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<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。

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       // 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。