本文原创作者:Cloud Chou. 欢迎转载，请注明出处和本文链接
前言
一直想研究Android完整的启动过程，网上看了不少资料，也看了书上的一些说明，对这些观点有些怀疑，于是自己分析了系统启动的完整过程。从内核启动第一个用户程序init开始说起，直到Home应用的启动，每一步都有源代码展示。希望能解除读者对Android系统启动过程中的困惑，若有什么疑问，欢迎留言交流。本研究基于CM10.1源码，读者若能对照源代码查看效果会更好。
1) init启动servicemanager和 zygote两个service
Android底层是Linux内核，和linux类似，内核初始化后启动的第一个用户进程是init，它会解析init.rc脚本，启动init.rc里声明的service，并执行一些action。在init.rc里有启动Andriod空间的一些关键服务，代码如下：
123456789101112131415161718
#…service servicemanager /system/bin/servicemanager class core user system group system critical onrestart restart zygote onrestart restart media onrestart restart surfaceflinger onrestart restart drmservice zygote /system/bin/app_process -Xzygote /system/bin --zygote --start-system-server class main socket zygote stream 660 root system onrestart write /sys/android_power/request_state wake onrestart write /sys/power/state on onrestart restart media onrestart restart netd#…
servicemanager负责管理所有的binder service, 这些binder service有native的，也有java的。native的binder service有surfaceflinger，drm，media等，java的binder service就有我们平常熟悉的很多管理服务了，ActivityManagerService，WindowManagerService，BatteryService，PowerManagerService，InputManagerService等等。service manager并不负责这些binder service的创建，native的binder service大多由init启动init.rc里的service时创建并启动，java层的binder service大多由zygote创建并启动的，接下来会详细这些service是如何被启动的。
2) zygote service启动java层的ZygoteInit
zygote服务是java层所有程序进程的父进程，它是Android空间程序的孵化器，Android空间所有程序都是由zygote进程启动的。zygote service对应/system/bin/app_process程序，源代码位于frameworks/base/cmds/app_process/app_main.cpp，启动时的main函数代码如下：
123456789101112131415161718192021222324252627282930313233343536373839404142434445464748495051525354555657585960616263
int main(int argc, const char* const argv[]){//.../*runtime就是dalvik虚拟机实例，启动Java层应用时， *会fork 一个子进程，复制虚拟机，许多书上将runtime看作一个进程， *然后再启动zygote进程，个人觉得这是错误的 */ AppRuntime runtime;//... while (i < argc) { const char* arg = argv[i++]; if (!parentDir) { parentDir = arg; /*init.rc启动app_main会设置参数--zygote*/ } else if (strcmp(arg, "--zygote") == 0) { zygote = true; niceName = "zygote"; //进程的名字 /*init.rc启动app_main会设置参数--start-system-server， *表示需启动systemserver */ } else if (strcmp(arg, "--start-system-server") == 0) { startSystemServer = true; /*启动应用时会使用--application参数*/ } else if (strcmp(arg, "--application") == 0) { application = true;/*--nice-name=参数表示要设置的进程名字*/ } else if (strncmp(arg, "--nice-name=", 12) == 0) { niceName = arg + 12; } else { className = arg; break; }} /*设置进程名*/if (niceName && *niceName) { setArgv0(argv0, niceName); set_process_name(niceName);}/*设置虚拟机运行环境的父目录*/runtime.mParentDir = parentDir;if (zygote) {/*虚拟机里启动com.android.internal.os.ZygoteInit， *并传递参数start-system-server */ runtime.start("com.android.internal.os.ZygoteInit", startSystemServer ? "start-system-server" : "");} else if (className) { /*若不是zygote，则启动的第一个类是com.android.internal.os.RuntimeInit， *RumtimeInit初始化后会启动mClassName */ runtime.mClassName = className; runtime.mArgC = argc - i; runtime.mArgV = argv + i; runtime.start("com.android.internal.os.RuntimeInit", application ? "application" : "tool");} else { fprintf(stderr, "Error: no class name or --zygote supplied.\n"); app_usage(); LOG_ALWAYS_FATAL("app_process: no class name or --zygote supplied."); return 10;}//...}通过上述代码可知道zygote service将运行dalvik虚拟机，并在虚拟机里执行com.android.internal.os.ZygoteInit，还给它传递了参数start-system-server
3) ZygoteInit启动SystemServer
ZygoteInit启动时的相关源代码：
12345678910111213141516171819202122232425262728293031323334
public static void main(String argv[]) {{try { //... //在某个描述符上监听连接请求， //其它Android空间的程序的启动都是通过连接zygote才孵化出来的 registerZygoteSocket(); //... if (argv[1].equals("start-system-server")) { //启动SystemServer startSystemServer(); } else if (!argv[1].equals("")) { throw new RuntimeException(argv[0] + USAGE_STRING); } //... /*ZYGOTE_FORK_MODE默认为false，如果为true的话，每收到一个连接请求， *就会建立一个新进程，然后再运行连接请求所要求执行的命令，此时会建立另一个新进程 */ if (ZYGOTE_FORK_MODE) { runForkMode(); } else { //使用Select poll的方式来建立新进程，收到连接请求后，也会建立进程启动某个程序 runSelectLoopMode(); } closeServerSocket();} catch (MethodAndArgsCaller caller) { caller.run();} catch (RuntimeException ex) { Log.e(TAG, "Zygote died with exception", ex); closeServerSocket(); throw ex;}}从上述代码可知道会调用startSystemServer以启动SystemServer,相关源代码如下：
1234567891011121314151617181920212223242526272829303132333435363738394041424344
private static boolean startSystemServer(){/* Hardcoded command line to start the system server *///启动SystemServer使用的参数String args[] = { "--setuid=1000", "--setgid=1000", "--setgroups=1001,1002,1003,1004,1005,1006,1007,1008,1009,1010,1018,1021,3001,3002,3003,3004,3006,3007,3009", "--capabilities=130104352,130104352", "--runtime-init", "--nice-name=system_server", //注意：就是在这里设置要启动的SystemServer包名及类名，故此后续才能启动SystemServer "com.android.server.SystemServer",};ZygoteConnection.Arguments parsedArgs = null;int pid;try { /*将args参数传给ZygoteConnection进行转化，--形式的参数将全部被接收 * 但是要启动的类的类名com.android.server.SystemServer会放在 *ZygoteConnection.Arguments的remainingArgs里，后来调用handleSystemServerProcess时会用到 */ parsedArgs = new ZygoteConnection.Arguments(args); /*添加额外运行参数*/ ZygoteConnection.applyDebuggerSystemProperty(parsedArgs); ZygoteConnection.applyInvokeWithSystemProperty(parsedArgs); /*开启新进程*/ pid = Zygote.forkSystemServer( parsedArgs.uid, parsedArgs.gid, parsedArgs.gids, parsedArgs.debugFlags, null, parsedArgs.permittedCapabilities, parsedArgs.effectiveCapabilities);} catch (IllegalArgumentException ex) { throw new RuntimeException(ex);} /* For child process */if (pid == 0) { /*调用handleSystemServerProcess会执行ZygoteConnection.Arguments的remainingArgs参数 *所指定的类，即com.android.server.SystemServer */ handleSystemServerProcess(parsedArgs);}}ZygoteInit的startSystemServer会调用forkSystemServer，然后：
ZygoteInit.forkSystemServer -> Zygote.nativeForkSystemServer-> dalvik_system_Zygote.cpp 里的Dalvik_dalvik_system_Zygote_forkSystemServer-> forkAndSpecializeCommon->fork建立新进程
ZygoteInit的startSystemServer会调用handleSystemServerProcess来真正启动systemserver，相关源代码如下：
12345678910111213141516171819
private static void handleSystemServerProcess( ZygoteConnection.Arguments parsedArgs) throws ZygoteInit.MethodAndArgsCaller {//... if (parsedArgs.niceName != null) { Process.setArgV0(parsedArgs.niceName);}//启动systemserver时invokeWith为nullif (parsedArgs.invokeWith != null) { WrapperInit.execApplication(parsedArgs.invokeWith, parsedArgs.niceName, parsedArgs.targetSdkVersion, null, parsedArgs.remainingArgs);} else { /* * 启动systemserver时，parsedArgs.remainingArgs为com.android.server.SystemServer. */ RuntimeInit.zygoteInit(parsedArgs.targetSdkVersion, parsedArgs.remainingArgs);}}然后的流程是
RuntimeInit.zygoteInit-> applicationInit，applicationInit的代码如下所示：
12345678910111213141516
private static void applicationInit(int targetSdkVersion, String[] argv){//...final Arguments args;try { //参数转换,系统启动时，argv里有一个参数是com.android.server.SystemServer args = new Arguments(argv);} catch (IllegalArgumentException ex) { Slog.e(TAG, ex.getMessage()); // let the process exit return;}//...//终于在此启动了SystemServerinvokeStaticMain(args.startClass, args.startArgs)}4) SystemServer 启动过程
执行com.android.server.SystemServer时，main函数里会调用init1函数，init1函数是一个本地函数，init1的实现放在frameworks/base/services/jni/com_android_server_SystemServer.cpp里，对应的jni函数是android_server_SystemServer_init1，在该函数里会调用system_init，而system_init的实现是在frameworks/base/cmds/system_server/library/system_init.cpp，该函数的实现代码如下所示：
1234567891011121314151617181920212223242526272829303132333435363738394041424344454647
extern "C" status_t system_init(){//... sp<ProcessState> proc(ProcessState::self());sp<IServiceManager> sm = defaultServiceManager();ALOGI("ServiceManager: %p\n", sm.get());sp<GrimReaper> grim = new GrimReaper();sm->asBinder()->linkToDeath(grim, grim.get(), 0);char propBuf[PROPERTY_VALUE_MAX];property_get("system_init.startsurfaceflinger", propBuf, "1");if (strcmp(propBuf, "1") == 0) { // Start the SurfaceFlinger SurfaceFlinger::instantiate();}property_get("system_init.startsensorservice", propBuf, "1");if (strcmp(propBuf, "1") == 0) { // Start the sensor service SensorService::instantiate();}// And now start the Android runtime. We have to do this bit// of nastiness because the Android runtime initialization requires// some of the core system services to already be started.// All other servers should just start the Android runtime at// the beginning of their processes's main(), before calling// the init function.ALOGI("System server: starting Android runtime.\n");AndroidRuntime* runtime = AndroidRuntime::getRuntime();ALOGI("System server: starting Android services.\n");JNIEnv* env = runtime->getJNIEnv();if (env == NULL) { return UNKNOWN_ERROR;}jclass clazz = env->FindClass("com/android/server/SystemServer");if (clazz == NULL) { return UNKNOWN_ERROR;}//反过来调用Java里SystemServer的init2函数jmethodID methodId = env->GetStaticMethodID(clazz, "init2", "()V");if (methodId == NULL) { return UNKNOWN_ERROR;}env->CallStaticVoidMethod(clazz, methodId);ALOGI("System server: entering thread pool.\n");ProcessState::self()->startThreadPool();IPCThreadState::self()->joinThreadPool();ALOGI("System server: exiting thread pool.\n");}5) 启动Java层的各种binder service
调用SystemServer的init2函数后，会开启新线程android.server.ServerThread，在新线程里会启动各种Java层的binder service，并在service manager里注册，这些Service大多开启了新线程运行，故此都是systemserver的子线程，添加的Service列表如下所示：
123456789101112131415161718192021222324252627282930313233343536373839404142434445464748495051
ServiceManager.addService("entropy", new EntropyMixer());ServiceManager.addService(Context.POWER_SERVICE, power);ServiceManager.addService(Context.DISPLAY_SERVICE, display, true);ServiceManager.addService("telephony.registry", telephonyRegistry);ServiceManager.addService(Context.SCHEDULING_POLICY_SERVICE,ServiceManager.addService(Context.USER_SERVICE,UserManagerService.getInstance());ServiceManager.addService(Context.ACCOUNT_SERVICE, accountManager);ServiceManager.addService("battery", battery);ServiceManager.addService("vibrator", vibrator);ServiceManager.addService(Context.ALARM_SERVICE, alarm);ServiceManager.addService(Context.WINDOW_SERVICE, wm);ServiceManager.addService(Context.INPUT_SERVICE, inputManager);ServiceManager.addService(BluetoothAdapter.BLUETOOTH_MANAGER_SERVICE, bluetooth);ServiceManager.addService(Context.INPUT_METHOD_SERVICE, imm);ServiceManager.addService(Context.ACCESSIBILITY_SERVICE,new AccessibilityManagerService(context));ServiceManager.addService("mount", mountService);ServiceManager.addService("lock_settings", lockSettings);ServiceManager.addService(Context.DEVICE_POLICY_SERVICE, devicePolicy);ServiceManager.addService(Context.STATUS_BAR_SERVICE, statusBar);ServiceManager.addService(Context.CLIPBOARD_SERVICE,new ClipboardService(context));ServiceManager.addService(Context.NETWORKMANAGEMENT_SERVICE, networkManagement);ServiceManager.addService(Context.TEXT_SERVICES_MANAGER_SERVICE, tsms);ServiceManager.addService(Context.NETWORK_STATS_SERVICE, networkStats);ServiceManager.addService(Context.NETWORK_POLICY_SERVICE, networkPolicy);ServiceManager.addService(Context.WIFI_P2P_SERVICE, wifiP2p);ServiceManager.addService(Context.WIFI_SERVICE, wifi);ServiceManager.addService(Context.CONNECTIVITY_SERVICE, connectivity);ServiceManager.addService(Context.NSD_SERVICE, serviceDiscovery);ServiceManager.addService(Context.THROTTLE_SERVICE, throttle);ServiceManager.addService("fm_receiver",new FmReceiverService(context));ServiceManager.addService("fm_transmitter",new FmTransmitterService(context));ServiceManager.addService(Context.UPDATE_LOCK_SERVICE,new UpdateLockService(context));ServiceManager.addService(Context.PROFILE_SERVICE, profile);ServiceManager.addService(Context.NOTIFICATION_SERVICE, notification);ServiceManager.addService(DeviceStorageMonitorService.SERVICE,ServiceManager.addService(Context.LOCATION_SERVICE, location);ServiceManager.addService(Context.COUNTRY_DETECTOR, countryDetector);ServiceManager.addService(Context.SEARCH_SERVICE,new SearchManagerService(context));ServiceManager.addService(Context.DROPBOX_SERVICE,new DropBoxManagerService(context, new File("/data/system/dropbox")));ServiceManager.addService(Context.WALLPAPER_SERVICE, wallpaper);ServiceManager.addService(Context.AUDIO_SERVICE, new AudioService(context));ServiceManager.addService(Context.USB_SERVICE, usb);ServiceManager.addService(Context.SERIAL_SERVICE, serial);ServiceManager.addService(Context.BACKUP_SERVICE,new BackupManagerService(context));ServiceManager.addService(Context.APPWIDGET_SERVICE, appWidget);ServiceManager.addService("diskstats", new DiskStatsService(context));ServiceManager.addService("samplingprofiler", new SamplingProfilerService(context));ServiceManager.addService("commontime_management", commonTimeMgmtService);ServiceManager.addService(DreamService.DREAM_SERVICE, dreamy);ServiceManager.addService("assetredirection", new AssetRedirectionManagerService(context));ServiceManager.addService("pieservice", pieService);上述并没有看到将ActivityManagerService添加到servicemanager管理，它的添加过程比较特别。在线程android.server.ServerThread里会调用ActivityManagerService.setSystemProcess();setSystemProcess函数的代码如下所示：
1234567891011121314151617
public static void setSystemProcess() {//…ActivityManagerService m = mSelf; ServiceManager.addService("activity", m, true);ServiceManager.addService("meminfo", new MemBinder(m));ServiceManager.addService("gfxinfo", new GraphicsBinder(m));ServiceManager.addService("dbinfo", new DbBinder(m));if (MONITOR_CPU_USAGE) { ServiceManager.addService("cpuinfo", new CpuBinder(m));}ServiceManager.addService("permission", new PermissionController(m));ApplicationInfo info = mSelf.mContext.getPackageManager().getApplicationInfo( "android", STOCK_PM_FLAGS);mSystemThread.installSystemApplicationInfo(info);//…}可以看到ActivityManagerService采用了单例模式，并调用ServiceManager.addService(“activity”, m, true);将ActivityManagerService交给servicemanager管理，在ActivityManagerService里还添加了别的binder service，像MemBinder，GraphicsBinder，DbBinder。
最后会调用Looper.loop();进入loop循环，等待和别的程序通信。
6) 启动系统界面
线程android.server.ServerThread里有如下代码：
12345678
ActivityManagerService.self().systemReady(new Runnable() { public void run() { Slog.i(TAG, "Making services ready"); if (!headless) startSystemUi(contextF); //... }}startSystemUi就是用于启动系统界面的，代码如下：
1234567
static final void startSystemUi(Context context) { Intent intent = new Intent(); intent.setComponent(new ComponentName("com.android.systemui", "com.android.systemui.SystemUIService")); //Slog.d(TAG, "Starting service: " + intent); context.startServiceAsUser(intent, UserHandle.OWNER);}这样便启动了com.android.systemui应用，该应用将启动PowerUI和RingtonePlayer两个线程。
7) 启动Home 程序
线程android.server.ServerThread里有如下代码：
1234567
//…ActivityManagerService.self().systemReady(new Runnable() { public void run() { //… } });//…ActivityManagerService.self().systemReady有如下代码：
123
//…mMainStack.resumeTopActivityLocked(null);//…
ActivityStack. resumeTopActivityLocked()有如下代码：
1
resumeTopActivityLocked(prev, null);
resumeTopActivityLocked的实现有如下代码：
12345678910
//…if (next == null) { // There are no more activities! Let's just start up the // Launcher... if (mMainStack) { ActivityOptions.abort(options); return mService.startHomeActivityLocked(mCurrentUser); }}//…mService类型是ActivityManagerService，ActivityManagerService. startHomeActivityLocked的实现有如下代码：
1234567891011121314151617181920212223242526272829
boolean startHomeActivityLocked(int userId) {//…Intent intent = new Intent( mTopAction, mTopData != null ? Uri.parse(mTopData) : null);intent.setComponent(mTopComponent);//这里便添加了Intent.CATEGORY_HOME，//所有的Home应用都会都带有该类型的Activity，只有这样才会被认为是Home应用if (mFactoryTest != SystemServer.FACTORY_TEST_LOW_LEVEL) { intent.addCategory(Intent.CATEGORY_HOME);}ActivityInfo aInfo = resolveActivityInfo(intent, STOCK_PM_FLAGS, userId);if (aInfo != null) { intent.setComponent(new ComponentName( aInfo.applicationInfo.packageName, aInfo.name)); // Don't do this if the home app is currently being // instrumented. aInfo = new ActivityInfo(aInfo); aInfo.applicationInfo = getAppInfoForUser(aInfo.applicationInfo, userId); ProcessRecord app = getProcessRecordLocked(aInfo.processName, aInfo.applicationInfo.uid); if (app == null || app.instrumentationClass == null) { intent.setFlags(intent.getFlags() | Intent.FLAG_ACTIVITY_NEW_TASK); mMainStack.startActivityLocked(null, intent, null, aInfo, null, null, 0, 0, 0, 0, null, false, null); }}}这样先找到使用Intent.CATEGORY_HOME声明的Activity组件，然后再调用mMainStack.startActivityLocked启动该Activity。
system server启动Home程序总结：
android.server.ServerThread->ActivityManagerService.self().systemReady->mMainStack.resumeTopActivityLocked->resumeTopActivityLocked-> mService.startHomeActivityLocked-> intent.addCategory(Intent.CATEGORY_HOME);mMainStack.startActivityLocked
总结
内核初始化好后，运行的第一个用户程序是init，init将启动init.rc里声明的多个service，跟Android空间相关的有servicemanager和zygote，servicemanager负责管理所有的binder service，zygote负责孵化所有Android空间的程序。zygote service对应的程序是app_process，不过加了一些启动参数，所以它会启动Java层的ZygoteInit，在ZygoteInit里会启动SystemServer，SystemServer分为两个阶段：本地的init1和Java层的init2，init2里会启动线程android.server.ServerThread。在android.server.ServerThread线程里会启动Java层的各种binder service，比如ActivityManagerService，PackageManagerService，WindowManagerService。然后调用ActivityManagerService的systemReady方法，在该方法里会启动系统界面以及Home程序。
