android开机启动流程说明

android开机启动流程说明

    第一步:启动linux

    1.Bootloader

    2.Kernel

    第二步android系统启动:入口为init.rc(system\core\rootdir)

    1./system/bin/service manager: Binder 守护进程;

    2.Runtime;

    3.Zygote :app-process/app-main;

    4.Start VM;

    5.Start server

    6.Start android service:Register to service Manager

    7.Start Launcher

    第三步:应用程序启动:运行package Manager

    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 media

    service vold /system/bin/vold

    socket vold stream 0660 root mount

    ioprio be 2

    service netd /system/bin/netd

    socket netd stream 0660 root system

    socket dnsproxyd stream 0660 root inet

    service debuggerd /system/bin/debuggerd

    service 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_rw

    service 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 netd

    service 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 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 EXCLUDED_CLASSES

    = new HashSet(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 SERVICES = new HashSet(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。

 

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