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;

    原文作者:伯努力不努力
    原文地址: https://blog.csdn.net/u012124438/article/details/71036659
    本文转自网络文章,转载此文章仅为分享知识,如有侵权,请联系博主进行删除。
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