Android Framework学习(三)之SyetemServer进程启动解析

从上篇博客中，我们知道了Zygote进程启动了SyetemServer进程，本篇博客我们就一起来学习SyetemServer进程。

SystemServer的作用

整个系统的android framework进程启动流程如下：

init进程 –> Zygote进程 –> SystemServer进程 –>各种应用进程

SystemServer进程主要的作用是启动各种系统服务，比如ActivityManagerService，PackageManagerService，WindowManagerService等服务，我们平时熟知的各种系统性的服务其实都是在SystemServer进程中启动的，而当我们的应用需要使用各种系统服务的时候其实也是通过与SystemServer进程通讯获取各种服务对象的句柄的进而执行相应的操作的。

SyetemServer进程启动流程

之前我们曾经说过，在ZygoteInit.Java的startSystemServer函数中启动了SyetemServer进程
frameworks/base/core/java/com/Android/internal/os/ZygoteInit.java

private static boolean startSystemServer(String abiList, String socketName)            throws MethodAndArgsCaller, RuntimeException {     ...        if (pid == 0) {            if (hasSecondZygote(abiList)) {                waitForSecondaryZygote(socketName);            }            handleSystemServerProcess(parsedArgs);        }        return true;    }

在startSystemServer函数中调用handleSystemServerProcess来启动SyetemServer进程。

handleSystemServerProcess

private static void handleSystemServerProcess(            ZygoteConnection.Arguments parsedArgs)            throws ZygoteInit.MethodAndArgsCaller {        closeServerSocket();//1      ...        if (parsedArgs.invokeWith != null) {           ...        } else {            ClassLoader cl = null;            if (systemServerClasspath != null) {                cl = createSystemServerClassLoader(systemServerClasspath,                                                   parsedArgs.targetSdkVersion);                Thread.currentThread().setContextClassLoader(cl);            }            RuntimeInit.zygoteInit(parsedArgs.targetSdkVersion, parsedArgs.remainingArgs, cl);//2        }    }

SyetemServer进程是复制了Zygote进程的地址空间，因此也会得到Zygote进程创建的Socket，这个Socket对于SyetemServer进程没有用处，因此，需要注释1处的代码来关闭该Socket。在注释2处调用RuntimeInit的zygoteInit函数

frameworks/base/core/java/com/android/internal/os/RuntimeInit.java

public static final void zygoteInit(int targetSdkVersion, String[] argv, ClassLoader classLoader)            throws ZygoteInit.MethodAndArgsCaller {        if (DEBUG) Slog.d(TAG, "RuntimeInit: Starting application from zygote");        Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "RuntimeInit");        redirectLogStreams();        commonInit();        nativeZygoteInit();//1        applicationInit(targetSdkVersion, argv, classLoader);//2    }

注释1处调用nativeZygoteInit函数，所以又调用了Native层的代码，从java代码又切换进native代码

启动Binder线程池
查看nativeZygoteInit函数对用的JNI文件
frameworks/base/core/jni/AndroidRuntime.cpp

static const JNINativeMethod gMethods[] = {    { "nativeFinishInit", "()V",        (void*) com_android_internal_os_RuntimeInit_nativeFinishInit },    { "nativeZygoteInit", "()V",        (void*) com_android_internal_os_RuntimeInit_nativeZygoteInit },    { "nativeSetExitWithoutCleanup", "(Z)V",        (void*) com_android_internal_os_RuntimeInit_nativeSetExitWithoutCleanup },};

通过JNI的gMethods数组，可以看出nativeZygoteInit函数对应的是JNI文件AndroidRuntime.cpp的com_android_internal_os_RuntimeInit_nativeZygoteInit函数：

...static AndroidRuntime* gCurRuntime = NULL;...static void com_android_internal_os_RuntimeInit_nativeZygoteInit(JNIEnv* env, jobject clazz){    gCurRuntime->onZygoteInit();}

这里gCurRuntime是AndroidRuntime类型的指针，AndroidRuntime的子类AppRuntime在app_main.cpp中定义

 virtual void onZygoteInit()    {        sp<ProcessState> proc = ProcessState::self();        ALOGV("App process: starting thread pool.\n");        proc->startThreadPool();//1    }

注释1处的代码用来启动一个Binder线程池，这样SyetemServer进程就可以使用Binder来与其他进程进行通信了。综合上述，可以看出RuntimeInit.java的nativeZygoteInit函数主要做的就是启动Binder线程池。

applicationInit
回到RuntimeInit.java的代码，在注释2处调用了applicationInit函数
frameworks/base/core/java/com/android/internal/os/RuntimeInit.java
applicationInit函数中主要调用了invokeStaticMain函数：

 private static void invokeStaticMain(String className, String[] argv, ClassLoader classLoader)            throws ZygoteInit.MethodAndArgsCaller {        Class<?> cl;        try {            cl = Class.forName(className, true, classLoader);//1        } catch (ClassNotFoundException ex) {            throw new RuntimeException(                    "Missing class when invoking static main " + className,                    ex);        }        Method m;        try {            m = cl.getMethod("main", new Class[] { String[].class });//2        } catch (NoSuchMethodException ex) {            throw new RuntimeException(                    "Missing static main on " + className, ex);        } catch (SecurityException ex) {            throw new RuntimeException(                    "Problem getting static main on " + className, ex);        }        int modifiers = m.getModifiers();        if (! (Modifier.isStatic(modifiers) && Modifier.isPublic(modifiers))) {            throw new RuntimeException(                    "Main method is not public and static on " + className);        }        throw new ZygoteInit.MethodAndArgsCaller(m, argv);//3    }

注释1处className为“com.android.server.SystemServer”，因此通过反射返回的cl为SystemServer类。注释2处找到SystemServer中的main函数。在注释3处将找到的main函数传入到MethodAndArgsCaller异常中并抛出该异常。截获MethodAndArgsCaller异常的代码在ZygoteInit.java的main函数中

frameworks/base/core/java/com/android/internal/os/ZygoteInit.java

public static void main(String argv[]) {       ...            closeServerSocket();        } catch (MethodAndArgsCaller caller) {            caller.run();//1        } catch (RuntimeException ex) {            Log.e(TAG, "Zygote died with exception", ex);            closeServerSocket();            throw ex;        }    }

在注释1处调用了MethodAndArgsCaller的run函数：

public void run() {        try {            mMethod.invoke(null, new Object[] { mArgs });        } catch (IllegalAccessException ex) {         ...        }    }

这里mMethod指的就是SystemServer的main函数，因此main函数被动态调用。

SyetemServer进程分析

SystemServer函数的入口同样是main函数：
frameworks/base/services/java/com/android/server/SystemServer.java

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

其中只是new出一个SystemServer对象并执行其run方法，查看SystemServer类的定义我们知道其实final类型的，所以我们不能重写或者继承SystemServer。

private void run() {        ...        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()) {//1            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());        // 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.        Build.ensureFingerprintProperty();        // Within the system server, it is an error to access Environment paths without        // explicitly specifying a user.        Environment.setUserRequired(true);        // Ensure binder calls into the system always run at foreground priority.        BinderInternal.disableBackgroundScheduling(true);        // 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");//2        // Check whether we failed to shut down last time we tried.        // This call may not return.        performPendingShutdown();        // Initialize the system context.        createSystemContext();//3        // Create the system service manager.        mSystemServiceManager = new SystemServiceManager(mSystemContext);//4        LocalServices.addService(SystemServiceManager.class, mSystemServiceManager);        // Start services.        try {            startBootstrapServices();//5            startCoreServices();//6            startOtherServices();//7        } catch (Throwable ex) {            Slog.e("System", "******************************************");            Slog.e("System", "************ Failure starting system services", ex);            throw ex;        }        // For debug builds, log event loop stalls to dropbox for analysis.        if (StrictMode.conditionallyEnableDebugLogging()) {            Slog.i(TAG, "Enabled StrictMode for system server main thread.");        }        // Loop forever.        Looper.loop();        throw new RuntimeException("Main thread loop unexpectedly exited");    }

1.设置系统的语言环境等属性

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", "");        }

2.加载libandroid_servers.so库
3.createSystemContext()

private void createSystemContext() {        ActivityThread activityThread = ActivityThread.systemMain();        mSystemContext = activityThread.getSystemContext();        mSystemContext.setTheme(android.R.style.Theme_DeviceDefault_Light_DarkActionBar);    }

可以看到在SystemServer进程中也存在着Context对象，并且是通过ActivityThread.systemMain方法创建context的，这一部分的逻辑以后会通过介绍Activity的启动流程来介绍，这里就不在扩展，这里我们只知道在SystemServer进程中也需要创建Context对象。

4.创建SystemServiceManager
创建SystemServiceManager，它会对系统的服务进行创建、启动和生命周期管理
然后调用：

LocalServices.addService(SystemServiceManager.class, mSystemServiceManager);

将SystemServiceManager对象保存SystemServer进程中的一个数据结构中。

Start services
创建SystemServiceManager后，会执行以下逻辑

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

里面主要涉及了是三个方法：
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

5.startBootstrapServices方法

private void startBootstrapServices() {        Installer installer = mSystemServiceManager.startService(Installer.class);//1        // Activity manager runs the show.        mActivityManagerService = mSystemServiceManager.startService(                ActivityManagerService.Lifecycle.class).getService();//2        mActivityManagerService.setSystemServiceManager(mSystemServiceManager);        mActivityManagerService.setInstaller(installer);        mPowerManagerService = mSystemServiceManager.startService(PowerManagerService.class);//3        // Now that the power manager has been started, let the activity manager        // initialize power management features.        mActivityManagerService.initPowerManagement();        // Manages LEDs and display backlight so we need it to bring up the display.        mSystemServiceManager.startService(LightsService.class);//4        // Display manager is needed to provide display metrics before package manager        // starts up.        mDisplayManagerService = mSystemServiceManager.startService(DisplayManagerService.class);//5        // 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.        Slog.i(TAG, "Package Manager");        mPackageManagerService = PackageManagerService.main(mSystemContext, installer,                mFactoryTestMode != FactoryTest.FACTORY_TEST_OFF, mOnlyCore);//6        mFirstBoot = mPackageManagerService.isFirstBoot();        mPackageManager = mSystemContext.getPackageManager();        Slog.i(TAG, "User Service");        ServiceManager.addService(Context.USER_SERVICE, UserManagerService.getInstance());//7        // 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();    }

注释1处执行了如下代码

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

mSystemServiceManager是系统服务管理对象，在main方法中已经创建完成，这里我们看一下其startService方法的具体实现：

public <T extends SystemService> T startService(Class<T> serviceClass) {        final String name = serviceClass.getName();        Slog.i(TAG, "Starting " + name);        // Create the service.        if (!SystemService.class.isAssignableFrom(serviceClass)) {            throw new RuntimeException("Failed to create " + name                    + ": service must extend " + SystemService.class.getName());        }        final T service;        try {            Constructor<T> constructor = serviceClass.getConstructor(Context.class);            service = constructor.newInstance(mContext);        } catch (InstantiationException ex) {            throw new RuntimeException("Failed to create service " + name                    + ": service could not be instantiated", ex);        } catch (IllegalAccessException ex) {            throw new RuntimeException("Failed to create service " + name                    + ": service must have a public constructor with a Context argument", ex);        } catch (NoSuchMethodException ex) {            throw new RuntimeException("Failed to create service " + name                    + ": service must have a public constructor with a Context argument", ex);        } catch (InvocationTargetException ex) {            throw new RuntimeException("Failed to create service " + name                    + ": service constructor threw an exception", ex);        }        // Register it.        mServices.add(service);        // Start it.        try {            service.onStart();        } catch (RuntimeException ex) {            throw new RuntimeException("Failed to start service " + name                    + ": onStart threw an exception", ex);        }        return service;    }

通过反射器构造方法创建出服务类，然后添加到SystemServiceManager的服务列表数据中，最后调用了service.onStart()方法，因为我们传递的是Installer.class，我们这里我们查看一下Installer的onStart方法：

@Override    public void onStart() {        Slog.i(TAG, "Waiting for installd to be ready.");        mInstaller.waitForConnection();    }

执行了mInstaller的waitForConnection方法，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。

然后查看waitForConnection（）方法：

public void waitForConnection() {        for (;;) {            if (execute("ping") >= 0) {                return;            }            Slog.w(TAG, "installd not ready");            SystemClock.sleep(1000);        }    }

通过追踪代码可以发现，其在不断的通过ping命令连接Zygote进程（SystemServer和Zygote进程通过socket方式通讯，其他进程通过Binder方式通讯）；

结论：在开始执行启动服务之前总是会先尝试通过socket方式连接Zygote进程，在成功连接之后才会开始启动其他服务。

继续回到startBootstrapServices方法，注释2：

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

启动ActivityManagerService服务，并为其设置SystemServiceManager和Installer。ActivityManagerService是系统中一个非常重要的服务，Activity，service，Broadcast，contentProvider都需要通过其余系统交互。

查看Lifecycle类的定义：

public static final class Lifecycle extends SystemService {        private final ActivityManagerService mService;        public Lifecycle(Context context) {            super(context);            mService = new ActivityManagerService(context);        }        @Override        public void onStart() {            mService.start();        }        public ActivityManagerService getService() {            return mService;        }    }

可以发现Lifecycle是ActivityManagerService是的一个静态内部类，在其构造方法中会创建一个ActivityManagerService，通过刚刚对Installer服务的分析我们知道，SystemServiceManager的startService方法会调用服务的onStart()方法，而在Lifecycle类的定义中我们看到其onStart（）方法直接调用了mService.start()方法，mService是Lifecycle类中对ActivityManagerService的引用，所以我们可以看一下ActivityManagerService的start方法的实现：

private void start() {        Process.removeAllProcessGroups();        mProcessCpuThread.start();        mBatteryStatsService.publish(mContext);        mAppOpsService.publish(mContext);        Slog.d("AppOps", "AppOpsService published");        LocalServices.addService(ActivityManagerInternal.class, new LocalService());    }

注释3处，启动PowerManagerService服务

mPowerManagerService = mSystemServiceManager.startService(PowerManagerService.class);

启动方式跟上面的ActivityManagerService服务相似都会调用其构造方法和onStart方法，PowerManagerService主要用于计算系统中和Power相关的计算，然后决策系统应该如何反应。同时协调Power如何与系统其它模块的交互，比如没有用户活动时，屏幕变暗等等。
注释4处，启动LightsService服务

mSystemServiceManager.startService(LightsService.class);

主要是手机中关于闪光灯，LED等相关的服务；也是会调用LightsService的构造方法和onStart方法；
注释5处，启动DisplayManagerService服务

mDisplayManagerService = mSystemServiceManager.startService(DisplayManagerService.class);

主要是手机显示方面的服务；
注释6处，启动PackageManagerService
启动PackageManagerService，该服务也是android系统中一个比较重要的服务，包括多apk文件的安装，解析，删除，卸载等等操作

Slog.i(TAG, "Package Manager");        mPackageManagerService = PackageManagerService.main(mSystemContext, installer,                mFactoryTestMode != FactoryTest.FACTORY_TEST_OFF, mOnlyCore);        mFirstBoot = mPackageManagerService.isFirstBoot();        mPackageManager = mSystemContext.getPackageManager();

可以看到PackageManagerService服务的启动方式与其他服务的启动方式有一些区别，直接调用了PackageManagerService的静态main方法，这里我们看一下其main方法的具体实现：

public static PackageManagerService main(Context context, Installer installer,            boolean factoryTest, boolean onlyCore) {        PackageManagerService m = new PackageManagerService(context, installer,                factoryTest, onlyCore);        ServiceManager.addService("package", m);        return m;    }

直接使用new的方式创建了一个PackageManagerService对象，并在其构造方法中初始化相关变量，最后调用了ServiceManager.addService方法，主要是通过Binder机制与JNI层交互

注释7处，启动UserManagerService和SensorService
至此startBootstrapServices方法执行完成。
6.startCoreServices() 方法

private void startCoreServices() {        // Tracks the battery level.  Requires LightService.        mSystemServiceManager.startService(BatteryService.class);        // Tracks application usage stats.        mSystemServiceManager.startService(UsageStatsService.class);        mActivityManagerService.setUsageStatsManager(                LocalServices.getService(UsageStatsManagerInternal.class));        // Update after UsageStatsService is available, needed before performBootDexOpt.        mPackageManagerService.getUsageStatsIfNoPackageUsageInfo();        // Tracks whether the updatable WebView is in a ready state and watches for update installs.        mSystemServiceManager.startService(WebViewUpdateService.class);    }

这里启动了BatteryService（电池相关服务），UsageStatsService，WebViewUpdateService服务等。

7.startOtherServices方法
startOtherServices方法，主要用于启动系统中其他的服务，CameraService、AlarmManagerService、VrManagerService等服务，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SyetemServer启动流程

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；