古之立大事者，不惟有超世之才，亦必有坚忍不拔之志。这篇文章主要讲述android代码启动流程2相关的知识，希望能为你提供帮助。
上回  说到，开始调用 ZygoteInit   main 函数，main 函数：

• registerZygoteServer：注册一个 zygote server socket，所有来自客户端的连接都通过 socket 方式连接；
• preload：预加载系统的类库和资源，这样其他程序启动将不再加载系统资源，只需加载自己程序的资源，这样就达到系统资源在程序之间共享；
• startSystemServer:

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private static boolean startSystemServer() throws MethodAndArgsCaller, RuntimeException { /* Hardcoded command line to start the system server */ //命令行参数，包括：uid，gid，group，process_name，process class String args[] = { " --setuid=1000" , " --setgid=1000" , " --setgroups=1001,1002,1003,1004,1005,1006,1007,1008,1009,1010,1018,3001,3002,3003,3006,3007" , " --capabilities=130104352,130104352" , " --runtime-init" , " --nice-name=system_server" , " com.android.server.SystemServer" , }; ZygoteConnection.Arguments parsedArgs = null; int pid; try { //解析命令行参数 parsedArgs = new ZygoteConnection.Arguments(args); ZygoteConnection.applyDebuggerSystemProperty(parsedArgs); ZygoteConnection.applyInvokeWithSystemProperty(parsedArgs); /* Request to fork the system server process */ //从 zygote 进程派生一个新的进程，fork 可参考：http://linux.die.net/man/2/fork ，不同的是该进程结束时，也会让 zygote 进程结束 //所以这里，会返回2次，一次返回的是 zygote 进程的 pid ，值 大于0；一次返回的是子进程 pid，值 等于0 // fork 返回在 zygote 进程返回的子进程 pid，非0，在子进程中返回0 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 */ //zygote 进程 pid 非0，直接返回，而子进程 pid = 0，对子进程进行设置 if (pid == 0) { handleSystemServerProcess(parsedArgs); }return true; }

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而 handleSystemServerProcess 将启动  com.android.server.SystemServer：

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private static void handleSystemServerProcess( ZygoteConnection.Arguments parsedArgs) throws ZygoteInit.MethodAndArgsCaller {//因为有 zygote 监听 socket，所以 system server 不监听 socket 连接，此处关闭 closeServerSocket(); // set umask to 0077 so new files and directories will default to owner-only permissions. FileUtils.setUMask(FileUtils.S_IRWXG | FileUtils.S_IRWXO); //设置进程名字，即从命令行参数获取的：system_server　 if (parsedArgs.niceName != null) { Process.setArgV0(parsedArgs.niceName); }if (parsedArgs.invokeWith != null) { WrapperInit.execApplication(parsedArgs.invokeWith, parsedArgs.niceName, parsedArgs.targetSdkVersion, null, parsedArgs.remainingArgs); } else { /* * Pass the remaining arguments to SystemServer. */ /* zygoteInit -> applicationInit：设置 sdktarget 版本 -> invokeStaticMain：得到 com.android.server.SystemServer main 方法 -> ZygoteInit.MethodAndArgsCaller * ZygoteInit.MethodAndArgsCaller 方法抛出异常 MethodAndArgsCaller，跳过了在 startSystemServer 下面的代码: * if (ZYGOTE_FORK_MODE) { *runForkMode(); *} else { *runSelectLoopMode(); *} */

RuntimeInit.zygoteInit(parsedArgs.targetSdkVersion, parsedArgs.remainingArgs); } /* should never reach here */

}

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  在对  MethodAndArgsCaller 异常 catch 语句里，直接调用了  com.android.server.SystemServer main 方法，而 zygote 进程因为 pid 不为0，执行 runSelectLoopMode 方法：

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private static void runSelectLoopMode() throws MethodAndArgsCaller { ArrayList< FileDescriptor> fds = new ArrayList(); ArrayList< ZygoteConnection> peers = new ArrayList(); FileDescriptor[] fdArray = new FileDescriptor[4]; fds.add(sServerSocket.getFileDescriptor()); peers.add(null); int loopCount = GC_LOOP_COUNT; //一直循环 while (true) { int index; /* * Call gc() before we block in select(). * It‘s work that has to be done anyway, and it‘s better * to avoid making every child do it.It will also * madvise() any free memory as a side-effect. * * Don‘t call it every time, because walking the entire * heap is a lot of overhead to free a few hundred bytes. */ if (loopCount < = 0) { gc(); loopCount = GC_LOOP_COUNT; } else { loopCount--; }//采用非阻塞方式，等待并取出 zygote 连接 try { fdArray = fds.toArray(fdArray); index = selectReadable(fdArray); } catch (IOException ex) { throw new RuntimeException(" Error in select()" , ex); }//selectReadable 返回值 小于0 ，有错误发生；值 等于0，有新的连接，加到 list 中；值 大于0，处理当前连接 if (index < 0) { throw new RuntimeException(" Error in select()" ); } else if (index == 0) { ZygoteConnection newPeer = acceptCommandPeer(); peers.add(newPeer); fds.add(newPeer.getFileDesciptor()); } else { boolean done; done = peers.get(index).runOnce(); if (done) { peers.remove(index); fds.remove(index); } } } }

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  在 zygote 进程等待连接的同时，com.android.server.SystemServer 已经启动：

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native public static void init1(String[] args); public static void main(String[] args) { ... //加载 jni ，init1 是本地方法 System.loadLibrary(" android_servers" ); // init1 -> frameworks/base/services/jni/com_android_server_SystemServer.cpp :: android_server_SystemServer_init1 -> //frameworks/base/cmds/system_server/library/system_init.cpp :: system_init init1(args); }// init1 将回调 init2 方法 public static final void init2() { Slog.i(TAG, " Entered the Android system server!" ); Thread thr = new ServerThread(); thr.setName(" android.server.ServerThread" ); thr.start(); }

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init1 方法最终调用的是 system_init 方法（代码：frameworks/base/cmds/system_server/library/system_init.cpp）

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extern " C" status_t system_init() { LOGI(" Entered system_init()" ); sp< ProcessState> proc(ProcessState::self()); sp< IServiceManager> sm = defaultServiceManager(); LOGI(" ServiceManager: %p\n" , sm.get()); sp< GrimReaper> grim = new GrimReaper(); sm-> asBinder()-> linkToDeath(grim, grim.get(), 0); //初始化 SurfaceFlinger 和传感器 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. LOGI(" System server: starting Android runtime.\n" ); AndroidRuntime* runtime = AndroidRuntime::getRuntime(); //回调 com.android.server.SystemServer init2 方法 LOGI(" 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; } jmethodID methodId = env-> GetStaticMethodID(clazz, " init2" , " ()V" ); if (methodId == NULL) { return UNKNOWN_ERROR; } env-> CallStaticVoidMethod(clazz, methodId); //启动线程池，为 binder 服务 LOGI(" System server: entering thread pool.\n" ); ProcessState::self()-> startThreadPool(); IPCThreadState::self()-> joinThreadPool(); LOGI(" System server: exiting thread pool.\n" ); return NO_ERROR; }

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  init2 启动 ServerThread 线程，它会启动 android 系统所有的服务：

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public void run() { EventLog.writeEvent(EventLogTags.BOOT_PROGRESS_SYSTEM_RUN, SystemClock.uptimeMillis()); Looper.prepare(); android.os.Process.setThreadPriority( android.os.Process.THREAD_PRIORITY_FOREGROUND); BinderInternal.disableBackgroundScheduling(true); android.os.Process.setCanSelfBackground(false); String factoryTestStr = SystemProperties.get(" ro.factorytest" ); int factoryTest = " " .equals(factoryTestStr) ? SystemServer.FACTORY_TEST_OFF : Integer.parseInt(factoryTestStr); //初始化服务，如：网络服务，Wifi服务，蓝牙，电源，等等，初始化完成以后，加到 ServiceManager 中， //所以我们用 Context.getSystemService (String name) 才获取到相应的服务 LightsService lights = null; PowerManagerService power = null; BatteryService battery = null; AlarmManagerService alarm = null; NetworkManagementService networkManagement = null; NetworkStatsService networkStats = null; NetworkPolicyManagerService networkPolicy = null; ConnectivityService connectivity = null; WifiP2pService wifiP2p = null; WifiService wifi = null; IPackageManager pm = null; Context context = null; WindowManagerService wm = null; BluetoothService bluetooth = null; BluetoothA2dpService bluetoothA2dp = null; DockObserver dock = null; UsbService usb = null; UiModeManagerService uiMode = null; RecognitionManagerService recognition = null; ThrottleService throttle = null; NetworkTimeUpdateService networkTimeUpdater = null; // Critical services... try { Slog.i(TAG, " Entropy Service" ); ServiceManager.addService(" entropy" , new EntropyService()); Slog.i(TAG, " Package Manager" ); // Only run " core" apps if we‘re encrypting the device.......//ActivityManagerService 是 android 系统最核心的服务之一　

//1.系统 context 的初始化，设置默认主题  android.R.style.Theme_Holo
//2.设置进程名字为  system_process
//3.初始化  ActivityStack
context = ActivityManagerService.main(factoryTest);
【android代码启动流程2】 
//往 service manager 里面添加一些服务，如：activity，meminfo，cupinfo，permission
ActivityManagerService.setSystemProcess();

 
//安装系统 content provider
Slog.i(TAG, " System Content Providers" );
ActivityManagerService.installSystemProviders();
 
//设置 windows manager
ActivityManagerService.self().setWindowManager(wm);            

......// We now tell the activity manager it is okay to run third party // code.It will call back into us once it has gotten to the state // where third party code can really run (but before it has actually // started launching the initial applications), for us to complete our // initialization.//代码到这里，表明系统已经就绪，可以运行第3方代码 ActivityManagerService.self().systemReady(new Runnable() { public void run() { Slog.i(TAG, " Making services ready" ); // systemui 是 3.0 以后添加的，因为没有物理键，提供虚拟键　 startSystemUi(contextF); //诸多服务开始启动 try { if (batteryF != null) batteryF.systemReady(); } catch (Throwable e) { reportWtf(" making Battery Service ready" , e); } try { if (networkManagementF != null) networkManagementF.systemReady(); } catch (Throwable e) { reportWtf(" making Network Managment Service ready" , e); } ...... } }); // For debug builds, log event loop stalls to dropbox for analysis. if (StrictMode.conditionallyEnableDebugLogging()) { Slog.i(TAG, " Enabled StrictMode for system server main thread." ); }Looper.loop(); Slog.d(TAG, " System ServerThread is exiting!" ); }

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  而要执行  ActivityManagerService.self().systemReady(new Runnable() ...) 参数里面 Runnable 的 run 方法，还必须等到  ActivityManagerService systemReady：

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public void systemReady(final Runnable goingCallback) { synchronized(this) { //mSystemReady = false if (mSystemReady) { if (goingCallback != null) goingCallback.run(); return; }// Check to see if there are any update receivers to run. if (!mDidUpdate) { if (mWaitingUpdate) { return; } //检测是否有 ACTION_PRE_BOOT_COMPLETED register，该广播在 ACTION_BOOT_COMPLETED 前发出 Intent intent = new Intent(Intent.ACTION_PRE_BOOT_COMPLETED); List< ResolveInfo> ris = null; try { ris = AppGlobals.getPackageManager().queryIntentReceivers( intent, null, 0); } catch (RemoteException e) { } if (ris != null) { for (int i=ris.size()-1; i> =0; i--) { //检测广播注册是否是系统程序 if ((ris.get(i).activityInfo.applicationInfo.flags & ApplicationInfo.FLAG_SYSTEM) == 0) { ris.remove(i); } } intent.addFlags(Intent.FLAG_RECEIVER_BOOT_UPGRADE); ArrayList< ComponentName> lastDoneReceivers = readLastDonePreBootReceivers(); final ArrayList< ComponentName> doneReceivers = new ArrayList< ComponentName> (); for (int i=0; i< ris.size(); i+ + ) { ActivityInfo ai = ris.get(i).activityInfo; ComponentName comp = new ComponentName(ai.packageName, ai.name); if (lastDoneReceivers.contains(comp)) { ris.remove(i); i--; } }for (int i=0; i< ris.size(); i+ + ) { ActivityInfo ai = ris.get(i).activityInfo; ComponentName comp = new ComponentName(ai.packageName, ai.name); doneReceivers.add(comp); intent.setComponent(comp); IIntentReceiver finisher = null; if (i == ris.size()-1) { finisher = new IIntentReceiver.Stub() { public void performReceive(Intent intent, int resultCode, String data, Bundle extras, boolean ordered, boolean sticky) { // The raw IIntentReceiver interface is called // with the AM lock held, so redispatch to // execute our code without the lock. mHandler.post(new Runnable() { public void run() { synchronized (ActivityManagerService.this) { mDidUpdate = true; } writeLastDonePreBootReceivers(doneReceivers); showBootMessage(mContext.getText( R.string.android_upgrading_complete), false); //如果有 ACTION_PRE_BOOT_COMPLETED，在处理完广播 receive 以后 ，还会再次走 systemRead(goingCallback) systemReady(goingCallback); } }); } }; } Slog.i(TAG, " Sending system update to: " + intent.getComponent()); broadcastIntentLocked(null, null, intent, null, finisher, 0, null, null, null, true, false, MY_PID, Process.SYSTEM_UID); if (finisher != null) { mWaitingUpdate = true; } } } if (mWaitingUpdate) { return; } mDidUpdate = true; }mSystemReady = true; //mStartRunning 已经在 ActivityManagerService.main(int factoryTest) 设置成 true if (!mStartRunning) { return; } }......retrieveSettings(); //开始执行 runnable 的 run 方法，执行完成以后，系统就绪 if (goingCallback != null) goingCallback.run(); synchronized (this) { if (mFactoryTest != SystemServer.FACTORY_TEST_LOW_LEVEL) { try { List apps = AppGlobals.getPackageManager(). getPersistentApplications(STOCK_PM_FLAGS); if (apps != null) { int N = apps.size(); int i; for (i=0; i< N; i+ + ) { ApplicationInfo info = (ApplicationInfo)apps.get(i); if (info != null & & !info.packageName.equals(" android" )) { addAppLocked(info); } } } } catch (RemoteException ex) { // pm is in same process, this will never happen. } }// Start up initial activity. mBooting = true; try { if (AppGlobals.getPackageManager().hasSystemUidErrors()) { Message msg = Message.obtain(); msg.what = SHOW_UID_ERROR_MSG; mHandler.sendMessage(msg); } } catch (RemoteException e) { }//恢复 top activity，因为现在没有任何启动的 activity， 将会启动 startHomeActivityLocked，启动 HOME mMainStack.resumeTopActivityLocked(null); } }

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  HOME 启动以后，ActivityManagerService 中 finishBooting 方法会发出 Intent.ACTION_BOOT_COMPLETED 广播，调用该方法的地方有很多，resume activity 的时候或者出错的时候，
  调用一次以后就不再调用。
 
  至此 android 就完成了整个启动工作，整个流程可以用下图简洁表示：

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