
                    throw ex;
                }
            } else {
                // Application has died. Receiver doesn't exist.
                throw new RemoteException("app.thread must not be null");
            }
        } else {
            receiver.performReceive(intent, resultCode, data, extras, ordered,
                    sticky, sendingUser);
        }
    }
    
    复制代码

辗转反侧,会去调用ApplicationThread的scheduleRegisteredReceiver方法,

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);
    }
    复制代码

注意,上面的IIntentReceiver是ReceiverDispatcher中的InnerReceiver对象,这个对象是拿来跨进程通信的.直接调用了InnerReceiver的performReceive方法,来看一下

@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();
        }
    
    
        rd.performReceive(intent, resultCode, data, extras,
            ordered, sticky, sendingUser);
    
    }
    复制代码

其实performReceive就是调用ReceiverDispatcher的performReceive,ReceiverDispatcher是InnerReceiver和BroadcastReceiver的桥梁,因为它里面有两者的引用,可以和方便地进行调用其方法.下面是ReceiverDispatcher的实现

public void performReceive(Intent intent, int resultCode, String data,
            Bundle extras, boolean ordered, boolean sticky, int sendingUser) {
        final Args args = new Args(intent, resultCode, data, extras, ordered,
                sticky, sendingUser);
        if (intent == null) {
            Log.wtf(TAG, "Null intent received");
        } else {
            if (ActivityThread.DEBUG_BROADCAST) {
                int seq = intent.getIntExtra("seq", -1);
                Slog.i(ActivityThread.TAG, "Enqueueing broadcast " + intent.getAction()
                        + " seq=" + seq + " to " + mReceiver);
            }
        }
        
        //注意这句代码  mActivityThread是一个Handler,主线程的Handler,
        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);
            }
        }
    }
    复制代码

performReceice里面就是利用主线程中的Handler运行一个Runnable.内容如下:

public final Runnable getRunnable() {
        return () -> {
            final BroadcastReceiver receiver = mReceiver;
            final boolean ordered = mOrdered;
    
            final IActivityManager mgr = ActivityManager.getService();
            final Intent intent = mCurIntent;
    
            mCurIntent = null;
            mDispatched = true;
            mPreviousRunStacktrace = new Throwable("Previous stacktrace");
            if (receiver == null || intent == null || mForgotten) {
                if (mRegistered && ordered) {
                    if (ActivityThread.DEBUG_BROADCAST) Slog.i(ActivityThread.TAG,
                            "Finishing null broadcast to " + mReceiver);
                    sendFinished(mgr);
                }
                return;
            }
    
            Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "broadcastReceiveReg");
            try {
                //注意了,BroadcastReceiver是在这个时候进行构建的,构建之后立刻调用它的onReceive方法,注意这是在主线程中调用的
                ClassLoader cl = mReceiver.getClass().getClassLoader();
                intent.setExtrasClassLoader(cl);
                intent.prepareToEnterProcess();
                setExtrasClassLoader(cl);
                receiver.setPendingResult(this);
                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);
        };
    }
    复制代码

可以看到,BroadcastReceiver也是通过反射构建出来的,构建出来之后调用它的onReceive方法,并且是在主线程中调用的.

到这里,广播的发送和接收都已讲解完毕.

转载于:https://juejin.im/post/5d0a4f706fb9a07eb15d5b20

[Android_Service]: https://juejin.im/post/5d026bd8e51d4556dc29361c
[Service]: https://juejin.im/post/5d050c106fb9a07ecb0ba66f