l? Init進程

Android系統在啟動時首先會啟動Linux系統，引導加載Linux Kernel并啟動init進程。Init進程是一個由內核啟動的用戶級進程，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，有如下代碼：

??? 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);

??? 這里會加載解析init.rc和init.hardware.rc兩個初始化腳本。*.rc文件定義了在init進程中需要啟動哪些進程服務和執行哪些動作。其詳細說明參見platform\system\core\init\reademe.txt。init.rc見如下定義：

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，動作添加到action_list中。

接下來在main函數中執行動作和啟動進程服務：

??????? execute_one_command();
??????? restart_processes()

通常init過程需要創建一些系統文件夾并啟動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進程是所有服務的管理器。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啟動。在main函數中有如下代碼：

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()把當前進程設置為ServiceManager。ServiceManager本身就是一個服務。

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

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

l? Zygote進程

???????? Zygote進程用于產生其他進程。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啟動。在main函數中有如下代碼：

??????? 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();
??????? }

??? 首先創建AppRuntime，即AndroidRuntime，建立了一個Dalvik虛擬機。通過這個runtime傳遞com.android.internal.os.ZygoteInit參數，從而由Dalvik虛擬機運行ZygoteInit.java的main()，開始創建Zygote進程。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);
??????????? }

??? 首先通過registerZygoteSocket()登記端口，接著preloadClasses()裝載相關類。這里大概要裝載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，有如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<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，當類的裝載時間大于1.25ms，則需要預裝載。

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如下定義：

??? /**
???? * 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"
??? ));

這幾個Binders和Thread是不會被預加載的。

??? 另外還有一些application需要裝載，要求滿足條件proc.fromZygote()且不是屬于常駐內存的服務。SERVICES定義如下：

??? /**
???? * 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是在下載源碼的時候生成，WritePreloadedClassFile類并沒有被用到，但可以通過這個類了解Android系統對預加載類的默認要求，參考修改preloaded-classes文件，減少開機初始化時要預加載的類，提高開機速度。

最后來通過startSystemServer()啟動SystemServer進程。見如下代碼：

??????? /* 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()最后會調用runSelectLoopMode()，進入while循環，由peers創建新的進程。

l? SystemService進程

?????? SystemService用于創建init.rc定義的服務之外的所有服務。在main()的最后有如下代碼：

??????? // 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，用于啟動native空間的服務，其實現在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()服務初始化創建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();
??? }

??? 最后通過如下代碼:

??? 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啟動一個線程，專門用來啟動java空間的所有服務。如下代碼所示啟動部分服務：

??????????? 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);

并且把這些服務添加到ServiceManager中，以便管理和進程間通訊。

在該線程后半部分，ActivityManagerService會等待AppWidget、WallPaper、IMM等systemReady后調用自身的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()最后會執行如下代碼：

??????????? mMainStack.resumeTopActivityLocked(null);

??? 由于Activity管理棧為空，因此啟動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()中創建一個帶Category為CATEGORY_HOME的Intent，由此去啟動相應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系統便啟動起來進入到待機界面。

?

?

轉載于:https://www.cnblogs.com/yuzaipiaofei/archive/2011/08/01/4124446.html

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