Activity服务启动大致可以分为以下几个阶段：

1. 准备阶段：做进程启动前的准备工作。
2. 进程启动阶段：通过Zygote启动进程。(当服务已经启动时，此步骤略）
3. Activity启动阶段：在新的进程里，启动Activity。

上面的流程图，和Activity应用启动相似，最后都分为服务进程已经启动和服务进程未启动两种情况，服务进程已经启动的情况下，不需要进程启动，直接到服务启动步骤。

从先前服务实例中，我们知道，当要调用一个服务时，需要先创建一个ServiceConnection，并在OnServiceConnected函数中，保存服务的Binder接口，以便调用服务的各种接口。然后，再绑定服务，再调用服务。

Client是调用Activity.bindService来绑定服务的，怎么会到ContextImpl.bindService中?上图中略去了一些步骤，这里结合代码说明：

Activity 继承自 ContextThemeWrapper ，而ContextThemeWrapper继承自 ContextWrapper，bindService就定义在ContextWraper中。

frameworks/base/core/java/android/content/ContextWrapper.java

public class ContextWrapper extends Context {
    Context mBase;

    public ContextWrapper(Context base) {
        mBase = base;
    }
    ...
    public boolean bindService(Intent service, ServiceConnection conn,
            int flags) {
        return mBase.bindService(service, conn, flags);
    }
    ...
}

再看ContextThemeWrapper

frameworks/base/core/java/android/view/ContextThemeWrapper.java

public class ContextThemeWrapper extends ContextWrapper {
    ...
    public ContextThemeWrapper() {
        super(null);
    }
    ...
}

而Activity没有构造函数，这说明我们new一个Activity时，mBase是null，那怎么bindService呢？一定有一个地方设置了mBase。在《Android进阶-Activity应用启动分析》一文中写到ActivityThread.performLaunchActivity函数有介绍。现在继续这个函数：

frameworks/base/core/java/android/app/ActivityThread.java

public ActivityThread{
    private Context createBaseContextForActivity(ActivityClientRecord r,
            final Activity activity) {
        ContextImpl appContext = new ContextImpl();
        appContext.init(r.packageInfo, r.token, this);
        appContext.setOuterContext(activity);

        ...
        Context baseContext = appContext;
        ...
        return baseContext;
    }
    ...
    private Activity performLaunchActivity(ActivityClientRecord r, Intent customIntent) {
        // System.out.println("##### [" + System.currentTimeMillis() + "] ActivityThread.performLaunchActivity(" + r + ")");
        ...

        try {
            Application app = r.packageInfo.makeApplication(false, mInstrumentation);
            ...
            if (activity != null) {
            	//创建上下文
                Context appContext = createBaseContextForActivity(r, activity);
                CharSequence title = r.activityInfo.loadLabel(appContext.getPackageManager());
                Configuration config = new Configuration(mCompatConfiguration);
                if (DEBUG_CONFIGURATION) Slog.v(TAG, "Launching activity "
                        + r.activityInfo.name + " with config " + config);
                //连接上下文
                activity.attach(appContext, this, getInstrumentation(), r.token,
                        r.ident, app, r.intent, r.activityInfo, title, r.parent,
                        r.embeddedID, r.lastNonConfigurationInstances, config);

               ...
            }
            r.paused = true;

            mActivities.put(r.token, r);

        } catch (SuperNotCalledException e) {
            throw e;

        } catch (Exception e) {
            if (!mInstrumentation.onException(activity, e)) {
                throw new RuntimeException(
                    "Unable to start activity " + component
                    + ": " + e.toString(), e);
            }
        }

        return activity;
    }
}

frameworks/base/core/java/android/app/Activity.java

public Activity{
	...
    final void attach(Context context, ActivityThread aThread,
            Instrumentation instr, IBinder token, int ident,
            Application application, Intent intent, ActivityInfo info,
            CharSequence title, Activity parent, String id,
            NonConfigurationInstances lastNonConfigurationInstances,
            Configuration config) {
        // 连接mBase
        attachBaseContext(context);

        mFragments.attachActivity(this, mContainer, null);
        ...
    }
}

frameworks/base/core/java/android/view/ContextThemeWrapper.java

public class ContextThemeWrapper{
	protected void attachBaseContext(Context newBase) {
		//连接mBase
        super.attachBaseContext(newBase);
        mBase = newBase;
    }
}

从上面的源代码可以看出，当载入Activity类后，便会调用createBaseContextForActivity来创建appContext，再用activity.attach来连接context。而appContext是用newContextImpl()来创建的，所以，Activity.mBase就是一个ContextImpl的类实例。

因此，上图从ContextImpl.bindService开始。

frameworks/base/core/java/android/app/ContextImpl.java

    private boolean bindServiceCommon(Intent service, ServiceConnection conn, int flags,
            UserHandle user) {
        IServiceConnection sd;
        if (conn == null) {
            throw new IllegalArgumentException("connection is null");
        }
        if (mPackageInfo != null) {
        	// 创建一个IServiceConnection对象，服务绑定后，需要调用此对象的connected函数，触发ServiceConnection.onServiceConnected事件
            // 此对象是一个LoadedApk.ServiceDispatcher.InnerConnection对象，见后面的LoadedApk的代码解释
            sd = mPackageInfo.getServiceDispatcher(conn, getOuterContext(),
                    mMainThread.getHandler(), flags);
        } else {
            throw new RuntimeException("Not supported in system context");
        }
        validateServiceIntent(service);
        try {
            ...
            // 通过ActivieyManagerProxy.bindService，经由Binder调用ActivityManagerService.bindService
            int res = ActivityManagerNative.getDefault().bindService(
                mMainThread.getApplicationThread(), getActivityToken(),
                service, service.resolveTypeIfNeeded(getContentResolver()),
                sd, flags, user.getIdentifier());
            if (res < 0) {
                throw new SecurityException(
                        "Not allowed to bind to service " + service);
            }
            return res != 0;
        } catch (RemoteException e) {
            return false;
        }
    }

frameworks/base/core/java/android/app/LoadedApk.java

public class LoadedApk{
	...
	static final class ServiceDispatcher {
		private final ServiceDispatcher.InnerConnection mIServiceConnection;
		...
		private static class InnerConnection extends IServiceConnection.Stub {
            ...
        }
        ...
        IServiceConnection getIServiceConnection() {
        	// 返回一个InnerConnection连接
            return mIServiceConnection;
        }
	}
	...
	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) {
            	// 创建ServiceDispatcher
                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);
            }
            // 返回ServiceDispatcher.getIServiceConnection()
            return sd.getIServiceConnection();
        }
    }
	...
}

本函数做两个事情：

1. 调用mPackageInfo.getServiceDispatcher创建IServiceConnection连接，返回的是ServiceDispatcher.InnerConnection对象，此对象作为bindService的参数，传给后续实现者。此对象可以通过connected函数，来触发ServiceConection.onServiceConnected，来通知调用方，服务已经绑定了，并传入IBinder对象，可以调用方通过此对象来调用服务的各种操作；
2. 通过ActivityManagerProxy.bindService，来调用ActivityManagerService中的bindService方法。说明：ActivityManagerNavite.getDefault()返回的是一个ActivityManagerProxy对象，这在《Android进阶- Activity应用启动分析》一文中已经有介绍。具体的过程见上图，Binder通信过程本文忽略。

ContextImpl通过ActivityManagerProxy，经由Binder驱动程序，将bindService的操作传入到了ActivityManagerService.bindService中，图中有较为清楚的表述，文字上不再细说。流程走到ActivityManagerService.bindService中后，又有两个关键动作，即ActiveServices.realStartServiceLocked和ActiveServices.reuqestServiceBindLocket。

frameworks/base/services/java/com/android/server/am/ActiveServices.java

public class ActiveServices{
	...
	private final String bringUpServiceLocked(ServiceRecord r,
            int intentFlags, boolean execInFg, boolean whileRestarting) {
        ...
        ProcessRecord app;

        if (!isolated) {
            app = mAm.getProcessRecordLocked(procName, r.appInfo.uid, false);
            if (DEBUG_MU) Slog.v(TAG_MU, "bringUpServiceLocked: appInfo.uid=" + r.appInfo.uid
                        + " app=" + app);
            if (app != null && app.thread != null) {
            	// 如果服务进程已经启动
                try {
                    app.addPackage(r.appInfo.packageName, mAm.mProcessStats);
                    realStartServiceLocked(r, app, execInFg);
                    return null;
                } 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.
            }
        } else {
            ...
        }

        // Not running -- get it started, and enqueue this service record
        // to be executed when the app comes up.
        if (app == null) {
        	// 如果服务进程未启动
            if ((app=mAm.startProcessLocked(procName, r.appInfo, true, intentFlags,
                    "service", r.name, false, isolated, false)) == null) {
                ...
                return msg;
            }
            if (isolated) {
                r.isolatedProc = app;
            }
        }
		...
        return null;
    }
}

上面的函数流程很简单，就是判断如果服务进程已经启动，则直接调用realStartServiceLocked启动服务，否则调用ActivityManagerService.startProcessLocked启动进程，进入进程启动阶段。

略。详见《Android 进阶 - 进程启动分析》一文。

上面的图看似比Activity启动要复杂，实际上大的步骤差不过，只不过，服务启动时，先要createService，再bindService，要发两次消息，而Activity启动只需要发一次消息。

如果服务进程已经启动的情况下，可直接从3.1:realStartServiceLocked一步往下看。

请先参看《Android 进阶 - Looper进程内通信》和《Android 进阶 -Activity应用启动分析》，从bindServiceLocked到sendMessage，再到消息进入到Looper.mainLooper.queue队列中，如果看了前面两篇文章，相信这一部分很容易看懂，这里不再讨论。流程主要发了两个消息H.CREATE_SERVICE和H.BIND_SERVICE，一个是创建服务的消息，一个是绑定服务的消息。这些消息会在Looper.loop函数依次处理。

H.CREATE_SERVICE的消息，经由H.dispatchMessage，会进入ActivityThread.handleCreateService函数。

frameworks/base/core/java/android/app/ActivityThread.java

public clas ActivityThread{
	...
	private void handleCreateService(CreateServiceData data) {
        // If we are getting ready to gc after going to the background, well
        // we are back active so skip it.
        unscheduleGcIdler();

        LoadedApk packageInfo = getPackageInfoNoCheck(
                data.info.applicationInfo, data.compatInfo);
        Service service = null;
        try {
            java.lang.ClassLoader cl = packageInfo.getClassLoader();
            service = (Service) cl.loadClass(data.info.name).newInstance();
        } catch (Exception e) {
            if (!mInstrumentation.onException(service, e)) {
                throw new RuntimeException(
                    "Unable to instantiate service " + data.info.name
                    + ": " + e.toString(), e);
            }
        }

        try {
            if (localLOGV) Slog.v(TAG, "Creating service " + data.info.name);

            ContextImpl context = new ContextImpl();
            context.init(packageInfo, null, this);

            Application app = packageInfo.makeApplication(false, mInstrumentation);
            context.setOuterContext(service);
            // 连接服务上下文
            service.attach(context, this, data.info.name, data.token, app,
                    ActivityManagerNative.getDefault());
            // 触发服务的onCreate事件
            service.onCreate();
            mServices.put(data.token, service);
            try {
                ActivityManagerNative.getDefault().serviceDoneExecuting(
                        data.token, 0, 0, 0);
            } catch (RemoteException e) {
                // nothing to do.
            }
        } catch (Exception e) {
            if (!mInstrumentation.onException(service, e)) {
                throw new RuntimeException(
                    "Unable to create service " + data.info.name
                    + ": " + e.toString(), e);
            }
        }
    }
}

H.BIND_SERVICE的消息，经由H.dispatchMessage，会进入ActivityThread.handleBindService函数。

frameworks/base/core/java/android/app/ActivityThread.java

public class ActivityThread{
	...
    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());
                try {
                    if (!data.rebind) {
                    	// 触发服务的onBind事件
                        IBinder binder = s.onBind(data.intent);
                        // 通过Binder，调用ActivityManagerService.publishService发布服务
                        ActivityManagerNative.getDefault().publishService(
                                data.token, data.intent, binder);
                    } else {
                        ...
                    }
                    ensureJitEnabled();
                } catch (RemoteException ex) {
                }
            } catch (Exception e) {
               ...
            }
        }
    }
}

上节的源码中说明，服务绑定完成之后，会通过ActivityManagerProxy代理，经由Binder，调用ActivityManagerService的publishService函数。frameworks/base/services/java/com/android/server/am/ActivityManagerService.java

pulbic class 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);
        }
    }
    ...
}
    

frameworks/base/services/java/com/android/server/am/ActiveServices.java

pulbic class ActiveService{
	...
	void publishServiceLocked(ServiceRecord r, Intent intent, IBinder service) {
        final long origId = Binder.clearCallingIdentity();
        try {
            if (DEBUG_SERVICE) Slog.v(TAG, "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);
                            ...
                            try {
                            	// c.conn 为 服务启动准备阶段 在ContextImpl.bindServiceCommon中创建的，为一个LoadedApk.ServiceDispatcher.InnerConnection的实例。
                                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);
        }
    }
    ...
}
    

frameworks/base/core/java/android/app/LoadedApk.java

public class LoadApk{
	...
	static final class ServiceDispatcher {
		private static class InnerConnection extends IServiceConnection.Stub {
            final WeakReference<LoadedApk.ServiceDispatcher> mDispatcher;

            InnerConnection(LoadedApk.ServiceDispatcher sd) {
                mDispatcher = new WeakReference<LoadedApk.ServiceDispatcher>(sd);
            }
			// 通知已经连接
            public void connected(ComponentName name, IBinder service) throws RemoteException {
                LoadedApk.ServiceDispatcher sd = mDispatcher.get();
                if (sd != null) {
                	// 进入下面的connected函数
                    sd.connected(name, service);
                }
            }
        }
        ...
        public void connected(ComponentName name, IBinder service) {
            if (mActivityThread != null) {
                mActivityThread.post(new RunConnection(name, service, 0));
            } else {
            	// 进入下面的doConnected函数
                doConnected(name, service);
            }
        }
        ...
        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) {
            	// 假如是服务，触发onServiceDisconnected事件
                mConnection.onServiceDisconnected(name);
            }
            // If there is a new service, it is now connected.
            if (service != null) {
            	// 假如是新服务，则触发onServiceConnected。mConnection为在Activity.bindService是传入的参数，也即是绑定服务前用户创建的ServiceConnection类实例。
                mConnection.onServiceConnected(name, service);
            }
        }
	}
	
}