Android应用程序注册广播接收器（registerReceiver）的过程分析

Android应用程序注册广播接收器（registerReceiver）的过程分析 - 老罗的Android之旅 - 博客频道 - CSDN.NET

        前面我们介绍了Android系统的广播机制，从本质来说，它是一种消息订阅/发布机制，因此，使用这种消息驱动模型的第一步便是订阅消息；而对Android应用程序来说，订阅消息其实就是注册广播接收器，本文将探讨Android应用程序是如何注册广播接收器以及把广播接收器注册到哪里去的。

        在Android的广播机制中，ActivityManagerService扮演着广播中心的角色，负责系统中所有广播的注册和发布操作，因此，Android应用程序注册广播接收器的过程就把是广播接收器注册到ActivityManagerService的过程。Android应用程序是通过调用ContextWrapper类的registerReceiver函数来把广播接收器BroadcastReceiver注册到ActivityManagerService中去的，而ContextWrapper类本身又借助ContextImpl类来注册广播接收器。

        在Android应用程序框架中，Activity和Service类都继承了ContextWrapper类，因此，我们可以在Activity或者Service的子类中调用registerReceiver函数来注册广播接收器。Activity、Service、ContextWrapper和ContextImpl这四个类的关系可以参考前面Android系统在新进程中启动自定义服务过程（startService）的原理分析一文中描述的Activity类图。

        这篇文章还是继续以实例来进行情景分析，所用到的例子便是上一篇文章Android系统中的广播（Broadcast）机制简要介绍和学习计划里面介绍的应用程序了，所以希望读者在继续阅读本文之前，先看看这篇文章；又由于Android应用程序是把广播接器注册到ActivityManagerService中去的，因此，这里又会涉入到Binder进程间通信机制，所以希望读者对Android系统的Binder进程间通信机制有所了解，具体请参考Android进程间通信（IPC）机制Binder简要介绍和学习计划一文。

        开始进入主题了，在Android系统中的广播（Broadcast）机制简要介绍和学习计划一文所介绍的例子中，注册广播接收器的操作是MainActivity发起的，我们先来看看注册过程的序列图：

        在分析这个序列图之前，我们先来看一下MainActivity是如何调用registerReceiver函数来注册广播接收器的：

public class MainActivity extends Activity implements OnClickListener {
    ......

    @Override
    public void onResume() {
        super.onResume();

        IntentFilter counterActionFilter = new IntentFilter(CounterService.BROADCAST_COUNTER_ACTION);
        registerReceiver(counterActionReceiver, counterActionFilter);
    }

    ......

}

public class MainActivity extends Activity implements OnClickListener {
	......

	@Override
	public void onResume() {
		super.onResume();  

		IntentFilter counterActionFilter = new IntentFilter(CounterService.BROADCAST_COUNTER_ACTION);
		registerReceiver(counterActionReceiver, counterActionFilter);
	} 

	......

}

        MainActivity在onResume函数里，通过其父类ContextWrapper的registerReceiver函数注册了一个BroadcastReceiver实例counterActionReceiver，并且通过IntentFilter实例counterActionFilter告诉ActivityManagerService，它要订阅的广播是CounterService.BROADCAST_COUNTER_ACTION类型的，这样，ActivityManagerService在收到CounterService.BROADCAST_COUNTER_ACTION类型的广播时，就会分发给counterActionReceiver实例的onReceive函数。

        接下来，就开始分析注册过程中的每一个步骤了。

        Step 1. ContextWrapper.registerReceiver

        这个函数实现在frameworks/base/core/java/android/content/ContextWrapper.java文件中：

public class ContextWrapper extends Context {
    Context mBase;
    ......

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

    ......

}

public class ContextWrapper extends Context {
	Context mBase;
	......

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

	......

}

        这里的成员变量mBase是一个ContextImpl实例，想知道为什么，可以回过头去看看Android应用程序启动过程源代码分析这篇文章>~<。

        Step 2. ContextImpl.registerReceiver

        这个函数实现在frameworks/base/core/java/android/app/ContextImpl.java文件中：

class ContextImpl extends Context {
    ......

    @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, filter, broadcastPermission,
            scheduler, getOuterContext());
    }

    private Intent registerReceiverInternal(BroadcastReceiver receiver,
            IntentFilter filter, String broadcastPermission,
            Handler scheduler, Context context) {
        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 {
                ......
            }
        }
        try {
            return ActivityManagerNative.getDefault().registerReceiver(
                    mMainThread.getApplicationThread(),
                    rd, filter, broadcastPermission);
        } catch (RemoteException e) {
                return null;
        }
    }

    ......

}

class ContextImpl extends Context {
	......

	@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, filter, broadcastPermission,
			scheduler, getOuterContext());
	}

	private Intent registerReceiverInternal(BroadcastReceiver receiver,
			IntentFilter filter, String broadcastPermission,
			Handler scheduler, Context context) {
		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 {
				......
			}
		}
		try {
			return ActivityManagerNative.getDefault().registerReceiver(
					mMainThread.getApplicationThread(),
					rd, filter, broadcastPermission);
		} catch (RemoteException e) {
				return null;
		}
	}

	......

}

        通过两个函数的中转，最终就进入到ContextImpl.registerReceiverInternal这个函数来了。这里的成员变量mPackageInfo是一个LoadedApk实例，它是用来负责处理广播的接收的，在后面一篇文章讲到广播的发送时（sendBroadcast），会详细描述。参数broadcastPermission和scheduler都为null，而参数context是上面的函数通过调用函数getOuterContext得到的，这里它就是指向MainActivity了，因为MainActivity是继承于Context类的，因此，这里用Context类型来引用。

        由于条件mPackageInfo != null和context != null都成立，而且条件scheduler == null也成立，于是就调用mMainThread.getHandler来获得一个Handler了，这个Hanlder是后面用来分发ActivityManagerService发送过的广播用的。这里的成员变量mMainThread是一个ActivityThread实例，在前面Android应用程序启动过程源代码分析这篇文章也描述过了。我们先来看看ActivityThread.getHandler函数的实现，然后再回过头来继续分析ContextImpl.registerReceiverInternal函数。

        Step 3. ActivityThread.getHandler

        这个函数实现在frameworks/base/core/java/android/app/ActivityThread.java文件中：

public final class ActivityThread {
    ......

    final H mH = new H();

    private final class H extends Handler {
        ......

        public void handleMessage(Message msg) {
            ......

            switch (msg.what) {
            ......
            }

            ......
        }

        ......

    }

    ......

    final Handler getHandler() {
        return mH;
    }

    ......

}

public final class ActivityThread {
	......

	final H mH = new H();

	private final class H extends Handler {
		......

		public void handleMessage(Message msg) {
			......

			switch (msg.what) {
			......
			}

			......
		}

		......

	}

	......

	final Handler getHandler() {
		return mH;
	}

	......

}

        有了这个Handler之后，就可以分发消息给应用程序处理了。

        再回到上一步的ContextImpl.registerReceiverInternal函数中，它通过mPackageInfo.getReceiverDispatcher函数获得一个IIntentReceiver接口对象rd，这是一个Binder对象，接下来会把它传给ActivityManagerService，ActivityManagerService在收到相应的广播时，就是通过这个Binder对象来通知MainActivity来接收的。

        我们也是先来看一下mPackageInfo.getReceiverDispatcher函数的实现，然后再回过头来继续分析ContextImpl.registerReceiverInternal函数。

        Step 4. LoadedApk.getReceiverDispatcher

        这个函数实现在frameworks/base/core/java/android/app/LoadedApk.java文件中：

final class LoadedApk {
    ......

    public IIntentReceiver getReceiverDispatcher(BroadcastReceiver r,
            Context context, Handler handler,
            Instrumentation instrumentation, boolean registered) {
        synchronized (mReceivers) {
            LoadedApk.ReceiverDispatcher rd = null;
            HashMap<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 HashMap<BroadcastReceiver, LoadedApk.ReceiverDispatcher>();
                        mReceivers.put(context, map);
                    }
                    map.put(r, rd);
                }
            } else {
                rd.validate(context, handler);
            }
            return rd.getIIntentReceiver();
        }
    }

    ......

    static final class ReceiverDispatcher {

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

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

            ......
        }

        ......

        final IIntentReceiver.Stub mIIntentReceiver;
        final Handler mActivityThread;

        ......

        ReceiverDispatcher(BroadcastReceiver receiver, Context context,
                Handler activityThread, Instrumentation instrumentation,
                boolean registered) {
            ......

            mIIntentReceiver = new InnerReceiver(this, !registered);
            mActivityThread = activityThread;

            ......
        }

        ......

        IIntentReceiver getIIntentReceiver() {
            return mIIntentReceiver;
        }

    }

    ......

}

final class LoadedApk {
	......

	public IIntentReceiver getReceiverDispatcher(BroadcastReceiver r,
			Context context, Handler handler,
			Instrumentation instrumentation, boolean registered) {
		synchronized (mReceivers) {
			LoadedApk.ReceiverDispatcher rd = null;
			HashMap<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 HashMap<BroadcastReceiver, LoadedApk.ReceiverDispatcher>();
						mReceivers.put(context, map);
					}
					map.put(r, rd);
				}
			} else {
				rd.validate(context, handler);
			}
			return rd.getIIntentReceiver();
		}
	}

	......

	static final class ReceiverDispatcher {

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

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

			......
		}

		......

		final IIntentReceiver.Stub mIIntentReceiver;
		final Handler mActivityThread;

		......

		ReceiverDispatcher(BroadcastReceiver receiver, Context context,
				Handler activityThread, Instrumentation instrumentation,
				boolean registered) {
			......

			mIIntentReceiver = new InnerReceiver(this, !registered);
			mActivityThread = activityThread;

			......
		}

		......

		IIntentReceiver getIIntentReceiver() {
			return mIIntentReceiver;
		}

	}

	......

}

        在LoadedApk.getReceiverDispatcher函数中，首先看一下参数r是不是已经有相应的ReceiverDispatcher存在了，如果有，就直接返回了，否则就新建一个ReceiverDispatcher，并且以r为Key值保在一个HashMap中，而这个HashMap以Context，这里即为MainActivity为Key值保存在LoadedApk的成员变量mReceivers中，这样，只要给定一个Activity和BroadcastReceiver，就可以查看LoadedApk里面是否已经存在相应的广播接收发布器ReceiverDispatcher了。

        在新建广播接收发布器ReceiverDispatcher时，会在构造函数里面创建一个InnerReceiver实例，这是一个Binder对象，实现了IIntentReceiver接口，可以通过ReceiverDispatcher.getIIntentReceiver函数来获得，获得后就会把它传给ActivityManagerService，以便接收广播。在ReceiverDispatcher类的构造函数中，还会把传进来的Handle类型的参数activityThread保存下来，以便后面在分发广播的时候使用。

        现在，再回到ContextImpl.registerReceiverInternal函数，在获得了IIntentReceiver类型的Binder对象后，就开始要把它注册到ActivityManagerService中去了。

        Step 5. ActivityManagerProxy.registerReceiver

        这个函数实现在frameworks/base/core/java/android/app/ActivityManagerNative.java文件中：

class ActivityManagerProxy implements IActivityManager
{
    ......

    public Intent registerReceiver(IApplicationThread caller,
            IIntentReceiver receiver,
            IntentFilter filter, String perm) throws RemoteException
    {
        Parcel data = Parcel.obtain();
        Parcel reply = Parcel.obtain();
        data.writeInterfaceToken(IActivityManager.descriptor);
        data.writeStrongBinder(caller != null ? caller.asBinder() : null);
        data.writeStrongBinder(receiver != null ? receiver.asBinder() : null);
        filter.writeToParcel(data, 0);
        data.writeString(perm);
        mRemote.transact(REGISTER_RECEIVER_TRANSACTION, data, reply, 0);
        reply.readException();
        Intent intent = null;
        int haveIntent = reply.readInt();
        if (haveIntent != 0) {
            intent = Intent.CREATOR.createFromParcel(reply);
        }
        reply.recycle();
        data.recycle();
        return intent;
    }

    ......

}

class ActivityManagerProxy implements IActivityManager
{
	......

	public Intent registerReceiver(IApplicationThread caller,
			IIntentReceiver receiver,
			IntentFilter filter, String perm) throws RemoteException
	{
		Parcel data = Parcel.obtain();
		Parcel reply = Parcel.obtain();
		data.writeInterfaceToken(IActivityManager.descriptor);
		data.writeStrongBinder(caller != null ? caller.asBinder() : null);
		data.writeStrongBinder(receiver != null ? receiver.asBinder() : null);
		filter.writeToParcel(data, 0);
		data.writeString(perm);
		mRemote.transact(REGISTER_RECEIVER_TRANSACTION, data, reply, 0);
		reply.readException();
		Intent intent = null;
		int haveIntent = reply.readInt();
		if (haveIntent != 0) {
			intent = Intent.CREATOR.createFromParcel(reply);
		}
		reply.recycle();
		data.recycle();
		return intent;
	}

	......

}

         这个函数通过Binder驱动程序就进入到ActivityManagerService中的registerReceiver函数中去了。

         Step 6. ActivityManagerService.registerReceiver

         这个函数实现在frameworks/base/services/java/com/android/server/am/ActivityManagerService.java文件中：

public final class ActivityManagerService extends ActivityManagerNative
        implements Watchdog.Monitor, BatteryStatsImpl.BatteryCallback {
    ......

    public Intent registerReceiver(IApplicationThread caller,
            IIntentReceiver receiver, IntentFilter filter, String permission) {
        synchronized(this) {
            ProcessRecord callerApp = null;
            if (caller != null) {
                callerApp = getRecordForAppLocked(caller);
                if (callerApp == null) {
                    ......
                }
            }

            List allSticky = null;

            // Look for any matching sticky broadcasts...
            Iterator actions = filter.actionsIterator();
            if (actions != null) {
                while (actions.hasNext()) {
                    String action = (String)actions.next();
                    allSticky = getStickiesLocked(action, filter, allSticky);
                }
            } else {
                ......
            }

            // The first sticky in the list is returned directly back to
            // the client.
            Intent sticky = allSticky != null ? (Intent)allSticky.get(0) : null;

            ......

            if (receiver == null) {
                return sticky;
            }

            ReceiverList rl
                = (ReceiverList)mRegisteredReceivers.get(receiver.asBinder());
            if (rl == null) {
                rl = new ReceiverList(this, callerApp,
                    Binder.getCallingPid(),
                    Binder.getCallingUid(), receiver);

                if (rl.app != null) {
                    rl.app.receivers.add(rl);
                } else {
                    ......
                }
                mRegisteredReceivers.put(receiver.asBinder(), rl);
            }

            BroadcastFilter bf = new BroadcastFilter(filter, rl, permission);
            rl.add(bf);
            ......
            mReceiverResolver.addFilter(bf);

            // Enqueue broadcasts for all existing stickies that match
            // this filter.
            if (allSticky != null) {
                ......
            }

            return sticky;
        }
    }

    ......

}

public final class ActivityManagerService extends ActivityManagerNative
		implements Watchdog.Monitor, BatteryStatsImpl.BatteryCallback {
	......

	public Intent registerReceiver(IApplicationThread caller,
			IIntentReceiver receiver, IntentFilter filter, String permission) {
		synchronized(this) {
			ProcessRecord callerApp = null;
			if (caller != null) {
				callerApp = getRecordForAppLocked(caller);
				if (callerApp == null) {
					......
				}
			}

			List allSticky = null;

			// Look for any matching sticky broadcasts...
			Iterator actions = filter.actionsIterator();
			if (actions != null) {
				while (actions.hasNext()) {
					String action = (String)actions.next();
					allSticky = getStickiesLocked(action, filter, allSticky);
				}
			} else {
				......
			}

			// The first sticky in the list is returned directly back to
			// the client.
			Intent sticky = allSticky != null ? (Intent)allSticky.get(0) : null;

			......

			if (receiver == null) {
				return sticky;
			}

			ReceiverList rl
				= (ReceiverList)mRegisteredReceivers.get(receiver.asBinder());
			if (rl == null) {
				rl = new ReceiverList(this, callerApp,
					Binder.getCallingPid(),
					Binder.getCallingUid(), receiver);

				if (rl.app != null) {
					rl.app.receivers.add(rl);
				} else {
					......
				}
				mRegisteredReceivers.put(receiver.asBinder(), rl);
			}

			BroadcastFilter bf = new BroadcastFilter(filter, rl, permission);
			rl.add(bf);
			......
			mReceiverResolver.addFilter(bf);

			// Enqueue broadcasts for all existing stickies that match
			// this filter.
			if (allSticky != null) {
				......
			}

			return sticky;
		}
	}

	......

}

         函数首先是获得调用registerReceiver函数的应用程序进程记录块：

   ProcessRecord callerApp = null;
   if (caller != null) {
callerApp = getRecordForAppLocked(caller);
if (callerApp == null) {
    ......
       }
   }

    ProcessRecord callerApp = null;
    if (caller != null) {
	callerApp = getRecordForAppLocked(caller);
	if (callerApp == null) {
	    ......
        }
    }

        这里得到的便是上一篇文章Android系统中的广播（Broadcast）机制简要介绍和学习计划里面介绍的应用程序Broadcast的进程记录块了，MainActivity就是在里面启动起来的。

        

   List allSticky = null;

   // Look for any matching sticky broadcasts...
   Iterator actions = filter.actionsIterator();
   if (actions != null) {
while (actions.hasNext()) {
    String action = (String)actions.next();
    allSticky = getStickiesLocked(action, filter, allSticky);
}
   } else {
......
   }

   // The first sticky in the list is returned directly back to
   // the client.
   Intent sticky = allSticky != null ? (Intent)allSticky.get(0) : null;

    List allSticky = null;

    // Look for any matching sticky broadcasts...
    Iterator actions = filter.actionsIterator();
    if (actions != null) {
	while (actions.hasNext()) {
		String action = (String)actions.next();
		allSticky = getStickiesLocked(action, filter, allSticky);
	}
    } else {
	......
    }

    // The first sticky in the list is returned directly back to
    // the client.
    Intent sticky = allSticky != null ? (Intent)allSticky.get(0) : null;

        这里传进来的filter只有一个action，就是前面描述的CounterService.BROADCAST_COUNTER_ACTION了，这里先通过getStickiesLocked函数查找一下有没有对应的sticky intent列表存在。什么是Sticky Intent呢？我们在最后一次调用sendStickyBroadcast函数来发送某个Action类型的广播时，系统会把代表这个广播的Intent保存下来，这样，后来调用registerReceiver来注册相同Action类型的广播接收器，就会得到这个最后发出的广播。这就是为什么叫做Sticky Intent了，这个最后发出的广播虽然被处理完了，但是仍然被粘住在ActivityManagerService中，以便下一个注册相应Action类型的广播接收器还能继承处理。

        这里，假设我们不使用sendStickyBroadcast来发送CounterService.BROADCAST_COUNTER_ACTION类型的广播，于是，这里得到的allSticky和sticky都为null了。

        继续往下看，这里传进来的receiver不为null，于是，继续往下执行：

   ReceiverList rl
= (ReceiverList)mRegisteredReceivers.get(receiver.asBinder());
   if (rl == null) {
rl = new ReceiverList(this, callerApp,
    Binder.getCallingPid(),
    Binder.getCallingUid(), receiver);

if (rl.app != null) {
    rl.app.receivers.add(rl);
} else {
    ......
}
mRegisteredReceivers.put(receiver.asBinder(), rl);
   }

    ReceiverList rl
	= (ReceiverList)mRegisteredReceivers.get(receiver.asBinder());
    if (rl == null) {
	rl = new ReceiverList(this, callerApp,
		Binder.getCallingPid(),
		Binder.getCallingUid(), receiver);

	if (rl.app != null) {
		rl.app.receivers.add(rl);
	} else {
		......
	}
	mRegisteredReceivers.put(receiver.asBinder(), rl);
    }

        这里其实就是把广播接收器receiver保存一个ReceiverList列表中，这个列表的宿主进程是rl.app，这里就是MainActivity所在的进程了，在ActivityManagerService中，用一个进程记录块来表示这个应用程序进程，它里面有一个列表receivers，专门用来保存这个进程注册的广播接收器。接着，又把这个ReceiverList列表以receiver为Key值保存在ActivityManagerService的成员变量mRegisteredReceivers中，这些都是为了方便在收到广播时，快速找到对应的广播接收器的。

        再往下看：

BroadcastFilter bf = new BroadcastFilter(filter, rl, permission);
rl.add(bf);
......
mReceiverResolver.addFilter(bf);

    BroadcastFilter bf = new BroadcastFilter(filter, rl, permission);
    rl.add(bf);
    ......
    mReceiverResolver.addFilter(bf);

        上面只是把广播接收器receiver保存起来了，但是还没有把它和filter关联起来，这里就创建一个BroadcastFilter来把广播接收器列表rl和filter关联起来，然后保存在ActivityManagerService中的成员变量mReceiverResolver中去。

        这样，广播接收器注册的过程就介绍完了，比较简单，但是工作又比较琐碎，主要就是将广播接收器receiver及其要接收的广播类型filter保存在ActivityManagerService中，以便以后能够接收到相应的广播并进行处理，在下一篇文章，我们将详细分析这个过程，敬请关注。