附上一張流程圖：

Service的繫結過程從ContextWrapper開始，如下：

  @Override
    public boolean bindService(Intent service, ServiceConnection conn,
            int flags) {
        return mBase.bindService(service, conn, flags);
    }

和啟動Service的過程一樣，直接看ContextImpl的bindService方法、bindServiceCommon方法。

@Override
    public boolean bindService(Intent service, ServiceConnection conn,
            int flags) {
        warnIfCallingFromSystemProcess();
        return bindServiceCommon(service, conn, flags, mMainThread.getHandler(),
                Process.myUserHandle());
    }

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();
        }
    }
 

bindServiceCommon方法完成兩件事：

首先要把客戶端的ServiceConnection物件轉換為ServiceDispatcher.InnerConnection。不直接使用ServiceConnection，服務的繫結有可能是跨程序的。ServiceConnection必須藉助Binder讓遠端服務端回撥自己的方法,ServiceDispatcher的內部類InnerConnection可以充當Binder角色。ServiceDispatcher的作用就是連線ServiceConnection與InnerConnection。檢視getServiceDispatcher原始碼：

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();
        }
    }





  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;
        }

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;
        }


 IServiceConnection getIServiceConnection() {
            return mIServiceConnection;
        }


       private final ArrayMap<Context, ArrayMap<ServiceConnection, LoadedApk.ServiceDispatcher>> mServices
        = new ArrayMap<Context, ArrayMap<ServiceConnection, LoadedApk.ServiceDispatcher>>();
 

在上面的程式碼中，mServices是一個ArrayMap,它儲存了一個應用當前活動的ServiceConnection和ServiceDispatcher的對映關係。

系統會先查詢是否有相同的ServiceConnection,不存在就重建一個ServiceDispatcher物件，並將其存在mServices中，key是ServiceConnection,value是ServiceDispatcher,在ServiceDispatcher的內部儲存了ServiceConnection和InnerConnection物件。當客戶端和Service建立聯絡後，系統通過InnerConnection來呼叫ServiceConnection的onServiceConnected方法（後面會單獨講解這個聯絡），這個過程可能是誇程序的。當ServiceDispatcher建好以後，getServiceDispatcher方法返回的就是其儲存的InnerConnection物件。

在看AMS的bindService方法：

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);
        }
    }

AMS呼叫ActiveServices的bindServiceLocked方法：

int bindServiceLocked(IApplicationThread caller, IBinder token, Intent service,
            String resolvedType, final IServiceConnection connection, int flags,
            String callingPackage, final int userId) throws TransactionTooLargeException {
	…….
 try {
                                        bringUpServiceLocked(serviceRecord,
                                                serviceIntent.getFlags(),
                                                callerFg, false, false);
                                    } catch (RemoteException e) {
                                        /* ignore - local call */
                                    }
	…….
}

ActiveServices呼叫bringUpServiceLocked方法，在呼叫realStartServiceLocked方法，realStartServiceLocked方法又呼叫ApplicationThread的scheduleCreateService方法，這裡的邏輯和Service進階篇（startService），就不在複述。和startService不同的是，Service的繫結過程會呼叫requestServiceBindingLocked。

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;
            } catch (TransactionTooLargeException e) {
                // Keep the executeNesting count accurate.
                if (DEBUG_SERVICE) Slog.v(TAG_SERVICE, "Crashed while binding " + r, e);
                final boolean inDestroying = mDestroyingServices.contains(r);
                serviceDoneExecutingLocked(r, inDestroying, inDestroying);
                throw e;
            } catch (RemoteException e) {
                if (DEBUG_SERVICE) Slog.v(TAG_SERVICE, "Crashed while binding " + r);
                // Keep the executeNesting count accurate.
                final boolean inDestroying = mDestroyingServices.contains(r);
                serviceDoneExecutingLocked(r, inDestroying, inDestroying);
                return false;
            }
        }
        return true;
    }

ApplicationThread呼叫scheduleBindService方法，有Handle處理，最終由Activity的handleBindService方法來處理。

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);
        }
 public void handleMessage(Message msg) {
	…..
	case BIND_SERVICE:
                    Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "serviceBind");
                    handleBindService((BindServiceData)msg.obj);
                    Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);
                    break;
	…….
}

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);
                }
            }
        }
    }

在handleBindService方法中，首先根據Service的token資料取出Service物件，然後呼叫Service的onBind方法，返回一個Binder物件給客戶端。Service呼叫了onBind方法，Service就處於繫結狀態了，但是客戶端不知道已成功連線Service，所以要呼叫客戶端的ServiceConnection的onServiceConnected方法，這個過程是由 ActivityManagerNative.getDefault().publishService來完成，也就是AMS的publishService方法。

Service有一個特性，多次繫結同一個Service，onBind方法只會執行一次，除非Service終止。Service繫結成功，系統還需要通知客戶端。

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);
        }
    }

AMS的publishService方法由ActiveServices的publishServiceLocked來處理。

void publishServiceLocked(ServiceRecord r, Intent intent, IBinder service) {
        final long origId = Binder.clearCallingIdentity();
	……
                                   ConnectionRecord c = clist.get(i);
	……
                           E) Slog.v(TAG_SERVICE, "Publishing to: " + c);
                            try {
                                c.conn.connected(r.name, service)
                            } catch (Exception e) {
                                Slog.w(TAG, "Failure sending service " + r.name +
                                      " to connection " + c.conn.asBinder() +
                                      " (in " + c.binding.client.processName + ")", e);
                            }
                        }
                    }
                }

                serviceDoneExecutingLocked(r, mDestroyingServices.contains(r), false);
            }
        } finally {
            Binder.restoreCallingIdentity(origId);
        }
    }


final class ConnectionRecord {
    final AppBindRecord binding;    // The application/service binding.
    final ActivityRecord activity;  // If non-null, the owning activity.
    final IServiceConnection conn; 
	……
}

核心程式碼： c.conn.connected(r.name, service)。c代表的是ConnectionRecord，c.conn的型別是ServiceDispatcher.InnerConnection。service是返回的Binder物件。檢視ServiceDispatcher.InnerConnection：

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);
                }
            }
        }

InnerConnection的connected方法，呼叫了ServiceDispatcher的connected方法：

public void connected(ComponentName name, IBinder service) {
            if (mActivityThread != null) {
                mActivityThread.post(new RunConnection(name, service, 0));
            } else {
                doConnected(name, service);
            }
        }

對於Service的繫結過程來說，ServiceDispatcher的mActivityThread是一個Handle,就是ActivityThread的H,從ServiceDispatcher的建立過程可知mActivityThread不為null，RunConnection由Handle的post方法執行在主執行緒。由此可知客戶端的ServiceConnection是在主執行緒被回撥的。

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;
        }

RunConnection的run方法呼叫了ServiceDispatcher的doConnectes方法。ServiceDispatcher內部儲存了ServiceConnection物件，很方便呼叫它的onServiceConnected方法。

public void doConnected(ComponentName name, IBinder service) {
            ServiceDispatcher.ConnectionInfo old;
            ServiceDispatcher.ConnectionInfo info;
	…….
 // 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);
            }
        }

客戶端的onServiceConnected方法執行後，Service的繫結過程也就完成了。