1、手机开机后，Android系统首先会创建一个Zygote（核心进程）。
2、由Zygote启动SystemServer。
3、SystemServer会启动系统运行所需的众多核心服务和普通服务、以及一些应用及数据。例如：SystemUI 启动就是从 SystemServer 里启动的。
4、进入锁屏界面，开机完成。

SystemServer 中有一个 main()方法为系统服务的入口；
frameworks/base/services/java/com/android/server/SystemServer.java

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

在SystemServer 中的 main()方法中，就一句代码生成 SystemServer 对象，执行run 方法。在run()方法里启动了各类服务；
frameworks/base/services/java/com/android/server/SystemServer.java

private void run() {//省略部分代码// Start services.try {traceBeginAndSlog("StartServices");startBootstrapServices();startCoreServices();startOtherServices();    // 在该方法里启动了 SystemUI的服务。SystemServerInitThreadPool.shutdown();} catch (Throwable ex) {Slog.e("System", "******************************************");Slog.e("System", "************ Failure starting system services", ex);throw ex;} finally {traceEnd();}//省略部分代码
}
private void startOtherServices() {//省略部分代码t.traceBegin("StartSystemUI");try {startSystemUi(context, windowManagerF);} catch (Throwable e) {reportWtf("starting System UI", e);}t.traceEnd();//省略部分代码
}
private static void startSystemUi(Context context, WindowManagerService windowManager) {PackageManagerInternal pm = LocalServices.getService(PackageManagerInternal.class);Intent intent = new Intent();intent.setComponent(pm.getSystemUiServiceComponent());intent.addFlags(Intent.FLAG_DEBUG_TRIAGED_MISSING);//Slog.d(TAG, "Starting service: " + intent);context.startServiceAsUser(intent, UserHandle.SYSTEM);windowManager.onSystemUiStarted();}

SystemServer执行流程图：

SystemUi进入到SystemUIService的onCreate()方法里；在onCreate()方法中获得 SystemUIApplication 对象并调用其 startServicesIfNeeded() 方法
frameworks/base/packages/SystemUI/src/com/android/systemui/SystemUIService.java

    @Overridepublic void onCreate() {super.onCreate();// Start all of SystemUI((SystemUIApplication) getApplication()).startServicesIfNeeded();// 省略部分代码...}/*** Makes sure that all the SystemUI services are running. If they are already running, this is a* no-op. This is needed to conditinally start all the services, as we only need to have it in* the main process.* <p>This method must only be called from the main thread.</p>*/public void startServicesIfNeeded() {String[] names = getResources().getStringArray(R.array.config_systemUIServiceComponents);startServicesIfNeeded(names);}

在SystemUIApplication中查看startServicesIfNeeded() 方法，其中其中 config_systemUIServiceComponents 值在frameworks/base/packages/SystemUI/res/values/config.xml 里：
frameworks/base/packages/SystemUI/src/com/android/systemui/SystemUIApplication.java

private void startServicesIfNeeded(String[] services) {if (mServicesStarted) {return;}mServices = new SystemUI[services.length];if (!mBootCompleted) {// check to see if maybe it was already completed long before we began// see ActivityManagerService.finishBooting()if ("1".equals(SystemProperties.get("sys.boot_completed"))) {mBootCompleted = true;if (DEBUG) Log.v(TAG, "BOOT_COMPLETED was already sent");}}Log.v(TAG, "Starting SystemUI services for user " +Process.myUserHandle().getIdentifier() + ".");TimingsTraceLog log = new TimingsTraceLog("SystemUIBootTiming",Trace.TRACE_TAG_APP);log.traceBegin("StartServices");final int N = services.length;for (int i = 0; i < N; i++) {String clsName = services[i];if (DEBUG) Log.d(TAG, "loading: " + clsName);log.traceBegin("StartServices" + clsName);long ti = System.currentTimeMillis();Class cls;try {cls = Class.forName(clsName);mServices[i] = (SystemUI) cls.newInstance();} catch(ClassNotFoundException ex){throw new RuntimeException(ex);} catch (IllegalAccessException ex) {throw new RuntimeException(ex);} catch (InstantiationException ex) {throw new RuntimeException(ex);}mServices[i].mContext = this;mServices[i].mComponents = mComponents;if (DEBUG) Log.d(TAG, "running: " + mServices[i]);mServices[i].start();log.traceEnd();//省略其他代码}
}

可以看到 startServicesIfNeeded() 循环 start 了config_systemUIServiceComponents 里的 Service，这些服务不是四大组件之一的 Service, 而是继承自 SystemUI 接口的服务，我们称之为 SystemUI服务。
到此SystemUI 启动流程分析完毕。