android开机启动流程说明

fmms 13年前
     <p>第一步:启动<span style="font-family:Times New Roman;">linux</span></p>    <p>1.Bootloader</p>    <p>2.Kernel</p>    <p>第二步 <span style="font-family:Times New Roman;">android</span><span style="font-family:宋体;">系统启动:入口为</span><span style="font-family:Times New Roman;">init.rc(system\core\rootdir)</span></p>    <p>1./system/bin/service manager: Binder <span style="font-family:宋体;">守护进程;</span></p>    <p>2.Runtime;</p>    <p>3.Zygote :app-process/app-main;</p>    <p>4.Start VM;</p>    <p>5.Start server</p>    <p>6.Start android service:Register to service Manager</p>    <p>7.Start Launcher</p>    <p>第三步:应用程序启动:运行<span style="font-family:Times New Roman;">package Manager</span></p>    <p><span style="font-family:Times New Roman;"><br /> </span></p>    <p><strong>l  Init进程</strong></p>    <p>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:</p>    <p>open_devnull_stdio();<br />     log_init();<br />     <br />     INFO("reading config file\n");<br />     init_parse_config_file("/init.rc");<br /> <span style="color:#33cc00;"><br /> </span><span style="color:#33cc00;">    /* pull the kernel commandline and ramdisk properties file in */</span><br />     import_kernel_cmdline(0);<br /> <br />     get_hardware_name(hardware, &revision);<br />     snprintf(tmp, sizeof(tmp), "/init.%s.rc", hardware);<br />     init_parse_config_file(tmp);</p>    <p>    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:</p>    <p>service servicemanager /system/bin/servicemanager<br />     user system<br />     critical<br />     onrestart restart zygote<br />     onrestart restart media<br /> <br /> service vold /system/bin/vold<br />     socket vold stream 0660 root mount<br />     ioprio be 2<br /> <br /> service netd /system/bin/netd<br />     socket netd stream 0660 root system<br />     socket dnsproxyd stream 0660 root inet<br /> <br /> service debuggerd /system/bin/debuggerd<br /> <br /> service ril-daemon /system/bin/rild<br />     socket rild stream 660 root radio<br />     socket rild-debug stream 660 radio system<br />     user root<br />     group radio cache inet misc audio sdcard_rw<br /> <br /> service zygote /system/bin/app_process -Xzygote /system/bin --zygote --start-system-server<br />     socket zygote stream 666<br />     onrestart write /sys/android_power/request_state wake<br />     onrestart write /sys/power/state on<br />     onrestart restart media<br />     onrestart restart netd<br /> <br /> service drm /system/bin/drmserver<br />     user drm<br />     group system root inet</p>    <p>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。</p>    <p> </p>    <p> </p>    <p>接下来在main函数中执行动作和启动进程服务:</p>    <p>      execute_one_command();<br />         restart_processes()</p>    <p>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。</p>    <p> </p>    <p><strong>l  ServiceManager进程</strong></p>    <p>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:</p>    <p>int main(int argc, char **argv)<br /> {<br />     struct binder_state *bs;<br />     void *svcmgr = BINDER_SERVICE_MANAGER;<br /> <br />     bs = binder_open(128*1024);<br /> <br />     if (binder_become_context_manager(bs)) {<br />         LOGE("cannot become context manager (%s)\n", strerror(errno));<br />         return -1;<br />     }<br /> <br />     svcmgr_handle = svcmgr;<br />     binder_loop(bs, svcmgr_handler);<br />     return 0;<br /> }</p>    <p>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。</p>    <p>int binder_become_context_manager(struct binder_state *bs)<br /> {<br />     return ioctl(bs->fd, BINDER_SET_CONTEXT_MGR, 0);<br /> }</p>    <p>    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。</p>    <p> </p>    <p><strong>l  Zygote进程</strong></p>    <p>         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:</p>    <p>       if (0 == strcmp("--zygote", arg)) {<br />             bool startSystemServer = (i < argc) ? <br />                     strcmp(argv[i], "--start-system-server") == 0 : false;<br />             setArgv0(argv0, "zygote");<br />             set_process_name("zygote");<br />             runtime.start("com.android.internal.os.ZygoteInit",<br />                 startSystemServer);<br />         } else {<br />             set_process_name(argv0);<br /> <br />             runtime.mClassName = arg;<br /> <br /> <span style="color:#33cc00;">      </span><span style="color:#33cc00;">      // Remainder of args get passed to startup class main()</span><br />             runtime.mArgC = argc-i;<br />             runtime.mArgV = argv+i;<br /> <br />             LOGV("App process is starting with pid=%d, class=%s.\n",<br />                  getpid(), runtime.getClassName());<br />             runtime.start();<br />         }</p>    <p>    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:</p>    <p>       registerZygoteSocket();<br />             EventLog.writeEvent(LOG_BOOT_PROGRESS_PRELOAD_START,<br />                 SystemClock.uptimeMillis());<br />             preloadClasses();<br />           <span style="color:#33cc00;">  //cacheRegisterMaps();</span><br />         preloadResources();<br />             EventLog.writeEvent(LOG_BOOT_PROGRESS_PRELOAD_END,<br />                 SystemClock.uptimeMillis());<br /> <span style="color:#009900;"><br /> </span><span style="color:#009900;">   <span style="color:#33cc00;">         // Finish profiling the zygote initialization.</span></span><br />           SamplingProfilerIntegration.writeZygoteSnapshot();<br /> <span style="color:#33cc00;"><br />        </span><span style="color:#33cc00;">     // Do an initial gc to clean up after startup</span><br />           gc();<br /> <br /> <span style="color:#33ff33;">         <span style="color:#33cc00;">   // If requested, start system server directly from Zygote</span></span><br />             if (argv.length != 2) {<br />                 throw new RuntimeException(argv[0] + USAGE_STRING);<br />             }<br /> <br />             if (argv[1].equals("true")) {<br />                 startSystemServer();<br />             } else if (!argv[1].equals("false")) {<br />                 throw new RuntimeException(argv[0] + USAGE_STRING);<br />             }</p>    <p>    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:</p>    <p> <span style="color:#33cc00;">      // Preload classes that were loaded by at least 2 processes. Hopefully,<br />         // the memory associated with these classes will be shared.</span><br />         for (LoadedClass loadedClass : root.loadedClasses.values()) {<br />             Set<String> names = loadedClass.processNames();<br />             if (!Policy.isPreloadable(loadedClass)) {<br />                 continue;<br />             }<br /> <br />             if (names.size() >= MIN_PROCESSES ||<br />                     (loadedClass.medianTimeMicros() > MIN_LOAD_TIME_MICROS && names.size() > 1)) {<br />                 toPreload.add(loadedClass);<br />             }<br />         }<br /> <br />         int initialSize = toPreload.size();<br />         System.out.println(initialSize<br />                 + " classses were loaded by more than one app.");<br /> <br />         // Preload eligable classes from applications (not long-running<br />         // services).<br />         for (Proc proc : root.processes.values()) {<br />             if (proc.fromZygote() && !Policy.isService(proc.name)) {<br />                 for (Operation operation : proc.operations) {<br />                     LoadedClass loadedClass = operation.loadedClass;<br />                     if (shouldPreload(loadedClass)) {<br />                         toPreload.add(loadedClass);<br />                     }<br />                 }<br />             }<br />         }</p>    <p>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。</p>    <p>Policy.isPreloadable()定于如下:</p>    <p><span style="color:#33cc00;">   /**Reports if the given class should be preloaded. */</span><br />     public static boolean isPreloadable(LoadedClass clazz) {<br />         return clazz.systemClass && !EXCLUDED_CLASSES.contains(clazz.name);<br />     }</p>    <p>其中EXCLUDED_CLASSES如下定义:</p>    <p>  <span style="color:#33cc00;"> /**<br />      * Classes which we shouldn't load from the Zygote.<br />      */</span><br />     private static final Set<String> EXCLUDED_CLASSES<br />             = new HashSet<String>(Arrays.asList(<br />    <span style="color:#33cc00;">     // Binders</span><br />         "android.app.AlarmManager",<br />         "android.app.SearchManager",<br />         "android.os.FileObserver",<br />         "com.android.server.PackageManagerService$AppDirObserver",<br /> <br /> <span style="color:#33cc00;">        // Threads</span><br />         "android.os.AsyncTask",<br />         "android.pim.ContactsAsyncHelper",<br />         "java.lang.ProcessManager"<br />     ));</p>    <p>这几个Binders和Thread是不会被预加载的。</p>    <p>    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:</p>    <p> <span style="color:#33cc00;">  /**<br />      * Long running services. These are restricted in their contribution to the <br />      * preloader because their launch time is less critical.<br />      */<br />     // TODO: Generate this automatically from package manager.</span><br />     private static final Set<String> SERVICES = new HashSet<String>(Arrays.asList(<br />         "system_server",<br />         "com.google.process.content",<br />         "android.process.media",<br />         "com.android.bluetooth",<br />         "com.android.calendar",<br />         "com.android.inputmethod.latin",<br />         "com.android.phone",<br />         "com.google.android.apps.maps.FriendService", // pre froyo<br />         "com.google.android.apps.maps:FriendService", // froyo<br />         "com.google.android.apps.maps.LocationFriendService",<br />         "com.google.android.deskclock",<br />         "com.google.process.gapps",<br />         "android.tts"<br />     ));</p>    <p>         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。</p>    <p>最后来通过startSystemServer()启动SystemServer进程。见如下代码:</p>    <p><span style="color:#33cc00;">       /* Hardcoded command line to start the system server */</span><br />         String args[] = {<br />             "--setuid=1000",<br />             "--setgid=1000",<br />             "--setgroups=1001,1002,1003,1004,1005,1006,1007,1008,1009,1010,1018,3001,3002,3003",<br />             "--capabilities=130104352,130104352",<br />             "--runtime-init",<br />             "--nice-name=system_server",<br />             "com.android.server.SystemServer",<br />         };<br />         ZygoteConnection.Arguments parsedArgs = null;<br /> <br />         int pid;<br /> <br />         try {<br />             parsedArgs = new ZygoteConnection.Arguments(args);<br /> <br />       <span style="color:#33cc00;">      /*<br />              * Enable debugging of the system process if *either* the command line flags<br />              * indicate it should be debuggable or the ro.debuggable system property<br />              * is set to "1"<br />              */</span><br />             int debugFlags = parsedArgs.debugFlags;<br />             if ("1".equals(SystemProperties.get("ro.debuggable")))<br />                 debugFlags |= Zygote.DEBUG_ENABLE_DEBUGGER;<br /> <br />             /* Request to fork the system server process */<br />             pid = Zygote.forkSystemServer(<br />                     parsedArgs.uid, parsedArgs.gid,<br />                     parsedArgs.gids, debugFlags, null,<br />                     parsedArgs.permittedCapabilities,<br />                     parsedArgs.effectiveCapabilities)</p>    <p>         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。</p>    <p>       main()最后会调用runSelectLoopMode(),进入while循环,由peers创建新的进程。</p>    <p> </p>    <p><strong>l  SystemService进程</strong></p>    <p>       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:</p>    <p>       // The system server has to run all of the time, so it needs to be<br />         // as efficient as possible with its memory usage.<br />         VMRuntime.getRuntime().setTargetHeapUtilization(0.8f);<br />         <br />         System.loadLibrary("android_servers");<br />         init1(args);</p>    <p>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():</p>    <p>static void android_server_SystemServer_init1(JNIEnv* env, jobject clazz)<br /> {<br />     system_init();<br /> }</p>    <p>    而system_init()服务初始化创建native层的各个服务:</p>    <p><span style="color:#33cc00;">   // Start the sensor service</span><br />   SensorService::instantiate();<br /> <span style="color:#33cc00;"><br />     // On the simulator, audioflinger et al don't get started the<br />     // same way as on the device, and we need to start them here</span><br />     if (!proc->supportsProcesses()) {<br /> <span style="color:#33cc00;"><br />         // Start the AudioFlinger</span><br />       AudioFlinger::instantiate();<br /> <span style="color:#33cc00;"><br />         // Start the media playback service</span><br />       MediaPlayerService::instantiate();<br /> <br />    <span style="color:#33cc00;">     // Start the camera service</span><br />       CameraService::instantiate();<br /> <br /> <span style="color:#33cc00;">        // Start the audio policy service</span><br />        AudioPolicyService::instantiate();<br />     }</p>    <p>    最后通过如下代码:</p>    <p>   LOGI("System server: starting Android services.\n");<br />     runtime->callStatic("com/android/server/SystemServer", "init2");</p>    <p>回到SystemServer.java,调用init2():</p>    <p>   public static final void init2() {<br />         Slog.i(TAG, "Entered the Android system server!");<br />         Thread thr = new ServerThread();<br />         thr.setName("android.server.ServerThread");<br />         thr.start();<br />     }</p>    <p>         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:</p>    <p>           Slog.i(TAG, "Content Manager");<br />             ContentService.main(context,<br />                     factoryTest == SystemServer.FACTORY_TEST_LOW_LEVEL);<br /> <br />             Slog.i(TAG, "System Content Providers");<br />             ActivityManagerService.installSystemProviders();<br /> <br />             Slog.i(TAG, "Battery Service");<br />             battery = new BatteryService(context);<br />             ServiceManager.addService("battery", battery);<br /> <br />             Slog.i(TAG, "Lights Service");<br />             lights = new LightsService(context);<br /> <br />             Slog.i(TAG, "Vibrator Service");<br />             ServiceManager.addService("vibrator", new VibratorService(context));<br /> <br />             // only initialize the power service after we have started the<br />             // lights service, content providers and the battery service.<br />           power.init(context, lights, ActivityManagerService.getDefault(), battery);<br /> <br />             Slog.i(TAG, "Alarm Manager");<br />             AlarmManagerService alarm = new AlarmManagerService(context);<br />             ServiceManager.addService(Context.ALARM_SERVICE, alarm);</p>    <p>并且把这些服务添加到ServiceManager中,以便管理和进程间通讯。</p>    <p>在该线程后半部分,ActivityManagerService会等待AppWidget、WallPaper、IMM等systemReady后调用自身的systemReady()。</p>    <p>           ((ActivityManagerService)ServiceManager.getService("activity"))<br />                     .setWindowManager(wm);<br /> <br />       <span style="color:#33cc00;">      // Skip Bluetooth if we have an emulator kernel<br />             // TODO: Use a more reliable check to see if this product should<br />             // support Bluetooth - see bug 988521</span><br />             if (SystemProperties.get("ro.kernel.qemu").equals("1")) {<br />                 Slog.i(TAG, "Registering null Bluetooth Service (emulator)");<br />                 ServiceManager.addService(BluetoothAdapter.BLUETOOTH_SERVICE, null);<br />             } else if (factoryTest == SystemServer.FACTORY_TEST_LOW_LEVEL) {<br />                 Slog.i(TAG, "Registering null Bluetooth Service (factory test)");<br />                 ServiceManager.addService(BluetoothAdapter.BLUETOOTH_SERVICE, null);<br />             } else {<br />                 Slog.i(TAG, "Bluetooth Service");<br />                 bluetooth = new BluetoothService(context);<br />                 ServiceManager.addService(BluetoothAdapter.BLUETOOTH_SERVICE, bluetooth);<br />                 bluetooth.initAfterRegistration();<br />                 bluetoothA2dp = new BluetoothA2dpService(context, bluetooth);<br />                 ServiceManager.addService(BluetoothA2dpService.BLUETOOTH_A2DP_SERVICE,<br />                                           bluetoothA2dp);<br /> <br />                 int bluetoothOn = Settings.Secure.getInt(mContentResolver,<br />                     Settings.Secure.BLUETOOTH_ON, 0);<br />                 if (bluetoothOn > 0) {<br />                     bluetooth.enable();<br />                 }<br />             }</p>    <p>    而在ActivityManagerService的systemReady()最后会执行如下代码:</p>    <p>           mMainStack.resumeTopActivityLocked(null);</p>    <p>    由于Activity管理栈为空,因此启动Launcher。</p>    <p>    <span style="color:#33cc00;">   // Find the first activity that is not finishing.</span><br />         ActivityRecord next = topRunningActivityLocked(null);<br /> <span style="color:#33cc00;"><br />         // Remember how we'll process this pause/resume situation, and ensure<br />         // that the state is reset however we wind up proceeding.</span><br />         final boolean userLeaving = mUserLeaving;<br />         mUserLeaving = false;<br /> <br />         if (next == null) {<br />     <span style="color:#33cc00;">        // There are no more activities!  Let's just start up the<br />             // Launcher...</span><br />             if (mMainStack) {<br />                 return mService.startHomeActivityLocked();<br />             }<br />         }</p>    <p>    在startHomeActivityLocked()中创建一个带Category为CATEGORY_HOME的Intent,由此去启动相应Activity,即Launcher。</p>    <p>       Intent intent = new Intent(<br />             mTopAction,<br />             mTopData != null ? Uri.parse(mTopData) : null);<br />         intent.setComponent(mTopComponent);<br />         if (mFactoryTest != SystemServer.FACTORY_TEST_LOW_LEVEL) {<br />             intent.addCategory(Intent.CATEGORY_HOME);<br />         }</p>    <p>    这样,Android系统便启动起来进入到待机界面。</p>