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。

// 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。