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：

@Override
public 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。