SystemServer进程源码分析

版本:Android O

SystemServer和Zygote是Android java世界的两大支柱,SystemServer是Zygote孵化出来的进程,进程名为system_server,几乎所有的系统服务都在该进程中,eg:AMS,PMS,WMS .etc

1 分析SystemServer代码

1.1 从SystemServer.java分析

源码位置:frameworks\base\services\java\com\android\server\SystemServer.java,从main()函数开始分析:

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

new出一个SystemServer类,执行其run()方法.

    public SystemServer() {        // Check for factory test mode.        mFactoryTestMode = FactoryTest.getMode();        // Remember if it's runtime restart(when sys.boot_completed is already set) or reboot        mRuntimeRestart = "1".equals(SystemProperties.get("sys.boot_completed"));    }

这是SystemServer的构造方法,具体没有研究,有blog说SystemServer.java是final的,不能被继承,但是在O中不是(也没有具体研究,研究了再补充).接下来看run()方法:

    private void run() {        try {            traceBeginAndSlog("InitBeforeStartServices");            // 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);            }            //            // Default the timezone property to GMT if not set.            //            String timezoneProperty =  SystemProperties.get("persist.sys.timezone");            if (timezoneProperty == null || timezoneProperty.isEmpty()) {                Slog.w(TAG, "Timezone not set; setting to GMT.");                SystemProperties.set("persist.sys.timezone", "GMT");            }            // If the system has "persist.sys.language" and friends set, replace them with            // "persist.sys.locale". Note that the default locale at this point is calculated            // using the "-Duser.locale" command line flag. That flag is usually populated by            // AndroidRuntime using the same set of system properties, but only the system_server            // and system apps are allowed to set them.            //            // NOTE: Most changes made here will need an equivalent change to            // core/jni/AndroidRuntime.cpp            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", "");            }            // The system server should never make non-oneway calls            Binder.setWarnOnBlocking(true);            // Here we go!            Slog.i(TAG, "Entered the Android system server!");            int uptimeMillis = (int) SystemClock.elapsedRealtime();            EventLog.writeEvent(EventLogTags.BOOT_PROGRESS_SYSTEM_RUN, uptimeMillis);            if (!mRuntimeRestart) {                MetricsLogger.histogram(null, "boot_system_server_init", uptimeMillis);            }            /// M: BOOTPROF            addBootEvent("Android:SysServerInit_START");            // In case the runtime switched since last boot (such as when            // the old runtime was removed in an OTA), set the system            // property so that it is in sync. We can | xq oqi't do this in            // libnativehelper's JniInvocation::Init code where we already            // had to fallback to a different runtime because it is            // running as root and we need to be the system user to set            // the property. http://b/11463182            SystemProperties.set("persist.sys.dalvik.vm.lib.2", VMRuntime.getRuntime().vmLibrary());            // 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);            }            // 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.            //指纹初始化属性值            Build.ensureFingerprintProperty();            // Within the system server, it is an error to access Environment paths without            // explicitly specifying a user.            Environment.setUserRequired(true);            // Within the system server, any incoming Bundles should be defused            // to avoid throwing BadParcelableException.            BaseBundle.setShouldDefuse(true);            // Ensure binder calls into the system always run at foreground priority.            BinderInternal.disableBackgroundScheduling(true);            // Increase the number of binder threads in system_server            BinderInternal.setMaxThreads(sMaxBinderThreads);            // 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");            // 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);            mSystemServiceManager.setRuntimeRestarted(mRuntimeRestart);            LocalServices.addService(SystemServiceManager.class, mSystemServiceManager);            // Prepare the thread pool for init tasks that can be parallelized            SystemServerInitThreadPool.get();        } finally {            traceEnd();  // InitBeforeStartServices        }        // Start services.        try {            traceBeginAndSlog("StartServices");            startBootstrapServices();            startCoreServices();            startOtherServices();            SystemServerInitThreadPool.shutdown();        } catch (Throwable ex) {            Slog.e("System", "******************************************");            Slog.e("System", "************ Failure starting system services", ex);            throw ex;        } finally {            traceEnd();        }        // For debug builds, log event loop stalls to dropbox for analysis.        if (StrictMode.conditionallyEnableDebugLogging()) {            Slog.i(TAG, "Enabled StrictMode for system server main thread.");        }        /// M: open wtf when load is user or userdebug.        if (!"eng".equals(Build.TYPE) && !mRuntimeRestart && !isFirstBootOrUpgrade()) {            int uptimeMillis = (int) SystemClock.elapsedRealtime();            MetricsLogger.histogram(null, "boot_system_server_ready", uptimeMillis);            final int MAX_UPTIME_MILLIS = 60 * 1000;            if (uptimeMillis > MAX_UPTIME_MILLIS) {                Slog.wtf(SYSTEM_SERVER_TIMING_TAG,                        "SystemServer init took too long. uptimeMillis=" + uptimeMillis);            }        }        /// M: BOOTPROF        addBootEvent("Android:SysServerInit_END");        // Loop forever.        Looper.loop();        throw new RuntimeException("Main thread loop unexpectedly exited");    }

最终的实现实在run方法里面,接下来分块分析run方法:

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

为了判断系统时钟是否早于1970/01/01/00:00,如果早于这个时间,系统后面的处理可能会出问题,所以如果早于该时间,统一设置为1970/01/01/00:00

            String timezoneProperty =  SystemProperties.get("persist.sys.timezone");            if (timezoneProperty == null || timezoneProperty.isEmpty()) {                Slog.w(TAG, "Timezone not set; setting to GMT.");                SystemProperties.set("persist.sys.timezone", "GMT");            }

如果没有设置时区,统一设置为GMT

            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", "");            }

设置系统语言环境;

            // Here we go!            Slog.i(TAG, "Entered the Android system server!");            int uptimeMillis = (int) SystemClock.elapsedRealtime();            EventLog.writeEvent(EventLogTags.BOOT_PROGRESS_SYSTEM_RUN, uptimeMillis);            if (!mRuntimeRestart) {                MetricsLogger.histogram(null, "boot_system_server_init", uptimeMillis);            }            /// M: BOOTPROF            addBootEvent("Android:SysServerInit_START");            // In case the runtime switched since last boot (such as when            // the old runtime was removed in an OTA), set the system            // property so that it is in sync. We can | xq oqi't do this in            // libnativehelper's JniInvocation::Init code where we already            // had to fallback to a different runtime because it is            // running as root and we need to be the system user to set            // the property. http://b/11463182            SystemProperties.set("persist.sys.dalvik.vm.lib.2", VMRuntime.getRuntime().vmLibrary());            // 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);            }            // 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.            //指纹            Build.ensureFingerprintProperty();            // Within the system server, it is an error to access Environment paths without            // explicitly specifying a user.            Environment.setUserRequired(true);            // Within the system server, any incoming Bundles should be defused            // to avoid throwing BadParcelableException.            BaseBundle.setShouldDefuse(true);            // Ensure binder calls into the system always run at foreground priority.            BinderInternal.disableBackgroundScheduling(true);            // Increase the number of binder threads in system_server            //设置binder最大线程数 sMaxBinderThreads=31            BinderInternal.setMaxThreads(sMaxBinderThreads);

这段代码的主要作用是设置虚拟机(VMRuntime)运行内存,相关的操作,(没有细致的研究过).

            // Prepare the main looper thread (this thread).            android.os.Process.setThreadPriority(                android.os.Process.THREAD_PRIORITY_FOREGROUND);            android.os.Process.setCanSelfBackground(false);            Looper.prepareMainLooper();

创建消息循环,和ActivityThread.java里的main()函数里建立异步消息循环一样.

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

加载了libandroid_servers.so文件

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

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这里给出performPendingShutdown()函数代码如下:

    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;            }            // If it's a pending reboot into recovery to apply an update,            // always make sure uncrypt gets executed properly when needed.            // If '/cache/recovery/block.map' hasn't been created, stop the            // reboot which will fail for sure, and get a chance to capture a            // bugreport when that's still feasible. (Bug: 26444951)            if (reason != null && reason.startsWith(PowerManager.REBOOT_RECOVERY_UPDATE)) {                File packageFile = new File(UNCRYPT_PACKAGE_FILE);                if (packageFile.exists()) {                    String filename = null;                    try {                        filename = FileUtils.readTextFile(packageFile, 0, null);                    } catch (IOException e) {                        Slog.e(TAG, "Error reading uncrypt package file", e);                    }                    if (filename != null && filename.startsWith("/data")) {                        if (!new File(BLOCK_MAP_FILE).exists()) {                            Slog.e(TAG, "Can't find block map file, uncrypt failed or " +                                       "unexpected runtime restart?");                            return;                        }                    }                }            }            ShutdownThread.rebootOrShutdown(null, reboot, reason);        }    }

依据注释,和代码名,最近一次关机操作(启动SystemServer是在开机过程中),是非正常状态,该代码将shutdown操作悬挂.(想要明白此处还要明白android的关机流程,还要努力!!!).

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回到run()方法接着分析;

            // Initialize the system context.            createSystemContext();

看看createSystemContext()做了什么

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

这里还是通过ActivityThread.java来操作,和启动一个应用进程类似,只不过应用进程是使用ActivityThread.java->main(),这里是使用ActivityThread.java->systemMain();这两者一定有区别,因为普通进程是要attach到AMS的,这里AMS还没有出生…可以对比ActivityThread.java里面的代码进行分析,对比.为了不偏离主线,这里就不深入分析了.接着往下看:

            // Create the system service manager.            mSystemServiceManager = new SystemServiceManager(mSystemContext);            mSystemServiceManager.setRuntimeRestarted(mRuntimeRestart);            LocalServices.addService(SystemServiceManager.class, mSystemServiceManager);            // Prepare the thread pool for init tasks that can be parallelized            SystemServerInitThreadPool.get();

启动AMS\PMS\WMS等等这些系统级的服务前,要把管理他们的服务SystemServiceManager启动起来吧,老铁,这没有问题吧~~~
LocalServices.addService(SystemServiceManager.class, mSystemServiceManager);将SystemServiceManager对象保存
SystemServer进程中的一个数据结构中.SystemServerInitThreadPool.get();初始化线程池,为了初始化任务能够并行处理.
这些启动系统级服务的准备工作都做好了,是不是该启动服务了:

        // Start services.        try {            traceBeginAndSlog("StartServices");            startBootstrapServices();            startCoreServices();            startOtherServices();            SystemServerInitThreadPool.shutdown();        } catch (Throwable ex) {            Slog.e("System", "******************************************");            Slog.e("System", "************ Failure starting system services", ex);            throw ex;        } finally {            traceEnd();        }

终于到启动这些重量级的系统级服务了,通过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.启动完成之后使用SystemServerInitThreadPool.shutdown()讲线程池关闭.
接着我们注意分析startBootstrapServices(),startCoreServices(),startOtherServices()这三个函数.

首先看startBootstrapServices():

    /**     * Starts the small tangle of critical services that are needed to get     * the system off the ground.  These services have complex mutual dependencies     * which is why we initialize them all in one place here.  Unless your service     * is also entwined in these dependencies, it should be initialized in one of     * the other functions.     */    private void startBootstrapServices() {       ......        //Installer提供安装、卸载App等服务       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();        // Activity manager runs the show.        //大名鼎鼎的ActivityManagerService在这里启动,提供Activity等组件的管理的服务        traceBeginAndSlog("StartActivityManager");        mActivityManagerService = mSystemServiceManager.startService(                ActivityManagerService.Lifecycle.class).getService();        mActivityManagerService.setSystemServiceManager(mSystemServiceManager);        mActivityManagerService.setInstaller(installer);        traceEnd();        // 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).        //电源管理的服务        traceBeginAndSlog("StartPowerManager");        mPowerManagerService = mSystemServiceManager.startService(PowerManagerService.class);        traceEnd();        // Now that the power manager has been started, let the activity manager        // initialize power management features.        traceBeginAndSlog("InitPowerManagement");        mActivityManagerService.initPowerManagement();        traceEnd();        // Bring up recovery system in case a rescue party needs a reboot        if (!SystemProperties.getBoolean("config.disable_noncore", false)) {            traceBeginAndSlog("StartRecoverySystemService");            mSystemServiceManager.startService(RecoverySystemService.class);            traceEnd();        }        // Now that we have the bare essentials of the OS up and running, take        // note that we just booted, which might send out a rescue party if        // we're stuck in a runtime restart loop.        RescueParty.noteBoot(mSystemContext);        // Manages LEDs and display backlight so we need it to bring up the display.        traceBeginAndSlog("StartLightsService");        //LED管理和背光显示的服务        mSystemServiceManager.startService(LightsService.class);        traceEnd();        // Display manager is needed to provide display metrics before package manager        // starts up.        traceBeginAndSlog("StartDisplayManager");        //提供显示的生命周期管理，根据物理显示设备当前的情况决定显示配置，在状态改变时发送通知给系统和应用等服务        mDisplayManagerService = mSystemServiceManager.startService(DisplayManagerService.class);        traceEnd();        // We need the default display before we can initialize the package manager.        traceBeginAndSlog("WaitForDisplay");        mSystemServiceManager.startBootPhase(SystemService.PHASE_WAIT_FOR_DEFAULT_DISPLAY);        traceEnd();        // 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.        //启动PackageManagerService,该服务用于管理所有的.apk        if (!mRuntimeRestart) {            MetricsLogger.histogram(null, "boot_package_manager_init_start",                    (int) SystemClock.elapsedRealtime());        }        traceBeginAndSlog("StartPackageManagerService");        mPackageManagerService = PackageManagerService.main(mSystemContext, installer,                mFactoryTestMode != FactoryTest.FACTORY_TEST_OFF, mOnlyCore);        mFirstBoot = mPackageManagerService.isFirstBoot();        mPackageManager = mSystemContext.getPackageManager();        traceEnd();        if (!mRuntimeRestart && !isFirstBootOrUpgrade()) {            MetricsLogger.histogram(null, "boot_package_manager_init_ready",                    (int) SystemClock.elapsedRealtime());        }        // 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 {                    traceEnd();                }            }        }        traceBeginAndSlog("StartUserManagerService");        //提供用户相关服务        mSystemServiceManager.startService(UserManagerService.LifeCycle.class);        traceEnd();        // Initialize attribute cache used to cache resources from packages.        traceBeginAndSlog("InitAttributerCache");        AttributeCache.init(mSystemContext);        traceEnd();        // Set up the Application instance for the system process and get started.        traceBeginAndSlog("SetSystemProcess");        mActivityManagerService.setSystemProcess();        traceEnd();        // DisplayManagerService needs to setup android.display scheduling related policies        // since setSystemProcess() would have overridden policies due to setProcessGroup        mDisplayManagerService.setupSchedulerPolicies();        /// M: CTA requirement - permission control  @{        /// M: MOTA for CTA permissions handling        /*         * This function is used for granting CTA permissions after OTA upgrade.         * This should be placed after AMS is added to ServiceManager and before         * starting other services since granting permissions needs AMS instance         * to do permission checking.         */        mPackageManagerService.onAmsAddedtoServiceMgr();        /// @}        // Manages Overlay packages        traceBeginAndSlog("StartOverlayManagerService");        mSystemServiceManager.startService(new OverlayManagerService(mSystemContext, installer));        traceEnd();        // The sensor service needs access to package manager service, app ops        // service, and permissions service, therefore we start it after them.        // Start sensor service in a separate thread. Completion should be checked        // before using it.        //通过startSensorService()本地方法启动Sensor服务        mSensorServiceStart = SystemServerInitThreadPool.get().submit(() -> {            BootTimingsTraceLog traceLog = new BootTimingsTraceLog(                    SYSTEM_SERVER_TIMING_ASYNC_TAG, Trace.TRACE_TAG_SYSTEM_SERVER);            traceLog.traceBegin(START_SENSOR_SERVICE);            startSensorService();            traceLog.traceEnd();        }, START_SENSOR_SERVICE);    }

首先分析

Installer installer = mSystemServiceManager.startService(Installer.class);

mSystemServiceManager是系统服务管理对象,在前面介绍的run()里面已经实例化了.这里简单介绍一下Installer类，
该类是系统安装apk时的一个服务类，继承SystemService（系统服务的一个抽象接口），我们需要在启动完成Installer
服务之后才能启动其他的系统服务.接着可以看到ActivityManagerService,PowerManagerService,RecoverySystemService,
LightsService,DisplayManagerService,UserManagerService这些服务都是使用mSystemServiceManager.startService()方法将服务启动起来.
发现PackageManagerService服务和其他的有点不一样,他是直接调用了静态方法main()方法实现了:

        // Start the package manager.        if (!mRuntimeRestart) {            MetricsLogger.histogram(null, "boot_package_manager_init_start",                    (int) SystemClock.elapsedRealtime());        }        traceBeginAndSlog("StartPackageManagerService");        mPackageManagerService = PackageManagerService.main(mSystemContext, installer,                mFactoryTestMode != FactoryTest.FACTORY_TEST_OFF, mOnlyCore);        mFirstBoot = mPackageManagerService.isFirstBoot();        mPackageManager = mSystemContext.getPackageManager();        traceEnd();        if (!mRuntimeRestart && !isFirstBootOrUpgrade()) {            MetricsLogger.histogram(null, "boot_package_manager_init_ready",                    (int) SystemClock.elapsedRealtime());        }

分析PackageManagerService.main()函数:

    public static PackageManagerService main(Context context, Installer installer,            boolean factoryTest, boolean onlyCore) {        // Self-check for initial settings.        PackageManagerServiceCompilerMapping.checkProperties();        PackageManagerService m = new PackageManagerService(context, installer,                factoryTest, onlyCore);        m.enableSystemUserPackages();        ServiceManager.addService("package", m);        return m;    }

这里是直接newPackageManagerService,然后调用ServiceManager.addService,通过了binder(以后再仔细研究).
对于这里涉及到的服务,会单独抽出来学习总结,这里简单介绍一下.

这里有一段涉及vold服务的:

        // 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;        }

android 4.0新增的一个功能，即设备加密（encrypting the device）,该功能由系统属性vold.decrypt指定.
涉及设备安全和加密的,vold(Volume Daemon)用于管理和控制Android平台外部存储设备的后台进程，这些管
理和控制，包括SD卡的插拔事件检测、SD卡挂载、卸载、格式化等。这里的设置是当我们的设备处于加密状态,
只启动核心服务,通过设置mOnlyCore来进行标示.vold服务也是一块知识点,可以单独总结.

接着分析startCoreServices()函数:

    /**     * Starts some essential services that are not tangled up in the bootstrap process.     */    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();    }

启动了DropBoxManagerService(系统出问题的调用栈信息服务),BatteryService(电池相关的服务),UsageStatsService(搜集用户使用app数据的信息服务类),
WebViewUpdateService.
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。

总结

• SystemServer进程是android中一个很重要的进程由Zygote进程启动,是Zygote的嫡长子,如果该进程崩溃,Zygote会调用方法kill掉自己；
• SystemServer进程主要用于启动系统中的服务；
• SystemServer进程启动服务的启动函数为main函数,其实真正干活的还是在run()方法里面；
• 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；
• 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;
• 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；
• 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；
• 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；

参考文章:
http://blog.csdn.net/qq_23547831/article/details/51105171