Service源碼淺析一
Service源碼淺析二
Service源碼淺析三
Service源碼淺析四

Service類的代碼量比較少，內(nèi)容沒什么可說的。
Service是一個(gè)抽象類。

Service

1. Start Service

    @Nullable
    public abstract ComponentName startService(Intent service);

start service其實(shí)調(diào)用的是Context的方法，Context的實(shí)現(xiàn)類是ContextWrapper，它的startService方法如下：

    @Override
    public @Nullable ComponentName startService(Intent service) {
        return mBase.startService(service);
    }

ContextWrapper使用代理模式，其中真正的實(shí)現(xiàn)類是mBase，mBase是ContextImpl的實(shí)例。
在ContextImpl中會(huì)依次調(diào)用 startService->startServiceCommon->ActivityManager.getService().startService

    @UnsupportedAppUsage
    public static IActivityManager getService() {
        return IActivityManagerSingleton.get();
    }

ActivityManager.getService方法如上，ActivityManager其實(shí)就是ActivityManagerService的客戶端類。

    @UnsupportedAppUsage
    private static final Singleton<IActivityManager> IActivityManagerSingleton =
            new Singleton<IActivityManager>() {
                @Override
                protected IActivityManager create() {
                    final IBinder b = ServiceManager.getService(Context.ACTIVITY_SERVICE);
                    final IActivityManager am = IActivityManager.Stub.asInterface(b);
                    return am;
                }
            };

這里用到了Singleton類，它是一個(gè)單例的工具類，在咱們得代碼中也可以使用，代碼我也貼一下

public abstract class Singleton<T> {

    @UnsupportedAppUsage
    public Singleton() {
    }

    @UnsupportedAppUsage
    private T mInstance;

    protected abstract T create();

    @UnsupportedAppUsage
    public final T get() {
        synchronized (this) {
            if (mInstance == null) {
                mInstance = create();
            }
            return mInstance;
        }
    }
}

通過IActivityManagerSingleton返回了IActivityManagerSingleton的Binder對(duì)象，猜測(cè)這里發(fā)生了進(jìn)程通信，詳細(xì)不深究了，后邊看AMS的時(shí)候補(bǔ)上，咱們先順流程。
接下來就進(jìn)入了ActivityManagerService進(jìn)程了，調(diào)用它的startService方法。
繼續(xù)走到ActiveServices的startServiceLocked，ActiveServices是ActivityManagerService的輔助類。
順著主線繼續(xù)走，ActiveServices#startServiceLocked -> startServiceInnerLocked -> bringUpServiceLocked -> bringUpServiceInnerLocked -> realStartServiceLocked
-> IApplicationThread#scheduleCreateService
走到這里有發(fā)生了一次進(jìn)程通信，已經(jīng)進(jìn)入到對(duì)應(yīng)進(jìn)程里了，這里仍然疑點(diǎn)重重：

1. 這里應(yīng)該發(fā)生了進(jìn)程創(chuàng)建；

2. 如果是同一個(gè)進(jìn)程怎么處理的；
   細(xì)節(jié)留到后邊再補(bǔ)，先順著流程走，在這之前先補(bǔ)個(gè)簡(jiǎn)圖，方便理解。

   
   
   startService

ApplicationThread是ActivityThread的內(nèi)部類，ActivityThread其實(shí)就是應(yīng)用的進(jìn)程入口，進(jìn)程創(chuàng)建時(shí)會(huì)觸發(fā)ActivityThread#main函數(shù)。
scheduleCreateService會(huì)向主線程的handler中發(fā)送創(chuàng)建服務(wù)的消息，最終調(diào)用handleCreateService，在此方法中，如果Application對(duì)象未創(chuàng)建，會(huì)創(chuàng)建Application對(duì)象并觸發(fā)其onCreate方法。同時(shí)實(shí)例化Service并調(diào)用其onCreate方法。

onCreate方法執(zhí)行之后咱們回到 ActiveServices#realStartServiceLocked，這里觸發(fā)了IApplicationThread#scheduleCreateService之后，后邊還觸發(fā)了sendServiceArgsLocked，在這個(gè)方法中最終會(huì)觸發(fā)r.app.getThread().scheduleServiceArgs，這里有一次進(jìn)程間通信再次來到ApplicationThread。在ApplicationThread中會(huì)觸發(fā) Service#onStartCommand。

到這里Start Service流程基本結(jié)束了。

2. Bind Service

    public abstract boolean bindService(@RequiresPermission @NonNull Intent service,
            @NonNull ServiceConnection conn, int flags);

我們按照同樣的方式繼續(xù)分析bindService
首先來到 ContextImpl#bindServiceCommon，在這里會(huì)調(diào)用ActivityManager.getService().bindServiceInstance，這里發(fā)生了進(jìn)程通信，進(jìn)入AMS進(jìn)程。
ActiveServices#bindServiceLocked -> bringUpServiceLocked -> bringUpServiceInnerLocked -> realStartServiceLocked
在realStartServiceLocked方法中會(huì)觸發(fā)thread.scheduleCreateService來創(chuàng)建Service實(shí)例。
Service onCreate執(zhí)行之后接下來繼續(xù)執(zhí)行requestServiceBindingsLocked -> r.app.getThread().scheduleBindService，又是進(jìn)程通信，同樣的套路觸發(fā)ApplicationThread#handleBindService，然后觸發(fā)Service的onBind返回一個(gè)IBinder用于進(jìn)程間通信。
接下來是AMS的publishService -> ActiveServices#publishServiceLocked，最終會(huì)觸發(fā)IServiceConnection#connected回調(diào)方法，這里要?jiǎng)x一腳車了，這個(gè)IServiceConnection是個(gè)什么東西？
這就不得不回到起點(diǎn)了，bindService時(shí)會(huì)傳入ServiceConnection，這個(gè)參數(shù)經(jīng)過LoadedApk#getServiceDispatcher -> getServiceDispatcherCommon，在這個(gè)方法中會(huì)創(chuàng)建ServiceDispatcher，然后又會(huì)創(chuàng)建InnerConnection，這個(gè)就是我們要找的IServiceConnection

        private static class InnerConnection extends IServiceConnection.Stub {
            @UnsupportedAppUsage(maxTargetSdk = Build.VERSION_CODES.R, trackingBug = 170729553)
            final WeakReference<LoadedApk.ServiceDispatcher> mDispatcher;

            InnerConnection(LoadedApk.ServiceDispatcher sd) {
                mDispatcher = new WeakReference<LoadedApk.ServiceDispatcher>(sd);
            }

            public void connected(ComponentName name, IBinder service, boolean dead)
                    throws RemoteException {
                LoadedApk.ServiceDispatcher sd = mDispatcher.get();
                if (sd != null) {
                    sd.connected(name, service, dead);
                }
            }
        }

觸發(fā)它的connected，最終會(huì)調(diào)用ServiceDispatcher#connected

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

在這三個(gè)分支中都會(huì)觸發(fā)doConnected，最終會(huì)調(diào)用onServiceConnected回調(diào)方法，至此bindService流程結(jié)束了。
補(bǔ)充一點(diǎn)：這里mActivityThread就是主線程的Handler，可以得出onServiceConnected最終會(huì)運(yùn)行在主線程。

3. Unbind Service

public abstract void unbindService(@NonNull ServiceConnection conn);

同樣執(zhí)行ContextImpl#unbindService方法

    @Override
    public void unbindService(ServiceConnection conn) {
        if (conn == null) {
            throw new IllegalArgumentException("connection is null");
        }
        if (mPackageInfo != null) {
            IServiceConnection sd = mPackageInfo.forgetServiceDispatcher(
                    getOuterContext(), conn);
            try {
                ActivityManager.getService().unbindService(sd);
            } catch (RemoteException e) {
                throw e.rethrowFromSystemServer();
            }
        } else {
            throw new RuntimeException("Not supported in system context");
        }
    }

這里的mPackageInfo就是LoadedApk實(shí)例。
AMS#unbindService -> ActiveServices#unbindServiceLocked -> removeConnectionLocked -> ActivityThread#scheduleUnbindService -> handleUnbindService -> Service#onUnbind -> AMS#unbindFinished -> ActiveServices#unbindFinishedLocked
回到ActiveServices#removeConnectionLocked -> removeConnectionLocked -> bringDownServiceIfNeededLocked -> bringDownServiceLocked -> ActivityThread#scheduleStopService -> Service#onDestroy