通常我们使用Service都要和它通信，当想要与Service通信的时候，那么Service要处于绑定状态的。然后客户端可以拿到一个Binder与服务端进行通信，这个过程是很自然的。
那你真的了解过Service的绑定过程吗？为什么可以是Binder和Service通信？
同样的先看一张图大致了解一下，灰色背景框起来的是同一个类的方法，如下：
我们知道调用Context的bindService方法即可绑定一个Service，而ContextImpl是Context的实现类。那接下来就从源码的角度分析Service的绑定过程。
当然是从ContextImpl的bindService方法开始，如下：
@Overridepublic boolean bindService(Intent service, ServiceConnection conn, int flags) { warnIfCallingFromSystemProcess(); return bindServiceCommon(service, conn, flags, mMainThread.getHandler(), Process.myUserHandle());}在bindService方法中又会转到bindServiceCommon方法，将Intent，ServiceConnection对象传进。
那就看看bindServiceCommon方法的实现。
private boolean bindServiceCommon(Intent service, ServiceConnection conn, int flags, Handler handler, UserHandle user) { IServiceConnection sd; if (conn == null) { throw new IllegalArgumentException("connection is null"); } if (mPackageInfo != null) { sd = mPackageInfo.getServiceDispatcher(conn, getOuterContext(), handler, flags); } else { throw new RuntimeException("Not supported in system context"); } validateServiceIntent(service); try { IBinder token = getActivityToken(); if (token == null && (flags&BIND_AUTO_CREATE) == 0 && mPackageInfo != null && mPackageInfo.getApplicationInfo().targetSdkVersion < android.os.Build.VERSION_CODES.ICE_CREAM_SANDWICH) { flags |= BIND_WAIVE_PRIORITY; } service.prepareToLeaveProcess(this); int res = ActivityManagerNative.getDefault().bindService( mMainThread.getApplicationThread(), getActivityToken(), service, service.resolveTypeIfNeeded(getContentResolver()), sd, flags, getOpPackageName(), user.getIdentifier()); if (res < 0) { throw new SecurityException( "Not allowed to bind to service " + service); } return res != 0; } catch (RemoteException e) { throw e.rethrowFromSystemServer(); }}在上述代码中，调用了mPackageInfo（LoadedApk对象）的getServiceDispatcher方法。从getServiceDispatcher方法的名字可以看出是获取一个“服务分发者”。其实是根据这个“服务分发者”获取到一个Binder对象的。
那现在就看到getServiceDispatcher方法的实现。
public final IServiceConnection getServiceDispatcher(ServiceConnection c, Context context, Handler handler, int flags) { synchronized (mServices) { LoadedApk.ServiceDispatcher sd = null; ArrayMap<ServiceConnection, LoadedApk.ServiceDispatcher> map = mServices.get(context); if (map != null) { sd = map.get(c); } if (sd == null) { sd = new ServiceDispatcher(c, context, handler, flags); if (map == null) { map = new ArrayMap<ServiceConnection, LoadedApk.ServiceDispatcher>(); mServices.put(context, map); } map.put(c, sd); } else { sd.validate(context, handler); } return sd.getIServiceConnection(); }}从getServiceDispatcher方法的实现可以知道，ServiceConnection和ServiceDispatcher构成了映射关系。当存储集合不为空的时候，根据传进的key，也就是ServiceConnection，来取出对应的ServiceDispatcher对象。
当取出ServiceDispatcher对象后，最后一行代码是关键，
return sd.getIServiceConnection();
调用了ServiceDispatcher对象的getIServiceConnection方法。这个方法肯定是获取一个IServiceConnection的。
IServiceConnection getIServiceConnection() { return mIServiceConnection;}那么mIServiceConnection是什么？现在就可以来看下ServiceDispatcher类了。ServiceDispatcher是LoadedApk的内部类，里面封装了InnerConnection和ServiceConnection。如下：
static final class ServiceDispatcher { private final ServiceDispatcher.InnerConnection mIServiceConnection; private final ServiceConnection mConnection; private final Context mContext; private final Handler mActivityThread; private final ServiceConnectionLeaked mLocation; private final int mFlags; private RuntimeException mUnbindLocation; private boolean mForgotten; private static class ConnectionInfo { IBinder binder; IBinder.DeathRecipient deathMonitor; } private static class InnerConnection extends IServiceConnection.Stub { final WeakReference<LoadedApk.ServiceDispatcher> mDispatcher; InnerConnection(LoadedApk.ServiceDispatcher sd) { mDispatcher = new WeakReference<LoadedApk.ServiceDispatcher>(sd); } public void connected(ComponentName name, IBinder service) throws RemoteException { LoadedApk.ServiceDispatcher sd = mDispatcher.get(); if (sd != null) { sd.connected(name, service); } } } private final ArrayMap<ComponentName, ServiceDispatcher.ConnectionInfo> mActiveConnections = new ArrayMap<ComponentName, ServiceDispatcher.ConnectionInfo>(); ServiceDispatcher(ServiceConnection conn, Context context, Handler activityThread, int flags) { mIServiceConnection = new InnerConnection(this); mConnection = conn; mContext = context; mActivityThread = activityThread; mLocation = new ServiceConnectionLeaked(null); mLocation.fillInStackTrace(); mFlags = flags; } //代码省略}先看到ServiceDispatcher的构造方法，一个ServiceDispatcher关联一个InnerConnection对象。而InnerConnection呢？，它是一个Binder，有一个很重要的connected方法。至于为什么要用Binder，因为与Service通信可能是跨进程的。
好，到了这里先总结一下：调用bindService方法绑定服务，会转到bindServiceCommon方法。
在bindServiceCommon方法中，会调用LoadedApk的getServiceDispatcher方法，并将ServiceConnection传进， 根据这个ServiceConnection取出与其映射的ServiceDispatcher对象，最后调用这个ServiceDispatcher对象的getIServiceConnection方法获取与其关联的InnerConnection对象并返回。简单点理解就是用ServiceConnection换来了InnerConnection。
现在回到bindServiceCommon方法，可以看到绑定Service的过程会转到ActivityManagerNative.getDefault()的bindService方法，其实从抛出的异常类型RemoteException也可以知道与Service通信可能是跨进程的，这个是很好理解的。
而ActivityManagerNative.getDefault()是ActivityManagerService，那么继续跟进ActivityManagerService的bindService方法即可，如下：
public int bindService(IApplicationThread caller, IBinder token, Intent service, String resolvedType, IServiceConnection connection, int flags, String callingPackage, int userId) throws TransactionTooLargeException { enforceNotIsolatedCaller("bindService"); // Refuse possible leaked file descriptors if (service != null && service.hasFileDescriptors() == true) { throw new IllegalArgumentException("File descriptors passed in Intent"); } if (callingPackage == null) { throw new IllegalArgumentException("callingPackage cannot be null"); } synchronized(this) { return mServices.bindServiceLocked(caller, token, service, resolvedType, connection, flags, callingPackage, userId); }}在上述代码中，绑定Service的过程转到ActiveServices的bindServiceLocked方法，那就跟进ActiveServices的bindServiceLocked方法瞧瞧。如下：
int bindServiceLocked(IApplicationThread caller, IBinder token, Intent service, String resolvedType, final IServiceConnection connection, int flags, String callingPackage, final int userId) throws TransactionTooLargeException { //代码省略 ConnectionRecord c = new ConnectionRecord(b, activity, connection, flags, clientLabel, clientIntent); IBinder binder = connection.asBinder(); ArrayList<ConnectionRecord> clist = s.connections.get(binder); if (clist == null) { clist = new ArrayList<ConnectionRecord>(); s.connections.put(binder, clist); } clist.add(c); //代码省略 if ((flags&Context.BIND_AUTO_CREATE) != 0) { s.lastActivity = SystemClock.uptimeMillis(); if (bringUpServiceLocked(s, service.getFlags(), callerFg, false, permissionsReviewRequired) != null) { return 0; } } //代码省略 return 1;}将connection对象封装在ConnectionRecord中，这里的connection就是上面提到的InnerConnection对象。这一步很重要的。
然后调用了bringUpServiceLocked方法，那么就探探这个bringUpServiceLocked方法，
private String bringUpServiceLocked(ServiceRecord r, int intentFlags, boolean execInFg, boolean whileRestarting, boolean permissionsReviewRequired) throws TransactionTooLargeException { //代码省略 if (app != null && app.thread != null) { try { app.addPackage(r.appInfo.packageName, r.appInfo.versionCode, mAm.mProcessStats); realStartServiceLocked(r, app, execInFg); return null; } catch (TransactionTooLargeException e) { throw e; } catch (RemoteException e) { Slog.w(TAG, "Exception when starting service " + r.shortName, e); } // If a dead object exception was thrown -- fall through to // restart the application. } //代码省略 return null;}可以看到调用了realStartServiceLocked方法，真正去启动Service了。
那么跟进realStartServiceLocked方法探探，如下：
private final void realStartServiceLocked(ServiceRecord r, ProcessRecord app, boolean execInFg) throws RemoteException { //代码省略 app.thread.scheduleCreateService(r, r.serviceInfo, mAm.compatibilityInfoForPackageLocked(r.serviceInfo.applicationInfo), app.repProcState); r.postNotification(); created = true; //代码省略 requestServiceBindingsLocked(r, execInFg); updateServiceClientActivitiesLocked(app, null, true); // If the service is in the started state, and there are no // pending arguments, then fake up one so its onStartCommand() will // be called. if (r.startRequested && r.callStart && r.pendingStarts.size() == 0) { r.pendingStarts.add(new ServiceRecord.StartItem(r, false, r.makeNextStartId(), null, null)); } sendServiceArgsLocked(r, execInFg, true);//代码省略}这里会调用app.thread的scheduleCreateService方法去创建一个Service，然后会回调Service的生命周期方法，然而绑定Service呢？
在上述代码中，找到一个requestServiceBindingsLocked方法，从名字看是请求绑定服务的意思，那么就是它没错了。
private final void requestServiceBindingsLocked(ServiceRecord r, boolean execInFg) throws TransactionTooLargeException { for (int i=r.bindings.size()-1; i>=0; i--) { IntentBindRecord ibr = r.bindings.valueAt(i); if (!requestServiceBindingLocked(r, ibr, execInFg, false)) { break; } }}咦，我再按住Ctrl+鼠标左键，点进去requestServiceBindingLocked方法。如下：
private final boolean requestServiceBindingLocked(ServiceRecord r, IntentBindRecord i, boolean execInFg, boolean rebind) throws TransactionTooLargeException { if (r.app == null || r.app.thread == null) { // If service is not currently running, can't yet bind. return false; } if ((!i.requested || rebind) && i.apps.size() > 0) { try { bumpServiceExecutingLocked(r, execInFg, "bind"); r.app.forceProcessStateUpTo(ActivityManager.PROCESS_STATE_SERVICE); r.app.thread.scheduleBindService(r, i.intent.getIntent(), rebind, r.app.repProcState); if (!rebind) { i.requested = true; } i.hasBound = true; i.doRebind = false; } //代码省略 return true;}r.app.thread调用了scheduleBindService方法来绑定服务，而r.app.thread是ApplicationThread，现在关注到 ApplicationThread即可，scheduleBindService方法如下：
public final void scheduleBindService(IBinder token, Intent intent, boolean rebind, int processState) { updateProcessState(processState, false); BindServiceData s = new BindServiceData(); s.token = token; s.intent = intent; s.rebind = rebind; if (DEBUG_SERVICE) Slog.v(TAG, "scheduleBindService token=" + token + " intent=" + intent + " uid=" + Binder.getCallingUid() + " pid=" + Binder.getCallingPid()); sendMessage(H.BIND_SERVICE, s);}封装了待绑定的Service的信息，并发送了一个消息给主线程，
public void handleMessage(Message msg) { if (DEBUG_MESSAGES) Slog.v(TAG, ">>> handling: " + codeToString(msg.what)); switch (msg.what) { //代码省略 case BIND_SERVICE: Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "serviceBind"); handleBindService((BindServiceData)msg.obj); Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER); break; //代码省略 }}调用了handleBindService方法，即将绑定完成啦。
private void handleBindService(BindServiceData data) { Service s = mServices.get(data.token); if (DEBUG_SERVICE) Slog.v(TAG, "handleBindService s=" + s + " rebind=" + data.rebind); if (s != null) { try { data.intent.setExtrasClassLoader(s.getClassLoader()); data.intent.prepareToEnterProcess(); try { if (!data.rebind) { IBinder binder = s.onBind(data.intent); ActivityManagerNative.getDefault().publishService( data.token, data.intent, binder); } else { s.onRebind(data.intent); ActivityManagerNative.getDefault().serviceDoneExecuting( data.token, SERVICE_DONE_EXECUTING_ANON, 0, 0); } ensureJitEnabled(); } catch (RemoteException ex) { throw ex.rethrowFromSystemServer(); } } catch (Exception e) { if (!mInstrumentation.onException(s, e)) { throw new RuntimeException( "Unable to bind to service " + s + " with " + data.intent + ": " + e.toString(), e); } } }}根据token获取到Service，然后Service回调onBind方法。结束了？
可是onBind方法返回了一个binder是用来干嘛的？
我们再看看ActivityManagerNative.getDefault()调用了publishService方法做了什么工作，此时又回到了ActivityManagerService。
public void publishService(IBinder token, Intent intent, IBinder service) { // Refuse possible leaked file descriptors if (intent != null && intent.hasFileDescriptors() == true) { throw new IllegalArgumentException("File descriptors passed in Intent"); } synchronized(this) { if (!(token instanceof ServiceRecord)) { throw new IllegalArgumentException("Invalid service token"); } mServices.publishServiceLocked((ServiceRecord)token, intent, service); }}又会交给ActiveServices处理，转到了publishServiceLocked方法，那看到ActiveServices的publishServiceLocked方法，
void publishServiceLocked(ServiceRecord r, Intent intent, IBinder service) { final long origId = Binder.clearCallingIdentity(); try { if (DEBUG_SERVICE) Slog.v(TAG_SERVICE, "PUBLISHING " + r + " " + intent + ": " + service); if (r != null) { Intent.FilterComparison filter = new Intent.FilterComparison(intent); IntentBindRecord b = r.bindings.get(filter); if (b != null && !b.received) { b.binder = service; b.requested = true; b.received = true; for (int conni=r.connections.size()-1; conni>=0; conni--) { ArrayList<ConnectionRecord> clist = r.connections.valueAt(conni); for (int i=0; i<clist.size(); i++) { ConnectionRecord c = clist.get(i); if (!filter.equals(c.binding.intent.intent)) { if (DEBUG_SERVICE) Slog.v( TAG_SERVICE, "Not publishing to: " + c); if (DEBUG_SERVICE) Slog.v( TAG_SERVICE, "Bound intent: " + c.binding.intent.intent); if (DEBUG_SERVICE) Slog.v( TAG_SERVICE, "Published intent: " + intent); continue; } if (DEBUG_SERVICE) Slog.v(TAG_SERVICE, "Publishing to: " + c); try { c.conn.connected(r.name, service); } //代码省略}c.conn是什么？ 它是一个InnerConnection对象，对，就是那个那个Binder，上面也贴出了代码，在ActiveServices的bindServiceLocked方法中，InnerConnection对象被封装在ConnectionRecord中。那么现在它调用了connected方法，就很好理解了。
InnerConnection的connected方法如下：
public void connected(ComponentName name, IBinder service) throws RemoteException { LoadedApk.ServiceDispatcher sd = mDispatcher.get(); if (sd != null) { sd.connected(name, service); }}会调用ServiceDispatcher 的connected方法，如下
public void connected(ComponentName name, IBinder service) { if (mActivityThread != null) { mActivityThread.post(new RunConnection(name, service, 0)); } else { doConnected(name, service); }}从而ActivityThread会投递一个消息到主线程，此时就解决了跨进程通信。
那么你应该猜到了RunConnection一定有在主线程回调的onServiceConnected方法，
private final class RunConnection implements Runnable { RunConnection(ComponentName name, IBinder service, int command) { mName = name; mService = service; mCommand = command; } public void run() { if (mCommand == 0) { doConnected(mName, mService); } else if (mCommand == 1) { doDeath(mName, mService); } } final ComponentName mName; final IBinder mService; final int mCommand;}咦，还不出现？继续跟进doConnected方法，
public void doConnected(ComponentName name, IBinder service) { ServiceDispatcher.ConnectionInfo old; ServiceDispatcher.ConnectionInfo info; synchronized (this) { if (mForgotten) { // We unbound before receiving the connection; ignore // any connection received. return; } old = mActiveConnections.get(name); if (old != null && old.binder == service) { // Huh, already have this one. Oh well! return; } if (service != null) { // A new service is being connected... set it all up. info = new ConnectionInfo(); info.binder = service; info.deathMonitor = new DeathMonitor(name, service); try { service.linkToDeath(info.deathMonitor, 0); mActiveConnections.put(name, info); } catch (RemoteException e) { // This service was dead before we got it... just // don't do anything with it. mActiveConnections.remove(name); return; } } else { // The named service is being disconnected... clean up. mActiveConnections.remove(name); } if (old != null) { old.binder.unlinkToDeath(old.deathMonitor, 0); } } // If there was an old service, it is not disconnected. if (old != null) { mConnection.onServiceDisconnected(name); } // If there is a new service, it is now connected. if (service != null) { mConnection.onServiceConnected(name, service); }}在最后一个if判断，终于找到了onServiceConnected方法！
总结一下，当Service回调onBind方法，其实还没有结束，会转到ActivityManagerService，然后又会在ActiveServices的publishServiceLocked方法中，从ConnectionRecord中取出InnerConnection对象。有了InnerConnection对象后，就回调了它的connected。在InnerConnection的connected方法中，又会调用ServiceDispatcher的connected方法，在ServiceDispatcher的connected方法向主线程扔了一个消息，切换到了主线程，之后就在主线程中回调了客户端传进的ServiceConnected对象的onServiceConnected方法。
至此， Service的绑定过程分析完毕。
以上就是本文的全部内容，希望对大家的学习有所帮助，也希望大家多多支持武林网。
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