App启动流程
init进程在启动Zygote进程时一般都会调用ZygoteInit类的main方法，因此这里看一下该方法的具体实现(基于android23源码)；
调用enableDdms()，设置DDMS可用，可以发现DDMS启动的时机还是比较早的，在整个Zygote进程刚刚开始要启动额时候就设置可用。之后初始化各种参数通过调用registerZygoteSocket方法，注册为Zygote进程注册Socket然后调用preload方法实现预加载各种资源然后通过调用startSystemServer开启SystemServer服务，这个是重点
public static void main(String argv[]) {try { //设置ddms可以用 RuntimeInit.enableDdms(); SamplingProfilerIntegration.start(); boolean startSystemServer = false; String socketName = "zygote"; String abiList = null; for (int i = 1; i < argv.length; i++) { if ("start-system-server".equals(argv[i])) { startSystemServer = true; } else if (argv[i].startsWith(ABI_LIST_ARG)) { abiList = argv[i].substring(ABI_LIST_ARG.length()); } else if (argv[i].startsWith(SOCKET_NAME_ARG)) { socketName = argv[i].substring(SOCKET_NAME_ARG.length()); } else { throw new RuntimeException("Unknown command line argument: " + argv[i]); } } if (abiList == null) { throw new RuntimeException("No ABI list supplied."); } registerZygoteSocket(socketName); EventLog.writeEvent(LOG_BOOT_PROGRESS_PRELOAD_START, SystemClock.uptimeMillis()); preload(); EventLog.writeEvent(LOG_BOOT_PROGRESS_PRELOAD_END, SystemClock.uptimeMillis()); SamplingProfilerIntegration.writeZygoteSnapshot(); gcAndFinalize(); Trace.setTracingEnabled(false); if (startSystemServer) { startSystemServer(abiList, socketName); } Log.i(TAG, "Accepting command socket connections"); runSelectLoop(abiList); closeServerSocket();} catch (MethodAndArgsCaller caller) { caller.run();} catch (RuntimeException ex) { Log.e(TAG, "Zygote died with exception", ex); closeServerSocket(); throw ex;}}2.3 registerZygoteSocket(socketName)分析
调用registerZygoteSocket（String socketName）为Zygote进程注册socket
private static void registerZygoteSocket(String socketName) { if (sServerSocket == null) { int fileDesc; final String fullSocketName = ANDROID_SOCKET_PREFIX + socketName; try { String env = System.getenv(fullSocketName); fileDesc = Integer.parseInt(env); } catch (RuntimeException ex) { throw new RuntimeException(fullSocketName + " unset or invalid", ex); } try { FileDescriptor fd = new FileDescriptor(); fd.setInt$(fileDesc); sServerSocket = new LocalServerSocket(fd); } catch (IOException ex) { throw new RuntimeException( "Error binding to local socket '" + fileDesc + "'", ex); } }}2.4 preLoad()方法分析源码如下所示
static void preload() { Log.d(TAG, "begin preload"); preloadClasses(); preloadResources(); preloadOpenGL(); preloadSharedLibraries(); preloadTextResources(); // Ask the WebViewFactory to do any initialization that must run in the zygote process, // for memory sharing purposes. WebViewFactory.prepareWebViewInZygote(); Log.d(TAG, "end preload");}大概操作是这样的：preloadClasses()用于初始化Zygote中需要的class类；preloadResources()用于初始化系统资源；preloadOpenGL()用于初始化OpenGL；preloadSharedLibraries()用于初始化系统libraries；preloadTextResources()用于初始化文字资源；prepareWebViewInZygote()用于初始化webview;2.5 startSystemServer()启动进程
这段逻辑的执行逻辑就是通过Zygote fork出SystemServer进程
private static boolean startSystemServer(String abiList, String socketName) throws MethodAndArgsCaller, RuntimeException { long capabilities = posixCapabilitiesAsBits( OsConstants.CAP_BLOCK_SUSPEND, OsConstants.CAP_KILL, OsConstants.CAP_NET_ADMIN, OsConstants.CAP_NET_BIND_SERVICE, OsConstants.CAP_NET_BROADCAST, OsConstants.CAP_NET_RAW, OsConstants.CAP_SYS_MODULE, OsConstants.CAP_SYS_NICE, OsConstants.CAP_SYS_RESOURCE, OsConstants.CAP_SYS_TIME, OsConstants.CAP_SYS_TTY_CONFIG ); /* Hardcoded command line to start the system server */ String args[] = { "--setuid=1000", "--setgid=1000", "--setgroups=1001,1002,1003,1004,1005,1006,1007,1008,1009,1010,1018,1021,1032,3001,3002,3003,3006,3007", "--capabilities=" + capabilities + "," + capabilities, "--nice-name=system_server", "--runtime-args", "com.android.server.SystemServer", }; ZygoteConnection.Arguments parsedArgs = null; int pid; try { parsedArgs = new ZygoteConnection.Arguments(args); ZygoteConnection.applyDebuggerSystemProperty(parsedArgs); ZygoteConnection.applyInvokeWithSystemProperty(parsedArgs); /* Request to fork the system server process */ 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) { if (hasSecondZygote(abiList)) { waitForSecondaryZygote(socketName); } handleSystemServerProcess(parsedArgs); } return true;}3.SystemServer进程启动流程3.1 SystemServer进程简介SystemServer进程主要的作用是在这个进程中启动各种系统服务，比如ActivityManagerService，PackageManagerService，WindowManagerService服务，以及各种系统性的服务其实都是在SystemServer进程中启动的，而当我们的应用需要使用各种系统服务的时候其实也是通过与SystemServer进程通讯获取各种服务对象的句柄的。3.2 SystemServer的main方法如下所示，比较简单，只是new出一个SystemServer对象并执行其run方法，查看SystemServer类的定义我们知道其实final类型的，所以我们一般不能重写或者继承。3.3 查看run方法
代码如下所示
首先判断系统当前时间，若当前时间小于1970年1月1日，则一些初始化操作可能会处所，所以当系统的当前时间小于1970年1月1日的时候，设置系统当前时间为该时间点。然后是设置系统的语言环境等接着设置虚拟机运行内存，加载运行库，设置SystemServer的异步消息
private void run() {if (System.currentTimeMillis() < EARLIEST_SUPPORTED_TIME) { Slog.w(TAG, "System clock is before 1970; setting to 1970."); SystemClock.setCurrentTimeMillis(EARLIEST_SUPPORTED_TIME);}if (!SystemProperties.get("persist.sys.language").isEmpty()) { final String languageTag = Locale.getDefault().toLanguageTag(); SystemProperties.set("persist.sys.locale", languageTag); SystemProperties.set("persist.sys.language", ""); SystemProperties.set("persist.sys.country", ""); SystemProperties.set("persist.sys.localevar", "");}Slog.i(TAG, "Entered the Android system server!");EventLog.writeEvent(EventLogTags.BOOT_PROGRESS_SYSTEM_RUN, SystemClock.uptimeMillis());SystemProperties.set("persist.sys.dalvik.vm.lib.2", VMRuntime.getRuntime().vmLibrary());if (SamplingProfilerIntegration.isEnabled()) { SamplingProfilerIntegration.start(); mProfilerSnapshotTimer = new Timer(); mProfilerSnapshotTimer.schedule(new TimerTask() { @Override public void run() { SamplingProfilerIntegration.writeSnapshot("system_server", null); } }, SNAPSHOT_INTERVAL, SNAPSHOT_INTERVAL);}// Mmmmmm... more memory!VMRuntime.getRuntime().clearGrowthLimit();// 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);// Some devices rely on runtime fingerprint generation, so make sure// we've defined it before booting further.Build.ensureFingerprintProperty();// Within the system server, it is an error to access Environment paths without// explicitly specifying a user.Environment.setUserRequired(true);// Ensure binder calls into the system always run at foreground priority.BinderInternal.disableBackgroundScheduling(true);// Prepare the main looper thread (this thread).android.os.Process.setThreadPriority( android.os.Process.THREAD_PRIORITY_FOREGROUND);android.os.Process.setCanSelfBackground(false);Looper.prepareMainLooper();// Initialize native services.System.loadLibrary("android_servers");// Check whether we failed to shut down last time we tried.// This call may not return.performPendingShutdown();// Initialize the system context.createSystemContext();// Create the system service manager.mSystemServiceManager = new SystemServiceManager(mSystemContext);LocalServices.addService(SystemServiceManager.class, mSystemServiceManager);// Start services.try { startBootstrapServices(); startCoreServices(); startOtherServices();} catch (Throwable ex) { Slog.e("System", "******************************************"); Slog.e("System", "************ Failure starting system services", ex); throw ex;}// For debug builds, log event loop stalls to dropbox for analysis.if (StrictMode.conditionallyEnableDebugLogging()) { Slog.i(TAG, "Enabled StrictMode for system server main thread.");}// Loop forever.Looper.loop();throw new RuntimeException("Main thread loop unexpectedly exited");}
然后下面的代码是：
// Initialize the system context.createSystemContext();// Create the system service manager.mSystemServiceManager = new SystemServiceManager(mSystemContext);LocalServices.addService(SystemServiceManager.class, mSystemServiceManager);// Start services.try { startBootstrapServices(); startCoreServices(); startOtherServices();} catch (Throwable ex) { Slog.e("System", "******************************************"); Slog.e("System", "************ Failure starting system services", ex); throw ex;}3.4 run方法中createSystemContext()解析调用createSystemContext()方法： 可以看到在SystemServer进程中也存在着Context对象，并且是通过ActivityThread.systemMain方法创建context的，这一部分的逻辑以后会通过介绍Activity的启动流程来介绍，这里就不在扩展，只知道在SystemServer进程中也需要创建Context对象。
private void createSystemContext() {ActivityThread activityThread = ActivityThread.systemMain();mSystemContext = activityThread.getSystemContext();mSystemContext.setTheme(android.R.style.Theme_DeviceDefault_Light_DarkActionBar);}3.5 mSystemServiceManager的创建看run方法中，通过SystemServiceManager的构造方法创建了一个新的SystemServiceManager对象，我们知道SystemServer进程主要是用来构建系统各种service服务的，而SystemServiceManager就是这些服务的管理对象。然后调用：将SystemServiceManager对象保存SystemServer进程中的一个数据结构中。
LocalServices.addService(SystemServiceManager.class, mSystemServiceManager);最后开始执行：
// Start services.try { startBootstrapServices(); startCoreServices(); startOtherServices();} catch (Throwable ex) { Slog.e("System", "******************************************"); Slog.e("System", "************ Failure starting system services", ex); throw ex;}里面主要涉及了是三个方法：startBootstrapServices() 主要用于启动系统Boot级服务startCoreServices() 主要用于启动系统核心的服务startOtherServices() 主要用于启动一些非紧要或者是非需要及时启动的服务4.启动服务4.1 启动哪些服务在开始执行启动服务之前总是会先尝试通过socket方式连接Zygote进程，在成功连接之后才会开始启动其他服务。 4.2 启动服务流程源码分析
首先看一下startBootstrapServices方法：
private void startBootstrapServices() { Installer installer = mSystemServiceManager.startService(Installer.class); mActivityManagerService = mSystemServiceManager.startService( ActivityManagerService.Lifecycle.class).getService(); mActivityManagerService.setSystemServiceManager(mSystemServiceManager); mActivityManagerService.setInstaller(installer); mPowerManagerService = mSystemServiceManager.startService(PowerManagerService.class); mActivityManagerService.initPowerManagement(); // Manages LEDs and display backlight so we need it to bring up the display. mSystemServiceManager.startService(LightsService.class); // Display manager is needed to provide display metrics before package manager // starts up. mDisplayManagerService = mSystemServiceManager.startService(DisplayManagerService.class); // We need the default display before we can initialize the package manager. mSystemServiceManager.startBootPhase(SystemService.PHASE_WAIT_FOR_DEFAULT_DISPLAY); // Only run "core" apps if we're encrypting the device. String cryptState = SystemProperties.get("vold.decrypt"); if (ENCRYPTING_STATE.equals(cryptState)) { Slog.w(TAG, "Detected encryption in progress - only parsing core apps"); mOnlyCore = true; } else if (ENCRYPTED_STATE.equals(cryptState)) { Slog.w(TAG, "Device encrypted - only parsing core apps"); mOnlyCore = true; } // Start the package manager. Slog.i(TAG, "Package Manager"); mPackageManagerService = PackageManagerService.main(mSystemContext, installer, mFactoryTestMode != FactoryTest.FACTORY_TEST_OFF, mOnlyCore); mFirstBoot = mPackageManagerService.isFirstBoot(); mPackageManager = mSystemContext.getPackageManager(); Slog.i(TAG, "User Service"); ServiceManager.addService(Context.USER_SERVICE, UserManagerService.getInstance()); // Initialize attribute cache used to cache resources from packages. AttributeCache.init(mSystemContext); // Set up the Application instance for the system process and get started. mActivityManagerService.setSystemProcess(); // The sensor service needs access to package manager service, app ops // service, and permissions service, therefore we start it after them. startSensorService();}先执行：
Installer installer = mSystemServiceManager.startService(Installer.class);
mSystemServiceManager是系统服务管理对象，在main方法中已经创建完成，这里我们看一下其startService方法的具体实现：
可以看到通过反射器构造方法创建出服务类，然后添加到SystemServiceManager的服务列表数据中，最后调用了service.onStart()方法，因为传递的是Installer.class
public <T extends SystemService> T startService(Class<T> serviceClass) {final String name = serviceClass.getName();Slog.i(TAG, "Starting " + name);// Create the service.if (!SystemService.class.isAssignableFrom(serviceClass)) { throw new RuntimeException("Failed to create " + name + ": service must extend " + SystemService.class.getName());}final T service;try { Constructor<T> constructor = serviceClass.getConstructor(Context.class); service = constructor.newInstance(mContext);} catch (InstantiationException ex) { throw new RuntimeException("Failed to create service " + name + ": service could not be instantiated", ex);} catch (IllegalAccessException ex) { throw new RuntimeException("Failed to create service " + name + ": service must have a public constructor with a Context argument", ex);} catch (NoSuchMethodException ex) { throw new RuntimeException("Failed to create service " + name + ": service must have a public constructor with a Context argument", ex);} catch (InvocationTargetException ex) { throw new RuntimeException("Failed to create service " + name + ": service constructor threw an exception", ex);}// Register it.mServices.add(service);// Start it.try { service.onStart();} catch (RuntimeException ex) { throw new RuntimeException("Failed to start service " + name + ": onStart threw an exception", ex);}return service;}看一下Installer的onStart方法：很简单就是执行了mInstaller的waitForConnection方法，这里简单介绍一下Installer类，该类是系统安装apk时的一个服务类，继承SystemService（系统服务的一个抽象接口），需要在启动完成Installer服务之后才能启动其他的系统服务。
@Overridepublic void onStart() {Slog.i(TAG, "Waiting for installd to be ready.");mInstaller.waitForConnection();}然后查看waitForConnection（）方法：通过追踪代码可以发现，其在不断的通过ping命令连接Zygote进程（SystemServer和Zygote进程通过socket方式通讯，其他进程通过Binder方式通讯）
public void waitForConnection() {for (;;) { if (execute("ping") >= 0) { return; } Slog.w(TAG, "installd not ready"); SystemClock.sleep(1000);}}继续看startBootstrapServices方法：这段代码主要是用于启动ActivityManagerService服务，并为其设置SysServiceManager和Installer。ActivityManagerService是系统中一个非常重要的服务，Activity，service，Broadcast，contentProvider都需要通过其余系统交互。
// Activity manager runs the show.mActivityManagerService = mSystemServiceManager.startService( ActivityManagerService.Lifecycle.class).getService();mActivityManagerService.setSystemServiceManager(mSystemServiceManager);mActivityManagerService.setInstaller(installer);
首先看一下Lifecycle类的定义：
可以看到其实ActivityManagerService的一个静态内部类，在其构造方法中会创建一个ActivityManagerService，通过刚刚对Installer服务的分析我们知道，SystemServiceManager的startService方法会调用服务的onStart()方法，而在Lifecycle类的定义中我们看到其onStart（）方法直接调用了mService.start()方法，mService是Lifecycle类中对ActivityManagerService的引用
public static final class Lifecycle extends SystemService {private final ActivityManagerService mService;public Lifecycle(Context context) { super(context); mService = new ActivityManagerService(context);}@Overridepublic void onStart() { mService.start();}public ActivityManagerService getService() { return mService;}}4.3 启动部分服务启动PowerManagerService服务：启动方式跟上面的ActivityManagerService服务相似都会调用其构造方法和onStart方法，PowerManagerService主要用于计算系统中和Power相关的计算，然后决策系统应该如何反应。同时协调Power如何与系统其它模块的交互，比如没有用户活动时，屏幕变暗等等。
mPowerManagerService = mSystemServiceManager.startService(PowerManagerService.class);然后是启动LightsService服务主要是手机中关于闪光灯，LED等相关的服务；也是会调用LightsService的构造方法和onStart方法；
mSystemServiceManager.startService(LightsService.class);然后是启动DisplayManagerService服务主要是手机显示方面的服务
mDisplayManagerService = mSystemServiceManager.startService(DisplayManagerService.class);然后是启动PackageManagerService，该服务也是android系统中一个比较重要的服务包括多apk文件的安装，解析，删除，卸载等等操作。可以看到PackageManagerService服务的启动方式与其他服务的启动方式有一些区别，直接调用了PackageManagerService的静态main方法
Slog.i(TAG, "Package Manager");mPackageManagerService = PackageManagerService.main(mSystemContext, installer, mFactoryTestMode != FactoryTest.FACTORY_TEST_OFF, mOnlyCore);mFirstBoot = mPackageManagerService.isFirstBoot();mPackageManager = mSystemContext.getPackageManager();看一下其main方法的具体实现：可以看到也是直接使用new的方式创建了一个PackageManagerService对象，并在其构造方法中初始化相关变量，最后调用了ServiceManager.addService方法，主要是通过Binder机制与JNI层交互
public static PackageManagerService main(Context context, Installer installer,boolean factoryTest, boolean onlyCore) {PackageManagerService m = new PackageManagerService(context, installer, factoryTest, onlyCore);ServiceManager.addService("package", m);return m;}
然后查看startCoreServices方法：
可以看到这里启动了BatteryService（电池相关服务），UsageStatsService，WebViewUpdateService服务等。
private void startCoreServices() {// Tracks the battery level. Requires LightService.mSystemServiceManager.startService(BatteryService.class);// Tracks application usage stats.mSystemServiceManager.startService(UsageStatsService.class);mActivityManagerService.setUsageStatsManager( LocalServices.getService(UsageStatsManagerInternal.class));// Update after UsageStatsService is available, needed before performBootDexOpt.mPackageManagerService.getUsageStatsIfNoPackageUsageInfo();// Tracks whether the updatable WebView is in a ready state and watches for update installs.mSystemServiceManager.startService(WebViewUpdateService.class);}总结：SystemServer进程是android中一个很重要的进程由Zygote进程启动；SystemServer进程主要用于启动系统中的服务；SystemServer进程启动服务的启动函数为main函数；SystemServer在执行过程中首先会初始化一些系统变量，加载类库，创建Context对象，创建SystemServiceManager对象等之后才开始启动系统服务；SystemServer进程将系统服务分为三类：boot服务，core服务和other服务，并逐步启动SertemServer进程在尝试启动服务之前会首先尝试与Zygote建立socket通讯，只有通讯成功之后才会开始尝试启动服务；创建的系统服务过程中主要通过SystemServiceManager对象来管理，通过调用服务对象的构造方法和onStart方法初始化服务的相关变量；服务对象都有自己的异步消息对象，并运行在单独的线程中；参考博客https://www.jianshu.com/p/064136533445http://blog.csdn.net/qq_23547831/article/details/51104873http://www.xuebuyuan.com/2178651.htmlhttps://www.jianshu.com/p/e69d22ec0582http://blog.csdn.net/luoshengyang/article/details/6768304http://blog.csdn.net/ericming200409/article/details/45566153关于其他内容介绍01.关于博客汇总链接1.技术博客汇总2.开源项目汇总3.生活博客汇总4.喜马拉雅音频汇总5.其他汇总02.关于我的博客我的个人站点：www.yczbj.org，www.ycbjie.cngithub：https://github.com/yangchong211知乎：https://www.zhihu.com/people/yang-chong-69-24/pins/posts简书：http://www.jianshu.com/u/b7b2c6ed9284csdn：http://my.csdn.net/m0_37700275喜马拉雅听书：http://www.ximalaya.com/zhubo/71989305/开源中国：https://my.oschina.net/zbj1618/blog泡在网上的日子：http://www.jcodecraeer.com/member/content_list.php?channelid=1邮箱：yangchong211@163.com阿里云博客：https://yq.aliyun.com/users/article?spm=5176.100- 239.headeruserinfo.3.dT4bcVsegmentfault头条：https://segmentfault.com/u/xiangjianyu/articles
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