· 这是《Android 系统源代码情景分析》 一书中第 11 章, Zygote 和 System 进程的启动过程,的读书摘要;
一、Zygote 进程的过程
Zygote 进程是由 Android 系统的第一个进程脚本 init 启动的;
Zygote 进程是通过复制自身的方式来创建 System 进程和应用程序的;
Android 系统中,所有的应用程序进程,以及用来运行系统关键服务的 System 进程都是 Zygote 进程负责创建的;
1、reigsterZyogteSocket()
创建一个 Server 段的本地 Socket,用来等待 ActivityManagerService 请求 Zygote 创建新的应用程序进程;
2、startSystemServer()
启动 System 进程,以便它可以将系统关键服务启动起来;
方法 Zygote.forkSystemServer() 创建子进程也就是 Android 系统的 System 进程;
System 进程的用户 ID 和用户组 ID 均为 1000, 并且它还具有用户组1001~1010, 1018以及3001~3003 的权限;
方法 handleSystemServiceProcess(...) 启动 System 进程
3、runSelectLoop()
等待 ActivityManagerService 请求 Zygote 进程创建新的应用程序进程;
至此, Zygote 进程的启动已经完成。
二、System 进程启动过程
commonInit() 设置 System 进程的时区,键盘布局等通用信息;
zygoteInitNative() 在 native 层启动一个 Bindler 线程池;
applicationInit(...) 调用 inovkeStaticMain(...) 方法, invokeStaticMain(...) 通过类加载器,加载 com.android.server.SystemServer, 进入 SystemServer.main() 方法, 启动 SystemServer
SystemServer.main() 方法会调用 System.run() 方法,在此方法里面启动一些相关的 Service;
private void run() {
...
// Prepare the main looper thread (this thread).
// 将线程的优先级设为前台线程,不能后台取消, 启动 主线程的 Looper
android.os.Process.setThreadPriority(android.os.Process.THREAD_PRIORITY_FOREGROUND);
android.os.Process.setCanSelfBackground(false);
Looper.prepareMainLooper();
// Initialize native services.
System.loadLibrary("android_servers");
nativeInit();
// Check whether we failed to shut down last time we tried.
// This call may not return.
performPendingShutdown();
// Initialize the system context.
// 创建化系统的 context
createSystemContext();
// Create the system service manager.
// 创建 系统服务管理
mSystemServiceManager = new SystemServiceManager(mSystemContext);
LocalServices.addService(SystemServiceManager.class, mSystemServiceManager);
// Start services.
// 启动服务
try {
startBootstrapServices();
startCoreServices();
startOtherServices();
} catch (Throwable ex) {
Slog.e("System", "******************************************");
Slog.e("System", "************ Failure starting system services", ex);
throw ex;
}
// Loop forever.
// 进入 Looper
Looper.loop();
...
}
创建系统的 context
// 创建系统的 context
private void createSystemContext() {
ActivityThread activityThread = ActivityThread.systemMain();
mSystemContext = activityThread.getSystemContext();
mSystemContext.setTheme(android.R.style.Theme_DeviceDefault_Light_DarkActionBar);
}
启动根服务
/**
* Starts the small tangle of critical services that are needed to get
* the system off the ground. These services have complex mutual dependencies
* which is why we initialize them all in one place here. Unless your service
* is also entwined in these dependencies, it should be initialized in one of
* the other functions.
*/
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.
// Activity 管理
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).
// 电池服务管理
mPowerManagerService = mSystemServiceManager.startService(PowerManagerService.class);
// Now that the power manager has been started, let the activity manager
// initialize power management features.
mActivityManagerService.initPowerManagement();
// Display manager is needed to provide display metrics before package manager
// starts up.
// 显示服务
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);
...
// Start the package manager.
// 启动 package 管理
mPackageManagerService = PackageManagerService.main(mSystemContext, installer, mFactoryTestMode != FactoryTest.FACTORY_TEST_OFF, mOnlyCore);
mFirstBoot = mPackageManagerService.isFirstBoot();
mPackageManager = mSystemContext.getPackageManager();
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.
// 启动 ActivityMangerService
mActivityManagerService.setSystemProcess();
}
启动一些核心服务
/**
* Starts some essential services that are not tangled up in the bootstrap process.
*/
private void startCoreServices() {
// Manages LEDs and display backlight.
// 启动屏幕灯光
mSystemServiceManager.startService(LightsService.class);
// Tracks the battery level. Requires LightService.
// 启动电池服务
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.
// 更新 WebView
mSystemServiceManager.startService(WebViewUpdateService.class);
}
startOtherService()
启动网络,蓝牙,电话等其他 Service;