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

    @Override
    public 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。