android開機啓動流程說明

android開機啓動流程說明

    第一步：啓動linux

    1.Bootloader

    2.Kernel

    第二步android系統啓動：入口爲init.rc(system\core\rootdir)

    1./system/bin/service manager: Binder 守護進程；

    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：應用程序啓動：運行package Manager

    l Init進程

    Android系統在啓動時首先會啓動Linux系統，引導加載Linux Kernel並啓動init進程。Init進程是一個由內核啓動的用戶級進程，是Android系統的第一個進程。該進程的相關代碼在platform\system\core\init\init.c。在main函數中，有以下代碼：

    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);

    這裏會加載解析init.rc和init.hardware.rc兩個初始化腳本。*.rc文件定義了在init進程中須要啓動哪些進程服務和執行哪些動做。其詳細說明參見platform\system\core\init\reademe.txt。init.rc見以下定義：

    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

    具體解析過程見platform\system\core\init\Init_parser.c。解析所得服務添加到service_list中，動做添加到action_list中。

    接下來在main函數中執行動做和啓動進程服務：

    execute_one_command();

    restart_processes()

    一般init過程須要建立一些系統文件夾並啓動USB守護進程、Android Debug Bridge守護進程、Debug守護進程、ServiceManager進程、Zygote進程等。

    l ServiceManager進程

    ServiceManager進程是全部服務的管理器。由init.rc對ServiceManager的描述service servicemanager /system/bin/servicemanager可知servicemanager進程從platform\frameworks\base\cmd\servicemanager\Service_manager.cpp啓動。在main函數中有以下代碼：

    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;

    }

    首先調用binder_open()打開Binder設備(/dev/binder)，調用binder_become_context_manager()把當前進程設置爲ServiceManager。ServiceManager自己就是一個服務。

    int binder_become_context_manager(struct binder_state *bs)

    {

    return ioctl(bs->fd, BINDER_SET_CONTEXT_MGR, 0);

    }

    最後binder_loop()進入循環狀態，並設置svcmgr_handler回調函數等待添加、查詢、獲取服務等請求。

    l Zygote進程

    Zygote進程用於產生其餘進程。由init.rc對zygote的描述service zygot /system/bin/app_process可知zygote進程從platfrom\frameworks\base\cmds\app_process\App_main.cpp啓動。在main函數中有以下代碼：

    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();

    }

    首先建立AppRuntime，即AndroidRuntime，創建了一個Dalvik虛擬機。經過這個runtime傳遞com.android.internal.os.ZygoteInit參數，從而由Dalvik虛擬機運行ZygoteInit.java的main()，開始建立Zygote進程。在其main()中，以下所示：

    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);

    }

    首先經過registerZygoteSocket()登記端口，接着preloadClasses()裝載相關類。這裏大概要裝載1000多個類，具體裝載類見platform\frameworks\base\preloaded-classes。這個文件有WritePreloadedClassFile類自動生成。分析該類的main函數，有以下一段篩選類的代碼：

    // 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等於1250，當類的裝載時間大於1.25ms，則須要預裝載。

    Policy.isPreloadable()定於以下：

    /**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以下定義：

    /**

    * 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"

    ));

    這幾個Binders和Thread是不會被預加載的。

    另外還有一些application須要裝載，要求知足條件proc.fromZygote()且不是屬於常駐內存的服務。SERVICES定義以下：

    /**

    * 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是在下載源碼的時候生成，WritePreloadedClassFile類並無被用到，但能夠經過這個類瞭解Android系統對預加載類的默認要求，參考修改preloaded-classes文件，減小開機初始化時要預加載的類，提升開機速度。

    最後來經過startSystemServer()啓動SystemServer進程。見以下代碼：

    /* 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包裝了Linux的fork。forkSystemServer()調用forkAndSpecialize()，最終穿過虛擬機調用platform\dalvik\vm\native\dalvik_system_Zygote.c中Dalvik_dalvik_system_Zygote_forkAndSpecialize()。由dalvik完成fork新的進程。

    main()最後會調用runSelectLoopMode()，進入while循環，由peers建立新的進程。

    l SystemService進程

    SystemService用於建立init.rc定義的服務以外的全部服務。在main()的最後有以下代碼：

    // 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空間實現的，用於啓動native空間的服務，其實如今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()服務初始化建立native層的各個服務：

    // 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();

    }

    最後經過以下代碼:

    LOGI("System server: starting Android services.\n");

    runtime->callStatic("com/android/server/SystemServer", "init2");

    回到SystemServer.java，調用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啓動一個線程，專門用來啓動java空間的全部服務。以下代碼所示啓動部分服務：

    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);

    而且把這些服務添加到ServiceManager中，以便管理和進程間通信。

    在該線程後半部分，ActivityManagerService會等待AppWidget、WallPaper、IMM等systemReady後調用自身的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()最後會執行以下代碼：

    mMainStack.resumeTopActivityLocked(null);

    因爲Activity管理棧爲空，所以啓動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()中建立一個帶Category爲CATEGORY_HOME的Intent，由此去啓動相應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);

    }

這樣，Android系統便啓動起來進入到待機界面。

 

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