详解Android广播Broadcast的启动流程
作者：新小梦
这篇文章主要为大家介绍了Android广播Broadcast启动流程详解，有需要的朋友可以借鉴参考下，希望能够有所帮助，祝大家多多进步，早日升职加薪
正文
本文整体阅读下来相对Activity和Service的启动流程较容易，比较贴近我们日常代码开发习惯。我们曾经有个整机项目，多个APP跨进程交互，本来想采用AIDL进行的，但最终考虑到项目工期和其他同事的能力，最终在采用广播方式进行IPC。
那时，自己也在想，这么多个APP相互发信息，数据量也大，对整机性能有影响么？会不会存在丢失和内存问题。一脸茫然，网上也不会有类似信息告诉总结这种情况，本文也不会总结这个答案，因为看完之后心中自然有数了。
在AMS中持有集合用于存储所有的广播，应用程序可以从向其注册和解注册广播。当应用发送广播时，AMS检查相关权限和特殊的Intent。然后再根据对应IntentFilter匹配到一个或多个Receiver，在应用进程回调其onReceive函数。
阅读源码本身就是一份苦活，不可能一次就读懂，或者了解透的。只有反复的阅读，输入与输出，才会越来越轻松。所以个人建议，先粗读，了解个大概的思路就行。收藏或点赞，等自己ready，再好好结合源码阅读。一定要Fuck Code!
广播的注册
我们常在Activity或Service、甚至Application中调用registerReceiver函数来注册动态广播，该函数其实来自它们共同的父类ContextWrapper中。ContextWrapper是Context的子类，我们会在介绍Context的文章介绍它们的关系。
public Intent registerReceiver(
BroadcastReceiver receiver, IntentFilter filter) {
return mBase.registerReceiver(receiver, filter);
}
这里Context类型的mBase，在Activity的创建过程实际被赋值为ContextImpl实例。
public Intent registerReceiver(BroadcastReceiver receiver, IntentFilter filter) {
return registerReceiver(receiver, filter, null, null);
}
public Intent registerReceiver(BroadcastReceiver receiver, IntentFilter filter,
String broadcastPermission, Handler scheduler) {
return registerReceiverInternal(receiver, getUserId(),
filter, broadcastPermission, scheduler, getOuterContext(), 0);
}
经过registerReceiver重载函数，调用了registerReceiverInternal函数。
private Intent registerReceiverInternal(BroadcastReceiver receiver, int userId,
IntentFilter filter, String broadcastPermission,
Handler scheduler, Context context, int flags) {
IIntentReceiver rd = null;
//分析一
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 {
分析二：
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();
}
}
分析一:
传递进来的BroadcastReceiver不为null，LoadedApk类型的mPackageInfo只要应用进程启动，该属性就会被赋值，context这里指向Activity。scheduler为null，赋值为主线程的H类型mH对象。分析一，主要通过上面的变量来获得IIntentReceiver类型rd对象。
getReceiverDispatcher函数先从缓存检测是否有相同类型的BroadcastReceiver对应的ReceiverDispatcher。没有的话，则新建并缓存起来。 一个context对应多个BroadcastReceiver，而一个BroadcastReceiver对应用一个ReceiverDispatcher。
ReceiverDispatcher是LoadedDispatcher的静态内部类，其内部还有一个AIDL类型本地实现静态类InnerReceiver。在ReceiverDispatcher的构造函数中会创建InnerReceiver的实例。
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;
if (registered) {
map = mReceivers.get(context);
if (map != null) {
rd = map.get(r);
}
}
if (rd == null) {
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();
}
}
回到registerReceiverInternal函数的分析二，调用了AMS的registerReceiverWithFeature函数。
该函数是Broadcast整个注册过程结束的地方，根据新注册的BroadcastReceiver，处理粘性广播的发送和当前注册Receiver的添加。
分析一：
粘性广播存储在AMS的SparseArray<ArrayMap<String, ArrayList<Intent>>>类型的 mStickyBroadcasts中。SparseArray的key为userId,而ArrayMap的key为action,value为Intent。即我们可以通过用户id在mStickyBroadcasts找到当前进程对应所有粘性广播（和针对所有进程的粘性广播），然后根据对应的action找到对应的Intent。这里将他们收集到stickyIntents集合中。
分析二：
所有广播的接收者BroacastReceiver存储在AMS类 HashMap<IBinder, ReceiverList>类型的mRegisteredReceivers中。这里的IBinder类型就是应用进程前面创建的InnerReceiver类实例在AMS的引用。因为广播接收者BroadcastReceiver对应一个或多个Broadcast,所以这里通过继承自ArrayList<BroadcastFilter>的ReceiverList来表达这种关系。通过BroadcastFilter来表示当前接收者感兴趣的广播。
分析三：
对匹配到的粘性Intent进入广播队列广播。
public Intent registerReceiverWithFeature(IApplicationThread caller, String callerPackage,
String callerFeatureId, IIntentReceiver receiver, IntentFilter filter,
String permission, int userId, int flags) {
enforceNotIsolatedCaller("registerReceiver");
//粘性Intent
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) {
//获得当前引用进程的ProcessRecord
callerApp = getRecordForAppLocked(caller);
...
callingUid = callerApp.info.uid;
callingPid = callerApp.pid;
} else {
callerPackage = null;
callingUid = Binder.getCallingUid();
callingPid = Binder.getCallingPid();
}
//是否快应用（类似小程序）
instantApp = isInstantApp(callerApp, callerPackage, callingUid);
userId = mUserController.handleIncomingUser(callingPid, callingUid, userId, true,
ALLOW_FULL_ONLY, "registerReceiver", callerPackage);
//分析一：当前注册广播中感兴趣的action列表
Iterator<String> actions = filter.actionsIterator();
if (actions == null) {
ArrayList<String> noAction = new ArrayList<String>(1);
noAction.add(null);
actions = noAction.iterator();
}
//从历史粘性广播中查找与当前注册的action一致的intent
//添加到stickyIntents
// Collect stickies of users
int[] userIds = { UserHandle.USER_ALL, UserHandle.getUserId(callingUid) };
while (actions.hasNext()) {
String action = actions.next();
for (int id : userIds) {
ArrayMap<String, ArrayList<Intent>> stickies = mStickyBroadcasts.get(id);
if (stickies != null) {
ArrayList<Intent> intents = stickies.get(action);
if (intents != null) {
if (stickyIntents == null) {
stickyIntents = new ArrayList<Intent>();
}
stickyIntents.addAll(intents);
}
}
}
}
}
//处理content类型的Intent
ArrayList<Intent> allSticky = null;
if (stickyIntents != null) {
final ContentResolver resolver = mContext.getContentResolver();
// Look for any matching sticky broadcasts...
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;
}
//当前注册广播IntentFilter是否与action一致的intent的匹配
//处理content类型
if (filter.match(resolver, intent, true, TAG) >= 0) {
if (allSticky == null) {
allSticky = new ArrayList<Intent>();
}
allSticky.add(intent);
}
}
}
//receiver为null，直接返回null或者第一个粘性intent
Intent sticky = allSticky != null ? allSticky.get(0) : null;
if (receiver == null) {
return sticky;
}
synchronized (this) {
...
//分析二：
//从缓存或新建ReceiverList对象，与Receiver绑定
ReceiverList rl = mRegisteredReceivers.get(receiver.asBinder());
if (rl == null) {
rl = new ReceiverList(this, callerApp, callingPid, callingUid,
userId, receiver);
if (rl.app != null) {
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);
}
//添加到ProcessRecord记录中
rl.app.receivers.add(rl);
} else {
try {
receiver.asBinder().linkToDeath(rl, 0);
} catch (RemoteException e) {
return sticky;
}
rl.linkedToDeath = true;
}
mRegisteredReceivers.put(receiver.asBinder(), rl);
}
...
//新建BroadcastFilter，并添加到BroadcastList
BroadcastFilter bf = new BroadcastFilter(filter, rl, callerPackage, callerFeatureId,
permission, callingUid, userId, instantApp, visibleToInstantApps);
if (rl.containsFilter(filter)) {
...
} else {
rl.add(bf);
//添加到接收者解析器
mReceiverResolver.addFilter(bf);
}
// Enqueue broadcasts for all existing stickies that match
// this filter.
//分析三：对匹配到action的粘性广播进行广播
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;
}
}
广播的解注册
回到ContextWrapper的unregisterReceiver函数。
#ContextWrapper
public void unregisterReceiver(BroadcastReceiver receiver) {
mBase.unregisterReceiver(receiver);
}
#ContextImpl
public void unregisterReceiver(BroadcastReceiver receiver) {
if (mPackageInfo != null) {
IIntentReceiver rd = mPackageInfo.forgetReceiverDispatcher(
getOuterContext(), receiver);
try {
ActivityManager.getService().unregisterReceiver(rd);
} catch (RemoteException e) {
throw e.rethrowFromSystemServer();
}
} else {
throw new RuntimeException("Not supported in system context");
}
}
这里通过receiver和context获得IIntentReceiver实例rd，然后调用AMS的unregisterReceiver函数。其中LoadedApk的forgetReceiverDispatcher函数，主要是从mReceivers获取IIntentReceiver的实例，并将receiver对应的内容从缓存移除。
AMS的unregisterReceiver函数。主要是将注册过程添加到mRegisteredReceivers、ProcessProcess.Receivers、mReceiverResolver中对应的内容移除。并终止正在发送的广播。
public void unregisterReceiver(IIntentReceiver receiver) {
...
final long origId = Binder.clearCallingIdentity();
try {
boolean doTrim = false;
synchronized(this) {
//获得当前对应的ReceiverList
ReceiverList rl = mRegisteredReceivers.get(receiver.asBinder());
if (rl != null) {
//默认情况为null，看看广播发送是否会赋值
//从处理逻辑来看，就是广播内容
final BroadcastRecord r = rl.curBroadcast;
if (r != null && r == r.queue.getMatchingOrderedReceiver(r)) {
final boolean doNext = r.queue.finishReceiverLocked(
r, r.resultCode, r.resultData, r.resultExtras,
r.resultAbort, false);
if (doNext) {
doTrim = true;
r.queue.processNextBroadcast(false);
}
}
//从processRecord中移除
if (rl.app != null) {
rl.app.receivers.remove(rl);
}
//从mRegisteredReceivers和mReceiverResolver移除
removeReceiverLocked(rl);
if (rl.linkedToDeath) {
rl.linkedToDeath = false;
rl.receiver.asBinder().unlinkToDeath(rl, 0);
}
}
}
// If we actually concluded any broadcasts, we might now be able
// to trim the recipients' apps from our working set
if (doTrim) {
trimApplications(OomAdjuster.OOM_ADJ_REASON_FINISH_RECEIVER);
return;
}
} finally {
Binder.restoreCallingIdentity(origId);
}
}
void removeReceiverLocked(ReceiverList rl) {
mRegisteredReceivers.remove(rl.receiver.asBinder());
for (int i = rl.size() - 1; i >= 0; i--) {
mReceiverResolver.removeFilter(rl.get(i));
}
}
广播的发送
定位到ContextWrapper的sendBroadcast函数。
public void sendBroadcast(Intent intent) {
mBase.sendBroadcast(intent);
}
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();
}
}
调用AMS的broadcastIntentWithFeature函数。内部又调用了broadcastIntentLocked函数。是所有Intent处理的地方，很长很长。有对特殊类型的Intent处理，例如Intent.ACTION_PACKAGE_REMOVED和Intent.ACTION_TIME_CHANGED。该函数主要将有序和无序广播接收者（匹配Intent）添加到receivers列表，并创建BroadcastRecord对象r，持有receivers列表。并根据intent获得对应的广播队列queue，将r添加到queue中，执行queue.scheduleBroadcastsLocked函数。
#AMS xxm
@GuardedBy("this")
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) {
intent = new Intent(intent);
...
int[] users;
if (userId == UserHandle.USER_ALL) {
// Caller wants broadcast to go to all started users.
users = mUserController.getStartedUserArray();
} else {
// Caller wants broadcast to go to one specific user.
users = new int[] {userId};
}
// Figure out who all will receive this broadcast.
List receivers = null;
List<BroadcastFilter> registeredReceivers = null;
// Need to resolve the intent to interested receivers...
if ((intent.getFlags()&Intent.FLAG_RECEIVER_REGISTERED_ONLY)
== 0) {
receivers = collectReceiverComponents(
intent, resolvedType, callingUid, users, broadcastWhitelist);
}
if (intent.getComponent() == null) {
if (userId == UserHandle.USER_ALL && callingUid == SHELL_UID) {
// Query one target user at a time, excluding shell-restricted users
for (int i = 0; i < users.length; i++) {
if (mUserController.hasUserRestriction(
UserManager.DISALLOW_DEBUGGING_FEATURES, users[i])) {
continue;
}
//查询已注册的Receiver，在注册过程会被添加
List<BroadcastFilter> registeredReceiversForUser =
mReceiverResolver.queryIntent(intent,
resolvedType, false /*defaultOnly*/, users[i]);
if (registeredReceivers == null) {
registeredReceivers = registeredReceiversForUser;
} else if (registeredReceiversForUser != null) {
registeredReceivers.addAll(registeredReceiversForUser);
}
}
} else {
//查询已注册的Receiver，在注册过程会被添加
registeredReceivers = mReceiverResolver.queryIntent(intent,
resolvedType, false /*defaultOnly*/, userId);
}
}
final boolean replacePending =
(intent.getFlags()&Intent.FLAG_RECEIVER_REPLACE_PENDING) != 0;
...
int NR = registeredReceivers != null ? registeredReceivers.size() : 0;
if (!ordered && NR > 0) {
...
//通过intent获得广播队列
final BroadcastQueue queue = broadcastQueueForIntent(intent);
//将所有数据都封装到BroadcastRecord中
BroadcastRecord r = new BroadcastRecord(queue, intent, callerApp, callerPackage,
callerFeatureId, callingPid, callingUid, callerInstantApp, resolvedType,
requiredPermissions, appOp, brOptions, registeredReceivers, resultTo,
resultCode, resultData, resultExtras, ordered, sticky, false, userId,
allowBackgroundActivityStarts, timeoutExempt);
...
final boolean replaced = replacePending
&& (queue.replaceParallelBroadcastLocked(r) != null);
if (!replaced) {
//将广播添加到queue的mParallelBroadcasts数组列表中
queue.enqueueParallelBroadcastLocked(r);
queue.scheduleBroadcastsLocked();
}
registeredReceivers = null;
NR = 0;
}
...
return ActivityManager.BROADCAST_SUCCESS;
}
enqueueParallelBroadcastLocked函数将BroadcastRecord对象r添加到ArrayList类型的mParallelBroadcasts,后续执行队列事务会从其中取出。
scheduleBroadcastsLocked函数。调用BroadcastHandler类型的mHandler发送一个BROADCAST_INTENT_MSG消息。
#BroadcastQueue xxm
public void scheduleBroadcastsLocked() {
if (mBroadcastsScheduled) {
return;
}
mHandler.sendMessage(mHandler.obtainMessage(BROADCAST_INTENT_MSG, this));
mBroadcastsScheduled = true;
}
BroadcastHandler的handleMessage函数。执行了processNextBroadcast函数。
#BroadcastHandler xxm
public void handleMessage(Message msg) {
switch (msg.what) {
case BROADCAST_INTENT_MSG: {
processNextBroadcast(true);
} break;
case BROADCAST_TIMEOUT_MSG: {
synchronized (mService) {
broadcastTimeoutLocked(true);
}
} break;
}
}
processNextBroadcast函数内容太长了，主要是将粘性广播和无序广播发送给接收者。这里只看函数前半部分对无序广播的处理。其中无序广播是从mParallelBroadcasts取出所有广播，并遍历每个广播的过滤器filter，将广播和广播filter传递给deliverToRegisteredReceiverLocked函数
#BroadcastQueue xxm
if (fromMsg) {
mBroadcastsScheduled = false;
}
//遍历无序广播数组
while (mParallelBroadcasts.size() > 0) {
r = mParallelBroadcasts.remove(0);
...
final int N = r.receivers.size();
...
for (int i=0; i<N; i++) {
Object target = r.receivers.get(i);
//开始传递广播
deliverToRegisteredReceiverLocked(r, (BroadcastFilter)target, false, i);
}
addBroadcastToHistoryLocked(r);
}
deliverToRegisteredReceiverLocked函数。主要进行权限检查。
#BroadcastQueue xxm
private void deliverToRegisteredReceiverLocked(BroadcastRecord r,
BroadcastFilter filter, boolean ordered, int index) {
boolean skip = false;
...
//广播filter进行权限检查，不通过skip=true
...
if (skip) {
r.delivery[index] = BroadcastRecord.DELIVERY_SKIPPED;
return;
}
...
r.delivery[index] = BroadcastRecord.DELIVERY_DELIVERED;
...
performReceiveLocked(filter.receiverList.app, filter.receiverList.receiver,
new Intent(r.intent), r.resultCode, r.resultData,
r.resultExtras, r.ordered, r.initialSticky, r.userId);
...
}
performReceiveLocked函数。如果接收者所在的进程已经启动，直接调用 app.thread.scheduleRegisteredReceiver,如果未启动，则直接回调 receiver.performReceive。
#BroadcastQueue xxm
void performReceiveLocked(ProcessRecord app, IIntentReceiver receiver,
Intent intent, int resultCode, String data, Bundle extras,
boolean ordered, boolean sticky, int sendingUser)
throws RemoteException {
if (app != null) {
if (app.thread != null) {
try {
app.thread.scheduleRegisteredReceiver(receiver, intent, resultCode,
data, extras, ordered, sticky, sendingUser, app.getReportedProcState());
} catch (RemoteException ex) {
synchronized (mService) {
app.scheduleCrash("can't deliver broadcast");
}
throw ex;
}
} else {
throw new RemoteException("app.thread must not be null");
}
} else {
receiver.performReceive(intent, resultCode, data, extras, ordered,
sticky, sendingUser);
}
}
如果进程已经启动，则调用ApplicationThread的scheduleRegisteredReceiver函数。
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);
}
调用了LoadedApk.ReceiverDispatcher.InnerReceiver类的performReceive函数。
public void performReceive(Intent intent, int resultCode, String data,
Bundle extras, boolean ordered, boolean sticky, int sendingUser) {
final LoadedApk.ReceiverDispatcher rd;
if (intent == null) {
rd = null;
} else {
rd = mDispatcher.get();
}
if (rd != null) {
//分析一
rd.performReceive(intent, resultCode, data, extras,
ordered, sticky, sendingUser);
} else {
IActivityManager mgr = ActivityManager.getService();
try {
if (extras != null) {
extras.setAllowFds(false);
}
mgr.finishReceiver(this, resultCode, data, extras, false, intent.getFlags());
} catch (RemoteException e) {
throw e.rethrowFromSystemServer();
}
}
}
调用了分析 一ReceiverDispatcher的performReceive函数
public void performReceive(Intent intent, int resultCode, String data,
Bundle extras, boolean ordered, boolean sticky, int sendingUser) {
//分析1
final Args args = new Args(intent, resultCode, data, extras, ordered,
sticky, sendingUser);
...
//分析2
if (intent == null || !mActivityThread.post(args.getRunnable())) {
if (mRegistered && ordered) {
IActivityManager mgr = ActivityManager.getService();
args.sendFinished(mgr);
}
}
}
ReceiverDispatcher.performReceive函数中分析1将相关数据封装成内部类Args类型的args,然后在分析2通过Handler类型的mActivityThread执行getRunnable返回的Runable对象的run函数。这时切换到应用进程的主线程。
Args.getRunnable函数回调了我们注册广播是复写的onReceiver函数。
public final Runnable getRunnable() {
return () -> {
...
ClassLoader cl = mReceiver.getClass().getClassLoader();
intent.setExtrasClassLoader(cl);
intent.prepareToEnterProcess();
setExtrasClassLoader(cl);
receiver.setPendingResult(this);
//调用广播接收者的onReceive
receiver.onReceive(mContext, intent);
...
}
}
总结
大道至简，所谓注册就是在每个地方维持一个集合，实现所谓的增删改查，根据业务需求增加不同逻辑，例如权限检查，接收者所在进程的状态。
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