Android SystemServer 启动流程

一 什么是SystemServer？

简单来说，systemServer就是系统用来启动各种service的入口，安卓系统在启动的时候，会初始化两个重要的部分，一个是zygote进程，另一个是由zygote进程fork出来的SystemServer进程，SystemServer会启动我们系统中所需要的一系列service，下面会做分析。

二 从源码出发，分析Systemserver

2.1 SystemServer调用的入口

public static void main(String[] args) {
    new SystemServer().run();
}

SystemServer的初始化入口在main（）方法中，我们进入run（）方法中，可以看到以下代码：

private void run() {
    try {
        ......
        if (System.currentTimeMillis() < EARLIEST_SUPPORTED_TIME) {
            Slog.w(TAG, "System clock is before 1970; setting to 1970.");
            SystemClock.setCurrentTimeMillis(EARLIEST_SUPPORTED_TIME);
        }

       
        if (!SystemProperties.get("persist.sys.language").isEmpty()) {
            final String languageTag = Locale.getDefault().toLanguageTag();

            SystemProperties.set("persist.sys.locale", languageTag);
            SystemProperties.set("persist.sys.language", "");
            SystemProperties.set("persist.sys.country", "");
            SystemProperties.set("persist.sys.localevar", "");
        }

        // Here we go!
        Slog.i(TAG, "Entered the Android system server!");
        EventLog.writeEvent(EventLogTags.BOOT_PROGRESS_SYSTEM_RUN, SystemClock.uptimeMillis());

        
        SystemProperties.set("persist.sys.dalvik.vm.lib.2", VMRuntime.getRuntime().vmLibrary());

        VMRuntime.getRuntime().clearGrowthLimit();
        VMRuntime.getRuntime().setTargetHeapUtilization(0.8f);
        Build.ensureFingerprintProperty();
        Environment.setUserRequired(true);
        BaseBundle.setShouldDefuse(true);
        BinderInternal.disableBackgroundScheduling(true);


        android.os.Process.setThreadPriority(
            android.os.Process.THREAD_PRIORITY_FOREGROUND);
        android.os.Process.setCanSelfBackground(false);
        Looper.prepareMainLooper();

        // Initialize native services.
        System.loadLibrary("android_servers");

        // Check whether we failed to shut down last time we tried.
        // This call may not return.
        performPendingShutdown();

        // Initialize the system context.
        createSystemContext();

        // Create the system service manager.
        mSystemServiceManager = new SystemServiceManager(mSystemContext);
        LocalServices.addService(SystemServiceManager.class, mSystemServiceManager);
    } finally {
        Trace.traceEnd(Trace.TRACE_TAG_SYSTEM_SERVER);
    }

    // Start services.
    try {
        Trace.traceBegin(Trace.TRACE_TAG_SYSTEM_SERVER, "StartServices");
        startBootstrapServices();
        startCoreServices();
        startOtherServices();
    } catch (Throwable ex) {
        Slog.e("System", "******************************************");
        Slog.e("System", "************ Failure starting system services", ex);
        throw ex;
    } finally {
        Trace.traceEnd(Trace.TRACE_TAG_SYSTEM_SERVER);
    }
    // Loop forever.
    Looper.loop();
    throw new RuntimeException("Main thread loop unexpectedly exited");
}

以上代码已经去除了部分无用代码，我们下面来一步一步的对上面的代码进行切割分析

if (System.currentTimeMillis() < EARLIEST_SUPPORTED_TIME) {
            Slog.w(TAG, "System clock is before 1970; setting to 1970.");
            SystemClock.setCurrentTimeMillis(EARLIEST_SUPPORTED_TIME);
        }

首先，SystemServer会去判断系统当前的时间，如果当前时间小于1970年那么，系统就会把当前的时间设置成1970年。
接下来，

        if (!SystemProperties.get("persist.sys.language").isEmpty()) {
            final String languageTag = Locale.getDefault().toLanguageTag();

            SystemProperties.set("persist.sys.locale", languageTag);
            SystemProperties.set("persist.sys.language", "");
            SystemProperties.set("persist.sys.country", "");
            SystemProperties.set("persist.sys.localevar", "");
        }

这代代码主要是初始化一些基本环境设置，比如说当前的语言，国家这些。
接着，

       SystemProperties.set("persist.sys.dalvik.vm.lib.2", VMRuntime.getRuntime().vmLibrary());

        VMRuntime.getRuntime().clearGrowthLimit();
        VMRuntime.getRuntime().setTargetHeapUtilization(0.8f);
        Build.ensureFingerprintProperty();
        Environment.setUserRequired(true);
        BaseBundle.setShouldDefuse(true);
        BinderInternal.disableBackgroundScheduling(true);


        android.os.Process.setThreadPriority(
            android.os.Process.THREAD_PRIORITY_FOREGROUND);
        android.os.Process.setCanSelfBackground(false);
        Looper.prepareMainLooper();

        // Initialize native services.
        System.loadLibrary("android_servers");

这一部分主要是加载一些虚拟机环境和一些运行库，这些都不是本文所要分析的重点，接下来还有一个方法：

  // Initialize the system context.
        createSystemContext();

 private void createSystemContext() {  
    ActivityThread activityThread = ActivityThread.systemMain();
    mSystemContext = activityThread.getSystemContext();
    mSystemContext.setTheme(DEFAULT_SYSTEM_THEME);
}

这个方法表示SystemServer需要获取当前系统的上下文
接下来，就会初始化系统的system Service Manager

mSystemServiceManager = new SystemServiceManager(mSystemContext);
        LocalServices.addService(SystemServiceManager.class, mSystemServiceManager);  

systemManagerService的作用是用来管理service的创建、开始或者SystemService下的其他生命周期事件。好了，接下来就是重头戏来了

try {
        startBootstrapServices();
        startCoreServices();
        startOtherServices();
    } catch (Throwable ex) {
        ......
        throw ex;
    } finally {
          ......
    }  

从谷歌给我们的注释我们给出以上三个方法简单的解释
1 startBootstrapServices() 主要是用来开启一些系统级别的service，这些service具有高度的复杂的相互依赖关系，所以我们需要把他们的初始化放在同一个地方
2 startCoreServices() 主要是启动一些核心的Service，但是这些service跟我们的bootservice没有相互依赖关系的，是相对独立的服务
2 startOtherServices() 正如英文所示，这是一些费关键非核心的service

Step 1 分析startBootstrapService()方法

private void startBootstrapServices() {
    Installer installer = mSystemServiceManager.startService(Installer.class);

    traceBeginAndSlog("DeviceIdentifiersPolicyService");
    mSystemServiceManager.startService(DeviceIdentifiersPolicyService.class);
    traceEnd();

    // Activity manager runs the show.
    mActivityManagerService = mSystemServiceManager.startService(
            ActivityManagerService.Lifecycle.class).getService();
    mActivityManagerService.setSystemServiceManager(mSystemServiceManager);
    mActivityManagerService.setInstaller(installer);

    // 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.
    Trace.traceBegin(Trace.TRACE_TAG_SYSTEM_SERVER, "InitPowerManagement");
    mActivityManagerService.initPowerManagement();
    Trace.traceEnd(Trace.TRACE_TAG_SYSTEM_SERVER);

    // Manages LEDs and display backlight so we need it to bring up the display.
    mSystemServiceManager.startService(LightsService.class);

    // 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;
    }

    // Start the package manager.
    traceBeginAndSlog("StartPackageManagerService");
    mPackageManagerService = PackageManagerService.main(mSystemContext, installer,
            mFactoryTestMode != FactoryTest.FACTORY_TEST_OFF, mOnlyCore);
    mFirstBoot = mPackageManagerService.isFirstBoot();
    mPackageManager = mSystemContext.getPackageManager();
    Trace.traceEnd(Trace.TRACE_TAG_SYSTEM_SERVER);

    // Manages A/B OTA dexopting. This is a bootstrap service as we need it to rename
    // A/B artifacts after boot, before anything else might touch/need them.
    // Note: this isn't needed during decryption (we don't have /data anyways).
    if (!mOnlyCore) {
        boolean disableOtaDexopt = SystemProperties.getBoolean("config.disable_otadexopt",
                false);
        if (!disableOtaDexopt) {
            traceBeginAndSlog("StartOtaDexOptService");
            try {
                OtaDexoptService.main(mSystemContext, mPackageManagerService);
            } catch (Throwable e) {
                reportWtf("starting OtaDexOptService", e);
            } finally {
                Trace.traceEnd(Trace.TRACE_TAG_SYSTEM_SERVER);
            }
        }
    }

    traceBeginAndSlog("StartUserManagerService");
    mSystemServiceManager.startService(UserManagerService.LifeCycle.class);
    Trace.traceEnd(Trace.TRACE_TAG_SYSTEM_SERVER);

    // 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();

    // The sensor service needs access to package manager service, app ops
    // service, and permissions service, therefore we start it after them.
    startSensorService();
}  

在这段代码中，我们先分析核心的代码：

    traceBeginAndSlog("StartInstaller");
    Installer installer = mSystemServiceManager.startService(Installer.class);
    traceEnd();

    // In some cases after launching an app we need to access device identifiers,
    // therefore register the device identifier policy before the activity manager.
    traceBeginAndSlog("DeviceIdentifiersPolicyService");
    mSystemServiceManager.startService(DeviceIdentifiersPolicyService.class);
    traceEnd();

这段代码的执行，就是为了先启动installer，这样android才有机会去创建一些关键的路径（data/user）,这些都需要在其他Service启动前完成。其次，通过mSystemServiceManager.startService(DeviceIdentifiersPolicyService.class);这段代码，android需要注册当前的设备表示，以防有一些特殊的时候需要用到。
我们进入install类的onStart()方法一看

@Override
public void onStart() {
    if (mIsolated) {
        mInstalld = null;
    } else {
        connect();
    }
}  
private void connect() {
    IBinder binder = ServiceManager.getService("installd");
    if (binder != null) {
        try {
            binder.linkToDeath(new DeathRecipient() {
                @Override
                public void binderDied() {
                    Slog.w(TAG, "installd died; reconnecting");
                    connect();
                }
            }, 0);
        } catch (RemoteException e) {
            binder = null;
        }
    }

    if (binder != null) {
        mInstalld = IInstalld.Stub.asInterface(binder);
        try {
            invalidateMounts();
        } catch (InstallerException ignored) {
        }
    } else {
        Slog.w(TAG, "installd not found; trying again");
        BackgroundThread.getHandler().postDelayed(() -> {
            connect();
        }, DateUtils.SECOND_IN_MILLIS);
    }
}

我们可以看到，其实这就是一个递归。install通过调用connect，去判断installer是否被初始化。只有installer被初始化了，才会继续往下掉用，初始化其他的服务。
接下来，启动ActivityManagerService。我们通过代码分析：

    mActivityManagerService = mSystemServiceManager.startService(
            ActivityManagerService.Lifecycle.class).getService();
    mActivityManagerService.setSystemServiceManager(mSystemServiceManager);
    mActivityManagerService.setInstaller(installer);

我们开启ams传入的是 ActivityManagerService.Lifecycle.class，同时把installer传入给ams。我们进入AMS查看代码，可以看到

final void finishBooting() {
    ...
    for (String abi : Build.SUPPORTED_ABIS) {
        zygoteProcess.establishZygoteConnectionForAbi(abi);
        final String instructionSet = VMRuntime.getInstructionSet(abi);
        if (!completedIsas.contains(instructionSet)) {
            try {
                mInstaller.markBootComplete(VMRuntime.getInstructionSet(abi));
            } catch (InstallerException e) {
                Slog.w(TAG, "Unable to mark boot complete for abi: " + abi + " (" +
                        e.getMessage() +")");
            }
            completedIsas.add(instructionSet);
        }
    }
  ...
 }

所以finishBooting是ams和zygote进行通讯的入口。
startBootstrapServices()方法中，还启动了很多其他的services，包括PowerManagerService、RecoverySystemService、LightsService、DisplayManagerService等等，这里就不重复做出分析了。

Step 2 分析startCoreServices()方法

private void startCoreServices() {
    // Records errors and logs, for example wtf()
    traceBeginAndSlog("StartDropBoxManager");
    mSystemServiceManager.startService(DropBoxManagerService.class);
    traceEnd();

    traceBeginAndSlog("StartBatteryService");
    // Tracks the battery level.  Requires LightService.
    mSystemServiceManager.startService(BatteryService.class);
    traceEnd();

    // Tracks application usage stats.
    traceBeginAndSlog("StartUsageService");
    mSystemServiceManager.startService(UsageStatsService.class);
    mActivityManagerService.setUsageStatsManager(
            LocalServices.getService(UsageStatsManagerInternal.class));
    traceEnd();

    // Tracks whether the updatable WebView is in a ready state and watches for update installs.
    traceBeginAndSlog("StartWebViewUpdateService");
    mWebViewUpdateService = mSystemServiceManager.startService(WebViewUpdateService.class);
    traceEnd();
}  

这个方法主要启动一些跟bootstrap进程无关的service，这里比较好玩的是，android会在这里去检测WebView是处于就绪状态和手动更新安装。
Step 3 分析startOtherServices()方法
这个方法过长，就不贴上源码的，这个方法主要是为了整理或者重构一些杂七杂八的包，不太重要，不做分析。

总结

至此，systemServer启动流程分析完毕，可能篇幅太长，大家需要自己好好整理，没事多复习。

（1）SystemServer进程是android中一个很重要的进程由Zygote进程启动；

（2）SystemServer进程主要用于启动系统中的服务；

（3）SystemServer进程启动服务的启动函数为main函数；

（4）SystemServer在执行过程中首先会初始化一些系统变量，加载类库，创建Context对象，创建SystemServiceManager对象等之后才开始启动系统服务；

（5）SystemServer进程将系统服务分为三类：boot服务，core服务和other服务，并逐步启动

（6）SertemServer进程在尝试启动服务之前会首先尝试与Zygote建立socket通讯，只有通讯成功之后才会开始尝试启动服务；

（7）创建的系统服务过程中主要通过SystemServiceManager对象来管理，通过调用服务对象的构造方法和onStart方法初始化服务的相关变量；

（8）服务对象都有自己的异步消息对象，并运行在单独的线程中；
（备注）以上8点的总结，参考的是http://blog.csdn.net/qq_23547831/article/details/51105171 在此感谢作者