Android开机启动流程初探

l  Init进程

Android系统在启动时首先会启动Linux系统，引导加载Linux Kernel并启动init进程。Init进程是一个由内核启动的用户级进程，是Android系统的第一个进程。该进程的相关代码在platform\system\core\init\init.c。在main函数中，有如下代码：

    open_devnull_stdio();    log_init();        INFO("reading config file\n");    init_parse_config_file("/init.rc");    /* pull the kernel commandline and ramdisk properties file in */    import_kernel_cmdline(0);    get_hardware_name(hardware, &revision);    snprintf(tmp, sizeof(tmp), "/init.%s.rc", hardware);    init_parse_config_file(tmp);

    这里会加载解析init.rc和init.hardware.rc两个初始化脚本。*.rc文件定义了在init进程中需要启动哪些进程服务和执行哪些动作。其详细说明参见platform\system\core\init\reademe.txt。init.rc见如下定义：

service servicemanager /system/bin/servicemanager    user system    critical    onrestart restart zygote    onrestart restart mediaservice vold /system/bin/vold    socket vold stream 0660 root mount    ioprio be 2service netd /system/bin/netd    socket netd stream 0660 root system    socket dnsproxyd stream 0660 root inetservice debuggerd /system/bin/debuggerdservice ril-daemon /system/bin/rild    socket rild stream 660 root radio    socket rild-debug stream 660 radio system    user root    group radio cache inet misc audio sdcard_rwservice zygote /system/bin/app_process -Xzygote /system/bin --zygote --start-system-server    socket zygote stream 666    onrestart write /sys/android_power/request_state wake    onrestart write /sys/power/state on    onrestart restart media    onrestart restart netdservice drm /system/bin/drmserver    user drm    group system root inet

具体解析过程见platform\system\core\init\Init_parser.c。解析所得服务添加到service_list中，动作添加到action_list中。

接下来在main函数中执行动作和启动进程服务：

        execute_one_command();        restart_processes()

通常init过程需要创建一些系统文件夹并启动USB守护进程、Android Debug Bridge守护进程、Debug守护进程、ServiceManager进程、Zygote进程等。

l  ServiceManager进程

ServiceManager进程是所有服务的管理器。由init.rc对ServiceManager的描述service servicemanager /system/bin/servicemanager可知servicemanager进程从platform\frameworks\base\cmd\servicemanager\Service_manager.cpp启动。在main函数中有如下代码：

int main(int argc, char **argv){    struct binder_state *bs;    void *svcmgr = BINDER_SERVICE_MANAGER;    bs = binder_open(128*1024);    if (binder_become_context_manager(bs)) {        LOGE("cannot become context manager (%s)\n", strerror(errno));        return -1;    }    svcmgr_handle = svcmgr;    binder_loop(bs, svcmgr_handler);    return 0;}

首先调用binder_open()打开Binder设备(/dev/binder)，调用binder_become_context_manager()把当前进程设置为ServiceManager。ServiceManager本身就是一个服务。

int binder_become_context_manager(struct binder_state *bs){    return ioctl(bs->fd, BINDER_SET_CONTEXT_MGR, 0);}

    最后binder_loop()进入循环状态，并设置svcmgr_handler回调函数等待添加、查询、获取服务等请求。

l  Zygote进程

         Zygote进程用于产生其他进程。由init.rc对zygote的描述service zygot /system/bin/app_process可知zygote进程从platfrom\frameworks\base\cmds\app_process\App_main.cpp启动。在main函数中有如下代码：

        if (0 == strcmp("--zygote", arg)) {            bool startSystemServer = (i < argc) ?                     strcmp(argv[i], "--start-system-server") == 0 : false;            setArgv0(argv0, "zygote");            set_process_name("zygote");            runtime.start("com.android.internal.os.ZygoteInit",                startSystemServer);        } else {            set_process_name(argv0);            runtime.mClassName = arg;            // Remainder of args get passed to startup class main()            runtime.mArgC = argc-i;            runtime.mArgV = argv+i;            LOGV("App process is starting with pid=%d, class=%s.\n",                 getpid(), runtime.getClassName());            runtime.start();        }

    首先创建AppRuntime，即AndroidRuntime，建立了一个Dalvik虚拟机。通过这个runtime传递com.android.internal.os.ZygoteInit参数，从而由Dalvik虚拟机运行ZygoteInit.java的main()，开始创建Zygote进程。在其main()中，如下所示：

            registerZygoteSocket();            EventLog.writeEvent(LOG_BOOT_PROGRESS_PRELOAD_START,                SystemClock.uptimeMillis());            preloadClasses();            //cacheRegisterMaps();            preloadResources();            EventLog.writeEvent(LOG_BOOT_PROGRESS_PRELOAD_END,                SystemClock.uptimeMillis());            // Finish profiling the zygote initialization.            SamplingProfilerIntegration.writeZygoteSnapshot();            // Do an initial gc to clean up after startup            gc();            // If requested, start system server directly from Zygote            if (argv.length != 2) {                throw new RuntimeException(argv[0] + USAGE_STRING);            }            if (argv[1].equals("true")) {                startSystemServer();            } else if (!argv[1].equals("false")) {                throw new RuntimeException(argv[0] + USAGE_STRING);            }

    首先通过registerZygoteSocket()登记端口，接着preloadClasses()装载相关类。这里大概要装载1000多个类，具体装载类见platform\frameworks\base\preloaded-classes。这个文件有WritePreloadedClassFile类自动生成。分析该类的main函数，有如下一段筛选类的代码：

        // Preload classes that were loaded by at least 2 processes. Hopefully,        // the memory associated with these classes will be shared.        for (LoadedClass loadedClass : root.loadedClasses.values()) {            Set<String> names = loadedClass.processNames();            if (!Policy.isPreloadable(loadedClass)) {                continue;            }            if (names.size() >= MIN_PROCESSES ||                    (loadedClass.medianTimeMicros() > MIN_LOAD_TIME_MICROS && names.size() > 1)) {                toPreload.add(loadedClass);            }        }        int initialSize = toPreload.size();        System.out.println(initialSize                + " classses were loaded by more than one app.");        // Preload eligable classes from applications (not long-running        // services).        for (Proc proc : root.processes.values()) {            if (proc.fromZygote() && !Policy.isService(proc.name)) {                for (Operation operation : proc.operations) {                    LoadedClass loadedClass = operation.loadedClass;                    if (shouldPreload(loadedClass)) {                        toPreload.add(loadedClass);                    }                }            }        }

其中MIN_LOAD_TIME_MICROS等于1250，当类的装载时间大于1.25ms，则需要预装载。

Policy.isPreloadable()定于如下：

    /**Reports if the given class should be preloaded. */    public static boolean isPreloadable(LoadedClass clazz) {        return clazz.systemClass && !EXCLUDED_CLASSES.contains(clazz.name);    }

其中EXCLUDED_CLASSES如下定义：

    /**     * Classes which we shouldn't load from the Zygote.     */    private static final Set<String> EXCLUDED_CLASSES            = new HashSet<String>(Arrays.asList(        // Binders        "android.app.AlarmManager",        "android.app.SearchManager",        "android.os.FileObserver",        "com.android.server.PackageManagerService$AppDirObserver",        // Threads        "android.os.AsyncTask",        "android.pim.ContactsAsyncHelper",        "java.lang.ProcessManager"    ));

这几个Binders和Thread是不会被预加载的。

    另外还有一些application需要装载，要求满足条件proc.fromZygote()且不是属于常驻内存的服务。SERVICES定义如下：

    /**     * Long running services. These are restricted in their contribution to the      * preloader because their launch time is less critical.     */    // TODO: Generate this automatically from package manager.    private static final Set<String> SERVICES = new HashSet<String>(Arrays.asList(        "system_server",        "com.google.process.content",        "android.process.media",        "com.android.bluetooth",        "com.android.calendar",        "com.android.inputmethod.latin",        "com.android.phone",        "com.google.android.apps.maps.FriendService", // pre froyo        "com.google.android.apps.maps:FriendService", // froyo        "com.google.android.apps.maps.LocationFriendService",        "com.google.android.deskclock",        "com.google.process.gapps",        "android.tts"    ));

         preloaded-classes是在下载源码的时候生成，WritePreloadedClassFile类并没有被用到，但可以通过这个类了解Android系统对预加载类的默认要求，参考修改preloaded-classes文件，减少开机初始化时要预加载的类，提高开机速度。

最后来通过startSystemServer()启动SystemServer进程。见如下代码：

        /* Hardcoded command line to start the system server */        String args[] = {            "--setuid=1000",            "--setgid=1000",            "--setgroups=1001,1002,1003,1004,1005,1006,1007,1008,1009,1010,1018,3001,3002,3003",            "--capabilities=130104352,130104352",            "--runtime-init",            "--nice-name=system_server",            "com.android.server.SystemServer",        };        ZygoteConnection.Arguments parsedArgs = null;        int pid;        try {            parsedArgs = new ZygoteConnection.Arguments(args);            /*             * Enable debugging of the system process if *either* the command line flags             * indicate it should be debuggable or the ro.debuggable system property             * is set to "1"             */            int debugFlags = parsedArgs.debugFlags;            if ("1".equals(SystemProperties.get("ro.debuggable")))                debugFlags |= Zygote.DEBUG_ENABLE_DEBUGGER;            /* Request to fork the system server process */            pid = Zygote.forkSystemServer(                    parsedArgs.uid, parsedArgs.gid,                    parsedArgs.gids, debugFlags, null,                    parsedArgs.permittedCapabilities,                    parsedArgs.effectiveCapabilities)

         Zygote包装了Linux的fork。forkSystemServer()调用forkAndSpecialize()，最终穿过虚拟机调用platform\dalvik\vm\native\dalvik_system_Zygote.c中Dalvik_dalvik_system_Zygote_forkAndSpecialize()。由dalvik完成fork新的进程。

       main()最后会调用runSelectLoopMode()，进入while循环，由peers创建新的进程。

l  SystemService进程

       SystemService用于创建init.rc定义的服务之外的所有服务。在main()的最后有如下代码：

        // 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);                System.loadLibrary("android_servers");        init1(args);

Init1()是在native空间实现的，用于启动native空间的服务，其实现在com_android_server_SystemServer.cpp中的android_server_SystemServer_init1()：

static void android_server_SystemServer_init1(JNIEnv* env, jobject clazz){    system_init();}

    而system_init()服务初始化创建native层的各个服务：

    // Start the sensor service    SensorService::instantiate();    // On the simulator, audioflinger et al don't get started the    // same way as on the device, and we need to start them here    if (!proc->supportsProcesses()) {        // Start the AudioFlinger        AudioFlinger::instantiate();        // Start the media playback service        MediaPlayerService::instantiate();        // Start the camera service        CameraService::instantiate();        // Start the audio policy service        AudioPolicyService::instantiate();    }

    最后通过如下代码:

    LOGI("System server: starting Android services.\n");    runtime->callStatic("com/android/server/SystemServer", "init2");

回到SystemServer.java，调用init2()：

    public static final void init2() {        Slog.i(TAG, "Entered the Android system server!");        Thread thr = new ServerThread();        thr.setName("android.server.ServerThread");        thr.start();    }

         Init2启动一个线程，专门用来启动java空间的所有服务。如下代码所示启动部分服务：

            Slog.i(TAG, "Content Manager");            ContentService.main(context,                    factoryTest == SystemServer.FACTORY_TEST_LOW_LEVEL);            Slog.i(TAG, "System Content Providers");            ActivityManagerService.installSystemProviders();            Slog.i(TAG, "Battery Service");            battery = new BatteryService(context);            ServiceManager.addService("battery", battery);            Slog.i(TAG, "Lights Service");            lights = new LightsService(context);            Slog.i(TAG, "Vibrator Service");            ServiceManager.addService("vibrator", new VibratorService(context));            // only initialize the power service after we have started the            // lights service, content providers and the battery service.            power.init(context, lights, ActivityManagerService.getDefault(), battery);            Slog.i(TAG, "Alarm Manager");            AlarmManagerService alarm = new AlarmManagerService(context);            ServiceManager.addService(Context.ALARM_SERVICE, alarm);

并且把这些服务添加到ServiceManager中，以便管理和进程间通讯。

在该线程后半部分，ActivityManagerService会等待AppWidget、WallPaper、IMM等systemReady后调用自身的systemReady()。

            ((ActivityManagerService)ServiceManager.getService("activity"))                    .setWindowManager(wm);            // Skip Bluetooth if we have an emulator kernel            // TODO: Use a more reliable check to see if this product should            // support Bluetooth - see bug 988521            if (SystemProperties.get("ro.kernel.qemu").equals("1")) {                Slog.i(TAG, "Registering null Bluetooth Service (emulator)");                ServiceManager.addService(BluetoothAdapter.BLUETOOTH_SERVICE, null);            } else if (factoryTest == SystemServer.FACTORY_TEST_LOW_LEVEL) {                Slog.i(TAG, "Registering null Bluetooth Service (factory test)");                ServiceManager.addService(BluetoothAdapter.BLUETOOTH_SERVICE, null);            } else {                Slog.i(TAG, "Bluetooth Service");                bluetooth = new BluetoothService(context);                ServiceManager.addService(BluetoothAdapter.BLUETOOTH_SERVICE, bluetooth);                bluetooth.initAfterRegistration();                bluetoothA2dp = new BluetoothA2dpService(context, bluetooth);                ServiceManager.addService(BluetoothA2dpService.BLUETOOTH_A2DP_SERVICE,                                          bluetoothA2dp);                int bluetoothOn = Settings.Secure.getInt(mContentResolver,                    Settings.Secure.BLUETOOTH_ON, 0);                if (bluetoothOn > 0) {                    bluetooth.enable();                }            }

    而在ActivityManagerService的systemReady()最后会执行如下代码：

            mMainStack.resumeTopActivityLocked(null);

    由于Activity管理栈为空，因此启动Launcher。

        // Find the first activity that is not finishing.        ActivityRecord next = topRunningActivityLocked(null);        // Remember how we'll process this pause/resume situation, and ensure        // that the state is reset however we wind up proceeding.        final boolean userLeaving = mUserLeaving;        mUserLeaving = false;        if (next == null) {            // There are no more activities!  Let's just start up the            // Launcher...            if (mMainStack) {                return mService.startHomeActivityLocked();            }        }

    在startHomeActivityLocked()中创建一个带Category为CATEGORY_HOME的Intent，由此去启动相应Activity，即Launcher。

        Intent intent = new Intent(            mTopAction,            mTopData != null ? Uri.parse(mTopData) : null);        intent.setComponent(mTopComponent);        if (mFactoryTest != SystemServer.FACTORY_TEST_LOW_LEVEL) {            intent.addCategory(Intent.CATEGORY_HOME);        }

    这样，Android系统便启动起来进入到待机界面。