SystemUI启动流程

首先我们来看一个时序图：

SystemServer前面的部分不再多说，有兴趣的朋友可以参考这篇博客《Android系统启动流程》

SystemServer 名为系统服务进程，负责启动 Android 系统的关键服务。SystemServer进程其实也是由Zygote进程fork出来的，并且执行其main方法，那么这里我们以android23的源码为例，看一下SystemServer的main方法的执行逻辑：SystemServer.main()：

vim frameworks/base/services/java/com/android/server/SystemServer.java

/**     * The main entry point from zygote.     */    public static void main(String[] args) {        new SystemServer().run();    }

可以看到main()中生成了SystemServer对象并执行了run方法。
SystemServer.run()：

private void run() {        //***1. 时间判断        // If a device's clock is before 1970 (before 0), a lot of        // APIs crash dealing with negative numbers, notably        // java.io.File#setLastModified, so instead we fake it and        // hope that time from cell towers or NTP fixes it shortly.        if (System.currentTimeMillis() < EARLIEST_SUPPORTED_TIME) {            Slog.w(TAG, "System clock is before 1970; setting to 1970.");            SystemClock.setCurrentTimeMillis(EARLIEST_SUPPORTED_TIME);        }       ....}

首先判断系统当前时间，若当前时间小于1970年1月1日，则一些初始化操作可能会处所，所以当系统的当前时间小于1970年1月1日的时候，设置系统当前时间为该时间点。

private void run() {        /***1. 时间判断 ***/        ...        /***2. 设置系统的语言环境 ***/        // Enable the sampling profiler.        if (SamplingProfilerIntegration.isEnabled()) {            SamplingProfilerIntegration.start();            mProfilerSnapshotTimer = new Timer();            mProfilerSnapshotTimer.schedule(new TimerTask() {                @Override                public void run() {                    SamplingProfilerIntegration.writeSnapshot("system_server", null);                }            }, SNAPSHOT_INTERVAL, SNAPSHOT_INTERVAL);        }        ...}

下面的主要是设置虚拟机运行内存，加载运行库，设置SystemServer的异步消息，具体的异步消息机制可参见《 android源码解析之（二）–>异步消息机制》

private void run() {        /***1. 时间判断 ***/        ...        /***2. 设置系统的语言环境 ***/        /***3. 设置运行内存等 ***/         // Mmmmmm... more memory!        VMRuntime.getRuntime().clearGrowthLimit();        // The system server has to run all of the time, so it needs to be        // as efficient as possible with its memory usage.        VMRuntime.getRuntime().setTargetHeapUtilization(0.8f);        // Some devices rely on runtime fingerprint generation, so make sure        // we've defined it before booting further.        Build.ensureFingerprintProperty();        // Within the system server, it is an error to access Environment paths without        // explicitly specifying a user.        Environment.setUserRequired(true);        // Ensure binder calls into the system always run at foreground priority.        BinderInternal.disableBackgroundScheduling(true);        // Prepare the main looper thread (this thread).        android.os.Process.setThreadPriority(                android.os.Process.THREAD_PRIORITY_FOREGROUND);        android.os.Process.setCanSelfBackground(false);        Looper.prepareMainLooper();        // Initialize native services.        System.loadLibrary("android_servers");        nativeInit();        ///M:Add for low storage feature,to delete the reserver file.@{        try {            Runtime.getRuntime().exec("rm -r /data/piggybank");        } catch (IOException e) {            Slog.e(TAG, "system server init delete piggybank fail" + e);        }        ///@}        // Check whether we failed to shut down last time we tried.        // This call may not return.        performPendingShutdown();         /***4.创建Context ***/        // Initialize the system context.        createSystemContext();        /***5.new 了一个SystemServiceManager ***/        // Create the system service manager.        mSystemServiceManager = new SystemServiceManager(mSystemContext);        /***6.把创建的SystemServiceManager加入到LocalServices ***/        LocalServices.addService(SystemServiceManager.class, mSystemServiceManager);         ... }

private void createSystemContext() {        ActivityThread activityThread = ActivityThread.systemMain();        mSystemContext = activityThread.getSystemContext();        mSystemContext.setTheme(android.R.style.Theme_DeviceDefault_Light_DarkActionBar);    }

可以看到在SystemServer进程中也存在着Context对象，并且是通过ActivityThread.systemMain方法创建context的，这一部分的逻辑以后会通过介绍Activity的启动流程来介绍，这里就不在扩展，这里我们只知道在SystemServer进程中也需要创建Context对象。

然后将SystemServiceManager对象保存SystemServer进程中的一个数据结构中。

继续分析：

private void run() {        /***1. 时间判断 ***/        ...        /***2. 设置系统的语言环境 ***/        /***3. 设置运行内存等 ***/        /***4.创建Context ***/        /***5.new 了一个SystemServiceManager ***/        /***6.把创建的SystemServiceManager加入到LocalServices ***/        /***7.启动服务 ***/        // Start services.        try {            startBootstrapServices();            startCoreServices();            startOtherServices();        } catch (Throwable ex) {            Slog.e("System", "******************************************");            Slog.e("System", "************ Failure starting system services", ex);            /// M: RecoveryManagerService  @{            if (mRecoveryManagerService != null && ex instanceof RuntimeException) {                mRecoveryManagerService.handleException((RuntimeException)ex, true);            }            /// @}        }

里面主要涉及了是三个方法：
startBootstrapServices() 主要用于启动系统Boot级服务
startCoreServices() 主要用于启动系统核心的服务
startOtherServices() 主要用于启动一些非紧要或者是非需要及时启动的服务

下面我们重点介绍这三个启动服务的方法，包括启动那些系统服务已经如何启动系统服务等。

private void startBootstrapServices() {        /*** 1. 获取installer ***/        Installer installer = mSystemServiceManager.startService(Installer.class);        /*** 2. 获取并配置 mActivityManagerService ***/        /*** 这段代码主要是用于启动ActivityManagerService服务，并为其设置SystemServiceManager和Installer。ActivityManagerService是系统中一个非常重要的服务，Activity，service，Broadcast，contentProvider都需要通过其与系统交互。 ***/        /*** Lifecycle可以看到其实ActivityManagerService的一个静态内部类，在其构造方法中会创建一个ActivityManagerService，通过刚刚对Installer服务的分析我们知道，SystemServiceManager的startService方法会调用服务的onStart()方法，而在Lifecycle类的定义中我们看到其onStart（）方法直接调用了mService.start()方法，mService是Lifecycle类中对ActivityManagerService的引用***/        // Activity manager runs the show.        // Activity manager runs the show.        mActivityManagerService = mSystemServiceManager.startService(                ActivityManagerService.Lifecycle.class).getService();        mActivityManagerService.setSystemServiceManager(mSystemServiceManager);        mActivityManagerService.setInstaller(installer);        /*** 3. 获取并配置 mPowerManagerService ***/        // Power manager needs to be started early because other services need it.        // Native daemons may be watching for it to be registered so it must be ready        // to handle incoming binder calls immediately (including being able to verify        // the permissions for those calls).        mPowerManagerService = mSystemServiceManager.startService(PowerManagerService.class);        // Now that the power manager has been started, let the activity manager        // initialize power management features.        mActivityManagerService.initPowerManagement();        /*** 4. 获取并配置 mDisplayManagerService 主要是手机显示方面的服务***/        // Display manager is needed to provide display metrics before package manager        // starts up.        mDisplayManagerService = mSystemServiceManager.startService(DisplayManagerService.class);         // We need the default display before we can initialize the package manager.        mSystemServiceManager.startBootPhase(SystemService.PHASE_WAIT_FOR_DEFAULT_DISPLAY);        // Only run "core" apps if we're encrypting the device.        String cryptState = SystemProperties.get("vold.decrypt");        if (ENCRYPTING_STATE.equals(cryptState)) {            Slog.w(TAG, "Detected encryption in progress - only parsing core apps");            mOnlyCore = true;        } else if (ENCRYPTED_STATE.equals(cryptState)) {            Slog.w(TAG, "Device encrypted - only parsing core apps");            mOnlyCore = true;        }        /// M: RecoveryManagerService  @{        boolean disabled = "0".equals(SystemProperties.get("ro.mtk_antibricking_level"));        if (!disabled) {            try {                Slog.i(TAG, "Recovery Manager");                mRecoveryManagerService = new RecoveryManagerService(mSystemContext);                if (mRecoveryManagerService != null) {                    ServiceManager.addService(Context.RECOVERY_SERVICE, mRecoveryManagerService.asBinder());                    mRecoveryManagerService.startBootMonitor();                }            } catch (Throwable e) {                reportWtf("Failure starting Recovery Manager", e);            }        }        /// @}        /*** 5. 开启PackageManagerService包管理服务  ***/        /***该服务也是android系统中一个比较重要的服务，包括多apk文件的安装，解析，删除，卸载等等操作。***/        /***PackageManagerService服务的启动方式与其他服务的启动方式有一些区别，直接调用了PackageManagerService的静态main方法，在静态方法里面也是直接使用new的方式创建了一个PackageManagerService对象，并在其构造方法中初始化相关变量。***/        /*** 最后调用了ServiceManager.addService方法，主要是通过Binder机制与JNI层交互，这里不再扩展 ***/        // Start the package manager.        Slog.i(TAG, "Package Manager");        mPackageManagerService = PackageManagerService.main(mSystemContext, installer,        mFactoryTestMode != FactoryTest.FACTORY_TEST_OFF, mOnlyCore);        mFirstBoot = mPackageManagerService.isFirstBoot();        mPackageManager = mSystemContext.getPackageManager();        /*** 6.然后启动UserManagerService和SensorService，至此startBootstrapServices方法执行完成。 ***/        Slog.i(TAG, "User Service");        ServiceManager.addService(Context.USER_SERVICE, UserManagerService.getInstance());        // Initialize attribute cache used to cache resources from packages.        AttributeCache.init(mSystemContext);        // Set up the Application instance for the system process and get started.        mActivityManagerService.setSystemProcess(); }

我们看到里面的服务都是通过SystemServiceManager启动的，调用startService方法，文件在frameworks/base/services/core/java/com/android/server/SystemServiceManager.java
SystemServiceManager的startService但是其实就是一句话，调用了所需要调用的Service中的onStart()方法，代码这里就不贴出来了；

然后查看startCoreServices方法：

/**     * Starts some essential services that are not tangled up in the bootstrap process.     */    private void startCoreServices() {        // Manages LEDs and display backlight.        mSystemServiceManager.startService(LightsService.class);        // Tracks the battery level.  Requires LightService.        mSystemServiceManager.startService(BatteryService.class);        // Tracks application usage stats.        mSystemServiceManager.startService(UsageStatsService.class);        mActivityManagerService.setUsageStatsManager(                LocalServices.getService(UsageStatsManagerInternal.class));        // Update after UsageStatsService is available, needed before performBootDexOpt.        mPackageManagerService.getUsageStatsIfNoPackageUsageInfo();        // Tracks whether the updatable WebView is in a ready state and watches for update installs.        mSystemServiceManager.startService(WebViewUpdateService.class);    }

可以看到这里启动了BatteryService（电池相关服务），UsageStatsService，WebViewUpdateService服务等。

最后看一下startOtherServices方法，主要用于启动系统中其他的服务，代码很多，这里就不贴代码了，启动的流程和ActivityManagerService的流程类似，会调用服务的构造方法与onStart方法初始化变量。

/*** 1.读配置 ***//*** 比如是否禁用SystemUI ****/...boolean disableSystemUI = SystemProperties.getBoolean("config.disable_systemui", false);.../***2.初始化各种服务****/...Slog.i(TAG, "Init Watchdog");final Watchdog watchdog = Watchdog.getInstance();watchdog.init(context, mActivityManagerService);Slog.i(TAG, "Input Manager");inputManager = new InputManagerService(context);.../***3.开启各种服务****/mSystemServiceManager.startService(NotificationManagerService.class);notification = INotificationManager.Stub.asInterface(          ServiceManager.getService(Context.NOTIFICATION_SERVICE));networkPolicy.bindNotificationManager(notification);mSystemServiceManager.startService(DeviceStorageMonitorService.class);/***调用各种服务的systemReady方法***/ mActivityManagerService.systemReady(new Runnable() {            @Override            public void run() {                Slog.i(TAG, "Making services ready");                ...                try {//SystemUIService就是在这里启动的  [e-mail]                    startSystemUi(context);                } catch (Throwable e) {                    reportWtf("starting System UI", e);                }                ...            }}

，到了这里我们知道SystemUIService是通过组件的方式启动的，这个组件则是在编译的时候清单文件配置的方式配置到了系统中，这个SystemUIService的源码在

frameworks/base/packages/SystemUI/src/com/android/systemui/SystemUIService.java

我们继续分析；
SystemUIService的onCreate方法里面调用了Application的startServicesIfNeeded方法，就是在SystemUIService启动的时候把所有SystemUI应用里面相关服务都重新启动启动一遍；
这些服务包括：

 com.android.systemui.keyguard.KeyguardViewMediator.class, com.android.systemui.recent.Recents.class, com.android.systemui.volume.VolumeUI.class, com.android.systemui.statusbar.SystemBars.class, com.android.systemui.usb.StorageNotification.class, com.android.systemui.power.PowerUI.class, com.android.systemui.media.RingtonePlayer.class

嗯，下一节我们来分析SystemUI内部启动流程；