应用启动流程
Android系统是基于Linux的，所以它的所有应用也是基于Linux的Init进程创建出来的，首先Init进程启动Zygote（受精卵）进程，然后再fork出其他进程（包括SystemServer），最后开启各种应用进程。也就是流程如下：
Init进程–>Zygote进程–>SystemServer进程–>应用进程
SystemServer进程中启动系统的各种服务（ActivityManagerService、PackageManagerService、WindowManagerService…）
Zygote进程启动流程（API 23）
Init进程启动Zygote进程时会首先来到ZygoteInit类的main方法：
public static void main(String argv[]) {
try {
RuntimeInit.enableDdms();
// Start profiling the zygote initialization.
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());
// Finish profiling the zygote initialization.
SamplingProfilerIntegration.writeZygoteSnapshot();
// Do an initial gc to clean up after startup
gcAndFinalize();
// Disable tracing so that forked processes do not inherit stale tracing tags from
// Zygote.
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;
}
}- 第一句enableDdms() 打开DDMS（Dalvik Debug Monitor Service虚拟机调试服务）
- SamplingProfilerIntegration.start() 开始分析Zygote
- 一个循环解析main方法的参数列表，判断是否开启系统服务，获取ABI列表，socket名称
- registerZygoteSocket(socketName) 注册socket（android间的进程通信是使用Binder,唯独Zygote和SystemService是使用socket）
- preload() 初始化
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");
}- preloadClasses()初始化相关的类
- preloadResources()初始化资源
- preloadOpenGL()初始化OpenGL
- preloadSharedLibraries()初始化系统的Lib
- preloadTextResources()初始化文字资源
- WebViewFactory.prepareWebViewInZygote()初始化webview
然后调用startSystemServer方法：
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;
}在调用Zygote.forkSystemServer方法后创建了SystemServer进程
SystemServer进程启动流程
在Zygote进程fork出了SystemServer进程后，来到SystemServer的main方法：
/**
* The main entry point from zygote.
*/
public static void main(String[] args) {
new SystemServer().run();
}就只有一句（还有注意下官方的注释），调用了run方法（删了一些无关代码）：
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", "");
}
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);
}
VMRuntime.getRuntime().clearGrowthLimit();
VMRuntime.getRuntime().setTargetHeapUtilization(0.8f);
Build.ensureFingerprintProperty();
Environment.setUserRequired(true);
BinderInternal.disableBackgroundScheduling(true);
android.os.Process.setThreadPriority(
android.os.Process.THREAD_PRIORITY_FOREGROUND);
android.os.Process.setCanSelfBackground(false);
Looper.prepareMainLooper();
System.loadLibrary("android_servers");
performPendingShutdown();
createSystemContext();
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;
}
if (StrictMode.conditionallyEnableDebugLogging()) {
Slog.i(TAG, "Enabled StrictMode for system server main thread.");
}
Looper.loop();
throw new RuntimeException("Main thread loop unexpectedly exited");
}- 首先判断系统的当前时间，如果小于1970.就设置为1970
- 使用SystemProperties设置系统的语言、虚拟机库、内存。。。
- 初始化主Looper（返回的Looper为单例）
- 调用createSystemContext()创建上下文（activityThread.getSystemContext()返回的Context为单例），并且设置了系统主题
private void createSystemContext() {
ActivityThread activityThread = ActivityThread.systemMain();
mSystemContext = activityThread.getSystemContext();
mSystemContext.setTheme(android.R.style.Theme_DeviceDefault_Light_DarkActionBar);
}接下来这两句：
mSystemServiceManager = new SystemServiceManager(mSystemContext);
LocalServices.addService(SystemServiceManager.class, mSystemServiceManager);创建了SystemServiceManager并且加到LocalServices(内部是个Map)来保存，来到最后：
startBootstrapServices();
startCoreServices();
startOtherServices();private void startBootstrapServices() {
// Wait for installd to finish starting up so that it has a chance to
// create critical directories such as /data/user with the appropriate
// permissions. We need this to complete before we initialize other services.
Installer installer = mSystemServiceManager.startService(Installer.class);
// Activity manager runs the show.
mActivityManagerService = mSystemServiceManager.startService(
ActivityManagerService.Lifecycle.class).getService();
mActivityManagerService.setSystemServiceManager(mSystemServiceManager);
mActivityManagerService.setInstaller(installer);
// Power manager needs to be started early because other services need it.
// Native daemons may be watching for it to be registered so it must be ready
// to handle incoming binder calls immediately (including being able to verify
// the permissions for those calls).
mPowerManagerService = mSystemServiceManager.startService(PowerManagerService.class);
// Now that the power manager has been started, let the activity manager
// initialize power management features.
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();
}首先调用mSystemServiceManager.startService(Installer.class)，这个服务是系统的安装应用服务，需要最先启动，来看看SystemServiceManager的startService方法：
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;
}通过反射创建实例，然后放到mServices数组中
private final ArrayList<SystemService> mServices = new ArrayList<SystemService>();最后调用service的onStart方法，刚才我们传入的是Installer，来到Installer的onStart方法：
private final InstallerConnection mInstaller;
...
public void onStart() {
Slog.i(TAG, "Waiting for installd to be ready.");
mInstaller.waitForConnection();
}来到InstallerConnection的waitForConnection方法:
public void waitForConnection() {
for (;;) {
if (execute("ping") >= 0) {
return;
}
Slog.w(TAG, "installd not ready");
SystemClock.sleep(1000);
}
}一个无限循环通过ping命令连接Zygote进程，连接成功后才开始启动服务
继续看其他服务的启动：
mActivityManagerService = mSystemServiceManager.startService(
ActivityManagerService.Lifecycle.class).getService();
mActivityManagerService.setSystemServiceManager(mSystemServiceManager);
mActivityManagerService.setInstaller(installer);接下里启动了ActivityManagerService，这个服务用来管理Android的四大组件(Activity,Service,Broadcast,ContentProvider)，来到ActivityManagerService.Lifecycle类：
public static final class Lifecycle extends SystemService {
private final ActivityManagerService mService;
public Lifecycle(Context context) {
super(context);
mService = new ActivityManagerService(context);
}
@Override
public void onStart() {
mService.start();
}
public ActivityManagerService getService() {
return mService;
}
}它的onStart调用了ActivityManagerService的start方法：
private void start() {
Process.removeAllProcessGroups();
mProcessCpuThread.start();
mBatteryStatsService.publish(mContext);
mAppOpsService.publish(mContext);
Slog.d("AppOps", "AppOpsService published");
LocalServices.addService(ActivityManagerInternal.class, new LocalService());
}这里面开启了操作CPU相关的子线程和电池状态相关的服务，应用操作相关的AppOpsService，最后添加到SystemService的存储数组中，具体的代码太复杂，就不赘述了
再继续看startBootstrapServices方法：
mPowerManagerService = mSystemServiceManager.startService(PowerManagerService.class);启动了PowerManagerService，电源管理服务，这个的作用就不多说了吧，来看看PowerManagerService的onStart方法:
public void onStart() {
publishBinderService(Context.POWER_SERVICE, new BinderService());
publishLocalService(PowerManagerInternal.class, new LocalService());
Watchdog.getInstance().addMonitor(this);
Watchdog.getInstance().addThread(mHandler);
}这几个方法就不详细看了，继续回到startBootstrapServices：
mActivityManagerService.initPowerManagement();还记得刚才在ActivityManagerService启动了电池状态相关服务么？现在有了电源管理服务，在ActivityManagerService进行一下初始化：
public void initPowerManagement() {
mStackSupervisor.initPowerManagement();
mBatteryStatsService.initPowerManagement();
mLocalPowerManager = LocalServices.getService(PowerManagerInternal.class);
PowerManager pm = (PowerManager)mContext.getSystemService(Context.POWER_SERVICE);
mVoiceWakeLock = pm.newWakeLock(PowerManager.PARTIAL_WAKE_LOCK, "*voice*");
mVoiceWakeLock.setReferenceCounted(false);
}继续：
mSystemServiceManager.startService(LightsService.class);开启LightsService，闪关灯LED相关的服务，看看onStart方法：
@Override
public void onStart() {
publishLocalService(LightsManager.class, mService);
}然后开启显示相关的服务DisplayManagerService：
mDisplayManagerService = mSystemServiceManager.startService(DisplayManagerService.class);也是来到DisplayManagerService的onStart方法，没什么特别之处：
@Override
public void onStart() {
mHandler.sendEmptyMessage(MSG_REGISTER_DEFAULT_DISPLAY_ADAPTER);
publishBinderService(Context.DISPLAY_SERVICE, new BinderService(),
true /*allowIsolated*/);
publishLocalService(DisplayManagerInternal.class, new LocalService());
}再回到startBootstrapServices，启动PackageManagerService：
mSystemServiceManager.startBootPhase(SystemService.PHASE_WAIT_FOR_DEFAULT_DISPLAY);//设置默认的显示界面
mPackageManagerService = PackageManagerService.main(mSystemContext, installer,
mFactoryTestMode != FactoryTest.FACTORY_TEST_OFF, mOnlyCore);
mFirstBoot = mPackageManagerService.isFirstBoot();
mPackageManager = mSystemContext.getPackageManager();这个的启动方法有所不同，直接调用了PackageManagerService.main：
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;
}通过构造方法new出了PackageManagerService
最后在startBootstrapServices方法中启动UserManagerService和SensorService，用户管理服务和传感器服务：
ServiceManager.addService(Context.USER_SERVICE, UserManagerService.getInstance());
......
startSensorService();//JNI方法到了这里startBootstrapServices方法就执行完了，在SystemServer的run方法中接下来调用startCoreServices()：
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);
}在这里启动了一些其他核心服务，流程跟刚才的启动流程一样，都是调用相关服务的onStart方法来启动，就不细看了，注意这里的BatteryService不同于刚才在ActivityManagerService的BatteryStatsService
最后调用startOtherServices() 启动一些其他服务：振动（VibratorService），网络管理（NetworkManagementService），网络状态（NetworkStatsService），窗口管理（WindowManagerService）。。。。。。
这个方法代码太长，就不贴过来了
总结
- Zygote进程是所有进程的父进程，它fork出了SystemServer进程
- SystemServer进程的main方法中调用run方法进行初始化
- 在run方法中创建了SystemServiceManager，并且依次调用startBootstrapServices();startCoreServices();startOtherServices(); 开启系统服务（三类服务：BootstrapServices–>CoreServices–>OtherServices）
- 在启动服务时调用服务类的onStart方法来初始化
附上系统服务启动顺序：
Installer–>ActivityManagerService–>PowerManagerService–>
ActivityManagerService–>DisplayManagerService–>PackageManagerService–>
UserManagerService–>SensorService–>BatteryService–>
UsageStatsService–>WebViewUpdateService–>OtherServices
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