05 Android系统启动-SystemServer下篇

基于Android 6.0的源码剖析， 分析Android启动过程的system_server进程

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

frameworks/base/services/core/java/com/android/server/
  - SystemServiceManager.java
  - ServiceThread.java
  - am/ActivityManagerService.java

frameworks/base/core/java/android/app/
  - ActivityThread.java
  - LoadedApk.java
  - ContextImpl.java

一. SystemServer启动

上篇文章 从Zygote一路启动到SystemServer的过程。 简单回顾下，在RuntimeInit.java中invokeStaticMain方法通过创建并抛出异常ZygoteInit.MethodAndArgsCaller，在ZygoteInit.java中的main()方法会捕捉该异常，并调用caller.run()，再通过反射便会调用到SystemServer.main()方法，该方法主要执行流程：

SystemServer.main
    SystemServer.run
        createSystemContext
        startBootstrapServices();
        startCoreServices();
        startOtherServices();
        Looper.loop();

接下来，从其main方法说起。

1.1 SystemServer.main

public final class SystemServer {
    ...
    public static void main(String[] args) {
        //先初始化SystemServer对象，再调用对象的run()方法， 【见小节1.2】
        new SystemServer().run();
    }
}

1.2 SystemServer.run

private void run() {
    //当系统时间比1970年更早，就设置当前系统时间为1970年
    if (System.currentTimeMillis() < EARLIEST_SUPPORTED_TIME) {
        SystemClock.setCurrentTimeMillis(EARLIEST_SUPPORTED_TIME);
    }

    //变更虚拟机的库文件，对于Android 6.0默认采用的是libart.so
    SystemProperties.set("persist.sys.dalvik.vm.lib.2", VMRuntime.getRuntime().vmLibrary());

    if (SamplingProfilerIntegration.isEnabled()) {
        ...
    }

    //清除vm内存增长上限，由于启动过程需要较多的虚拟机内存空间
    VMRuntime.getRuntime().clearGrowthLimit();

    //设置内存的可能有效使用率为0.8
    VMRuntime.getRuntime().setTargetHeapUtilization(0.8f);
    // 针对部分设备依赖于运行时就产生指纹信息，因此需要在开机完成前已经定义
    Build.ensureFingerprintProperty();

    //访问环境变量前，需要明确地指定用户
    Environment.setUserRequired(true);

    //确保当前系统进程的binder调用，总是运行在前台优先级(foreground priority)
    BinderInternal.disableBackgroundScheduling(true);
    android.os.Process.setThreadPriority(android.os.Process.THREAD_PRIORITY_FOREGROUND);
    android.os.Process.setCanSelfBackground(false);

    // 主线程looper就在当前线程运行
    Looper.prepareMainLooper();

    //加载android_servers.so库，该库包含的源码在frameworks/base/services/目录下
    System.loadLibrary("android_servers");

    //检测上次关机过程是否失败，该方法可能不会返回[见小节1.2.1]
    performPendingShutdown();

    //初始化系统上下文 【见小节1.3】
    createSystemContext();

    //创建系统服务管理
    mSystemServiceManager = new SystemServiceManager(mSystemContext);
    //将mSystemServiceManager添加到本地服务的成员sLocalServiceObjects
    LocalServices.addService(SystemServiceManager.class, mSystemServiceManager);

    //启动各种系统服务
    try {
        startBootstrapServices(); // 启动引导服务【见小节1.4】
        startCoreServices();      // 启动核心服务【见小节1.5】
        startOtherServices();     // 启动其他服务【见小节1.6】
    } catch (Throwable ex) {
        Slog.e("System", "************ Failure starting system services", ex);
        throw ex;
    }

    //用于debug版本，将log事件不断循环地输出到dropbox（用于分析）
    if (StrictMode.conditionallyEnableDebugLogging()) {
        Slog.i(TAG, "Enabled StrictMode for system server main thread.");
    }
    //一直循环执行
    Looper.loop();
    throw new RuntimeException("Main thread loop unexpectedly exited");
}

LocalServices通过用静态Map变量sLocalServiceObjects，来保存以服务类名为key，以具体服务对象为value的Map结构。

1.2.1 performPendingShutdown

[–>SystemServer.java]

private void performPendingShutdown() {
    final String shutdownAction = SystemProperties.get(
            ShutdownThread.SHUTDOWN_ACTION_PROPERTY, "");
    if (shutdownAction != null && shutdownAction.length() > 0) {
        boolean reboot = (shutdownAction.charAt(0) == '1');

        final String reason;
        if (shutdownAction.length() > 1) {
            reason = shutdownAction.substring(1, shutdownAction.length());
        } else {
            reason = null;
        }
        // 当"sys.shutdown.requested"值不为空,则会重启或者关机
        ShutdownThread.rebootOrShutdown(null, reboot, reason);
    }
}

1.3 createSystemContext

[–>SystemServer.java]

private void createSystemContext() {
    //创建system_server进程的上下文信息
    ActivityThread activityThread = ActivityThread.systemMain();
    mSystemContext = activityThread.getSystemContext();
    //设置主题
    mSystemContext.setTheme(android.R.style.Theme_DeviceDefault_Light_DarkActionBar);
}

理解Application创建过程已介绍过createSystemContext()过程， 该过程会创建对象有ActivityThread，Instrumentation, ContextImpl，LoadedApk，Application。

1.4 startBootstrapServices

[–>SystemServer.java]

private void startBootstrapServices() {
    //阻塞等待与installd建立socket通道
    Installer installer = mSystemServiceManager.startService(Installer.class);

    //启动服务ActivityManagerService
    mActivityManagerService = mSystemServiceManager.startService(
            ActivityManagerService.Lifecycle.class).getService();
    mActivityManagerService.setSystemServiceManager(mSystemServiceManager);
    mActivityManagerService.setInstaller(installer);

    //启动服务PowerManagerService
    mPowerManagerService = mSystemServiceManager.startService(PowerManagerService.class);

    //初始化power management
    mActivityManagerService.initPowerManagement();

    //启动服务LightsService
    mSystemServiceManager.startService(LightsService.class);

    //启动服务DisplayManagerService
    mDisplayManagerService = mSystemServiceManager.startService(DisplayManagerService.class);

    //Phase100: 在初始化package manager之前，需要默认的显示.
    mSystemServiceManager.startBootPhase(SystemService.PHASE_WAIT_FOR_DEFAULT_DISPLAY);

    //当设备正在加密时，仅运行核心
    String cryptState = SystemProperties.get("vold.decrypt");
    if (ENCRYPTING_STATE.equals(cryptState)) {
        mOnlyCore = true;
    } else if (ENCRYPTED_STATE.equals(cryptState)) {
        mOnlyCore = true;
    }

    //启动服务PackageManagerService
    mPackageManagerService = PackageManagerService.main(mSystemContext, installer,
            mFactoryTestMode != FactoryTest.FACTORY_TEST_OFF, mOnlyCore);
    mFirstBoot = mPackageManagerService.isFirstBoot();
    mPackageManager = mSystemContext.getPackageManager();

    //启动服务UserManagerService，新建目录/data/user/
    ServiceManager.addService(Context.USER_SERVICE, UserManagerService.getInstance());

    AttributeCache.init(mSystemContext);

    //设置AMS
    mActivityManagerService.setSystemProcess();

    //启动传感器服务
    startSensorService();
}

该方法所创建的服务：ActivityManagerService, PowerManagerService, LightsService, DisplayManagerService， PackageManagerService， UserManagerService， sensor服务.

1.5 startCoreServices

private void startCoreServices() {
    //启动服务BatteryService，用于统计电池电量，需要LightService.
    mSystemServiceManager.startService(BatteryService.class);

    //启动服务UsageStatsService，用于统计应用使用情况
    mSystemServiceManager.startService(UsageStatsService.class);
    mActivityManagerService.setUsageStatsManager(
            LocalServices.getService(UsageStatsManagerInternal.class));

    mPackageManagerService.getUsageStatsIfNoPackageUsageInfo();

    //启动服务WebViewUpdateService
    mSystemServiceManager.startService(WebViewUpdateService.class);
}

启动服务BatteryService，UsageStatsService，WebViewUpdateService。

1.6 startOtherServices

该方法比较长，有近千行代码，逻辑很简单，主要是启动一系列的服务，这里就不具体列举源码了，在第四节直接对其中的服务进行一个简单分类。

    private void startOtherServices() {
        ...
        SystemConfig.getInstance();
        mContentResolver = context.getContentResolver(); // resolver
        ...
        mActivityManagerService.installSystemProviders(); //provider
        mSystemServiceManager.startService(AlarmManagerService.class); // alarm
        // watchdog
        watchdog.init(context, mActivityManagerService); 
        inputManager = new InputManagerService(context); // input
        wm = WindowManagerService.main(...); // window
        inputManager.start();  //启动input
        mDisplayManagerService.windowManagerAndInputReady();
        ...
        mSystemServiceManager.startService(MOUNT_SERVICE_CLASS); // mount
        mPackageManagerService.performBootDexOpt();  // dexopt操作
        ActivityManagerNative.getDefault().showBootMessage(...); //显示启动界面
        ...
        statusBar = new StatusBarManagerService(context, wm); //statusBar
        //dropbox
        ServiceManager.addService(Context.DROPBOX_SERVICE,
                    new DropBoxManagerService(context, new File("/data/system/dropbox")));
         mSystemServiceManager.startService(JobSchedulerService.class); //JobScheduler
         lockSettings.systemReady(); //lockSettings

        //phase480 和phase500
        mSystemServiceManager.startBootPhase(SystemService.PHASE_LOCK_SETTINGS_READY);
        mSystemServiceManager.startBootPhase(SystemService.PHASE_SYSTEM_SERVICES_READY);
        ...
        // 准备好window, power, package, display服务
        wm.systemReady();
        mPowerManagerService.systemReady(...);
        mPackageManagerService.systemReady();
        mDisplayManagerService.systemReady(...);

        //重头戏[见小节2.1]
        mActivityManagerService.systemReady(new Runnable() {
            public void run() {
              ...
            }
        });
    }

SystemServer启动各种服务中最后的一个环节便是AMS.systemReady()，详见ActivityManagerService启动过程.

到此, System_server主线程的启动工作总算完成, 进入Looper.loop()状态,等待其他线程通过handler发送消息到主线再处理.

二、服务启动阶段

SystemServiceManager的startBootPhase()贯穿system_server进程的整个启动过程：

system_server服务启动流程

其中PHASE_BOOT_COMPLETED=1000，该阶段是发生在Boot完成和home应用启动完毕。系统服务更倾向于监听该阶段，而不是注册广播ACTION_BOOT_COMPLETED，从而降低系统延迟。

各个启动阶段所在源码的大致位置：

public final class SystemServer {

    private void startBootstrapServices() {
      ...
      //phase100
      mSystemServiceManager.startBootPhase(SystemService.PHASE_WAIT_FOR_DEFAULT_DISPLAY);
      ...
    }

    private void startCoreServices() {
      ...
    }

    private void startOtherServices() {
      ...
      //phase480 && 500
      mSystemServiceManager.startBootPhase(SystemService.PHASE_LOCK_SETTINGS_READY);
      mSystemServiceManager.startBootPhase(SystemService.PHASE_SYSTEM_SERVICES_READY);

      ...
      mActivityManagerService.systemReady(new Runnable() {
         public void run() {
             //phase550
             mSystemServiceManager.startBootPhase(
                     SystemService.PHASE_ACTIVITY_MANAGER_READY);
             ...
             //phase600
             mSystemServiceManager.startBootPhase(
                     SystemService.PHASE_THIRD_PARTY_APPS_CAN_START);
          }
      }
    }
}

接下来再说说简单每个阶段的大概完成的工作：

2.1 Phase0

创建四大引导服务:

• ActivityManagerService
• PowerManagerService
• LightsService
• DisplayManagerService

2.2 Phase100

进入阶段PHASE_WAIT_FOR_DEFAULT_DISPLAY=100回调服务

onBootPhase(100)

• DisplayManagerService

然后创建大量服务下面列举部分:

• PackageManagerService
• WindowManagerService
• InputManagerService
• NetworkManagerService
• DropBoxManagerService
• FingerprintService
• LauncherAppsService
• …

2.3 Phase480

进入阶段PHASE_LOCK_SETTINGS_READY=480回调服务

onBootPhase(480)

• DevicePolicyManagerService

阶段480后马上就进入阶段500.

2.4 Phase500

PHASE_SYSTEM_SERVICES_READY=500，进入该阶段服务能安全地调用核心系统服务.

onBootPhase(500)

• AlarmManagerService

• JobSchedulerService

• NotificationManagerService

• BackupManagerService

• UsageStatsService

• DeviceIdleController

• TrustManagerService

• UiModeManagerService

• BluetoothService

• BluetoothManagerService

• EthernetService

• WifiP2pService

• WifiScanningService

• WifiService

• RttService

各大服务执行systemReady():

• WindowManagerService.systemReady():
• PowerManagerService.systemReady():
• PackageManagerService.systemReady():
• DisplayManagerService.systemReady():

接下来就绪AMS.systemReady方法.

2.5 Phase550

PHASE_ACTIVITY_MANAGER_READY=550， AMS.mSystemReady=true, 已准备就绪,进入该阶段服务能广播Intent;但是system_server主线程并没有就绪.

onBootPhase(550)

• MountService
• TelecomLoaderService
• UsbService
• WebViewUpdateService
• DockObserver
• BatteryService

接下来执行: (AMS启动native crash监控, 加载WebView，启动SystemUi等),如下

• mActivityManagerService.startObservingNativeCrashes();

• WebViewFactory.prepareWebViewInSystemServer();

• startSystemUi(context);

• networkScoreF.systemReady();

• networkManagementF.systemReady();

• networkStatsF.systemReady();

• networkPolicyF.systemReady();

• connectivityF.systemReady();

• audioServiceF.systemReady();

• Watchdog.getInstance().start();

2.6 Phase600

PHASE_THIRD_PARTY_APPS_CAN_START=600

onBootPhase(600)

• JobSchedulerService
• NotificationManagerService
• BackupManagerService
• AppWidgetService
• GestureLauncherService
• DreamManagerService
• TrustManagerService
• VoiceInteractionManagerService

接下来,各种服务的systemRunning过程:

WallpaperManagerService、InputMethodManagerService、LocationManagerService、CountryDetectorService、NetworkTimeUpdateService、CommonTimeManagementService、TextServicesManagerService、AssetAtlasService、InputManagerService、TelephonyRegistry、MediaRouterService、MmsServiceBroker这些服务依次执行其systemRunning()方法。

2.7 Phase1000

在经过一系列流程，再调用AMS.finishBooting()时，则进入阶段Phase1000。

到此，系统服务启动阶段完成就绪，system_server进程启动完成则进入Looper.loop()状态，随时待命，等待消息队列MessageQueue中的消息到来，则马上进入执行状态。

三、服务类别

system_server进程，从源码角度划分为引导服务、核心服务、其他服务3类。 以下这些系统服务的注册过程, 见Android系统服务的注册方式

1. 引导服务(7个)：ActivityManagerService、PowerManagerService、LightsService、DisplayManagerService、PackageManagerService、UserManagerService、SensorService；
2. 核心服务(3个)：BatteryService、UsageStatsService、WebViewUpdateService；
3. 其他服务(70个+)：AlarmManagerService、VibratorService等。

合计总大约80个系统服务：

| ActivityManagerService | PackageManagerService | WindowManagerService |
| PowerManagerService | BatteryService | BatteryStatsService |
| DreamManagerService | DropBoxManagerService | SamplingProfilerService |
| UsageStatsService | DiskStatsService | DeviceStorageMonitorService |
| SchedulingPolicyService | AlarmManagerService | DeviceIdleController |
| ThermalObserver | JobSchedulerService | AccessibilityManagerService |
| DisplayManagerService | LightsService | GraphicsStatsService |
| StatusBarManagerService | NotificationManagerService | WallpaperManagerService |
| UiModeManagerService | AppWidgetService | LauncherAppsService |
| TextServicesManagerService | ContentService | LockSettingsService |
| InputMethodManagerService | InputManagerService | MountService |
| FingerprintService | TvInputManagerService | DockObserver |
| NetworkManagementService | NetworkScoreService | NetworkStatsService |
| NetworkPolicyManagerService | ConnectivityService | BluetoothService |
| WifiP2pService | WifiService | WifiScanningService |
| AudioService | MediaRouterService | VoiceInteractionManagerService |
| MediaProjectionManagerService | MediaSessionService | |
| DevicePolicyManagerService | PrintManagerService | BackupManagerService |
| UserManagerService | AccountManagerService | TrustManagerService |
| SensorService | LocationManagerService | VibratorService |
| CountryDetectorService | GestureLauncherService | PersistentDataBlockService |
| EthernetService | WebViewUpdateService | ClipboardService |
| TelephonyRegistry | TelecomLoaderService | NsdService |
| UpdateLockService | SerialService | SearchManagerService |
| CommonTimeManagementService | AssetAtlasService | ConsumerIrService |
| MidiServiceCameraService | TwilightService | RestrictionsManagerService |
| MmsServiceBroker | RttService | UsbService |