Android开机启动流程初探

Wince.Android

Android开机启动流程初探 [复制链接]

Android开机启动流程初探

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：

[url=" title=][/url]
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);[url=" title=][/url]

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：

[url=" title=][/url]
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[url=" title=][/url]

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：

[url=" title=][/url]
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;
}[url=" title=][/url]

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：

[url=" title=][/url]
      if (0 == strcmp("--zygote", arg)) {
         bool startSystemServer = (i < argc) ?
                  strcmp(argv, "--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();
      }[url=" title=][/url]

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：
[url=" title=][/url]
         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);
         }[url=" title=][/url]

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：
[url=" title=][/url]
      // 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);
                  }
            }
         }
      }[url=" title=][/url]

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：
[url=" title=][/url]
/**
   * 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"
));[url=" title=][/url]

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：
[url=" title=][/url]
/**
   * 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"
));[url=" title=][/url]

      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：
[url=" title=][/url]
      /* 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)[url=" title=][/url]

      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：

[url=" title=][/url]
// 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();
}[url=" title=][/url]

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：
[url=" title=][/url]
         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);[url=" title=][/url]

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()。

[url=" title=][/url]
         ((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();
            }
         }[url=" title=][/url]

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。
[url=" title=][/url]
      // 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();
         }
      }[url=" title=][/url]

在startHomeActivityLocked()中创建一个带Category为CATEGORY_HOME的Intent，由此去启动相应Activity，即Launcher。
[url=" title=][/url]
      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);
      }[url=" title=][/url]

这样，Android系统便启动起来进入到待机界面。

upc_arm

还没学android mark

zhaojun_xf

不错，学习了。

gooogleman

原帖由 upc_arm 于 2013-7-11 13:22 发表
还没学android mark

呵呵，我也是初学者，以前搞wince的，现在过来不简单。现在也算是入门级别。高级的还不会玩。

Android开机启动流程初探 [复制链接]

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