Zygote家的大儿子 —— SystemServer
本文基于 Android 9.0 , 代码仓库地址 ： android_9.0.0_r45
文中源码链接：
首先来回顾一下上篇文章 Java 世界的盘古和女娲 —— Zygote ，主要介绍了 Android 世界中的第一个 Java 进程 Zygote，它的主要工作流程如下：
registerServerSocketFromEnv()， 注册服务端 socket，用于和客户端进程通信preload()，预加载一系列资源，提高应用启动速度forkSystemServer()，创建system_server进程- 功成身退，调用
runSelectLoop()等待响应客户端请求，创建应用进程
本篇文章的主角 system_server 进程是 Zygote 进程 fork 出的第一个进程，它负责管理和启动整个 Framework 层。
再来看看 Gityuan 的这张图片，找一下 System Server 的位置，它承载了各类系统服务的创建和启动。关于 system_server 进程的创建流程，上篇文章中已经做了详细介绍，这里再简单看一下流程图：
最终会调用到 SystemServer.main() 方法。下面就以此为起点，来具体分析 SystemServer 都做了些什么。
SystemServer 启动流程
public static void main(String[] args) {
new SystemServer().run();
}
接着看 run() 方法。
private void run() {
try {
......
// 如果设备时间早于 1970 年，很多 API 处理负数时会 crash。所以直接设置为 1970 年 1 月 1 日
if (System.currentTimeMillis() < EARLIEST_SUPPORTED_TIME) {
Slog.w(TAG, "System clock is before 1970; setting to 1970.");
SystemClock.setCurrentTimeMillis(EARLIEST_SUPPORTED_TIME);
}
// 未设置时区的话默认设为 GMT
String timezoneProperty = SystemProperties.get("persist.sys.timezone");
if (timezoneProperty == null || timezoneProperty.isEmpty()) {
Slog.w(TAG, "Timezone not set; setting to GMT.");
SystemProperties.set("persist.sys.timezone", "GMT");
}
// 语言地区设置
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", "");
}
// The system server should never make non-oneway calls
Binder.setWarnOnBlocking(true);
// The system server should always load safe labels
PackageItemInfo.setForceSafeLabels(true);
// Default to FULL within the system server.
SQLiteGlobal.sDefaultSyncMode = SQLiteGlobal.SYNC_MODE_FULL;
// Deactivate SQLiteCompatibilityWalFlags until settings provider is initialized
SQLiteCompatibilityWalFlags.init(null);
// Here we go!
Slog.i(TAG, "Entered the Android system server!");
int uptimeMillis = (int) SystemClock.elapsedRealtime();
EventLog.writeEvent(EventLogTags.BOOT_PROGRESS_SYSTEM_RUN, uptimeMillis);
if (!mRuntimeRestart) {
MetricsLogger.histogram(null, "boot_system_server_init", uptimeMillis);
}
// 设置虚拟机运行库路径
SystemProperties.set("persist.sys.dalvik.vm.lib.2", VMRuntime.getRuntime().vmLibrary());
// Mmmmmm... more memory!
// 清除虚拟机内存增长限制，允许应用申请更多内存
VMRuntime.getRuntime().clearGrowthLimit();
// 设置堆内存的有效利用率为 0.8，(可能被忽略)
VMRuntime.getRuntime().setTargetHeapUtilization(0.8f);
// 确保指纹信息已经定义
Build.ensureFingerprintProperty();
// Within the system server, it is an error to access Environment paths without
// explicitly specifying a user.
Environment.setUserRequired(true);
// Within the system server, any incoming Bundles should be defused
// to avoid throwing BadParcelableException.
BaseBundle.setShouldDefuse(true);
// Within the system server, when parceling exceptions, include the stack trace
Parcel.setStackTraceParceling(true);
// 确保系统的 Binder 调用总是运行在前台优先级
BinderInternal.disableBackgroundScheduling(true);
// Increase the number of binder threads in system_server
BinderInternal.setMaxThreads(sMaxBinderThreads);
// Prepare the main looper thread (this thread).
android.os.Process.setThreadPriority(
android.os.Process.THREAD_PRIORITY_FOREGROUND);
android.os.Process.setCanSelfBackground(false);
// 1. 创建主线程 Looper
Looper.prepareMainLooper();
Looper.getMainLooper().setSlowLogThresholdMs(
SLOW_DISPATCH_THRESHOLD_MS, SLOW_DELIVERY_THRESHOLD_MS);
// 初始化 native 服务，加载 libandroid_servers.so
System.loadLibrary("android_servers");
// 检查上次关机是否失败，可能不会有返回值
performPendingShutdown();
// 2. 初始化系统上下文
createSystemContext();
// 3. 创建系统服务管理 SystemServiceManager
// 并将 mSystemServiceManager 注册到 sLocalServiceObjects 中
mSystemServiceManager = new SystemServiceManager(mSystemContext);
mSystemServiceManager.setStartInfo(mRuntimeRestart,
mRuntimeStartElapsedTime, mRuntimeStartUptime);
LocalServices.addService(SystemServiceManager.class, mSystemServiceManager);
// Prepare the thread pool for init tasks that can be parallelized
SystemServerInitThreadPool.get();
} finally {
traceEnd(); // InitBeforeStartServices
}
// Start services.
try {
traceBeginAndSlog("StartServices");
startBootstrapServices(); // 4. 启动系统引导服务
startCoreServices(); // 5. 启动系统核心服务
startOtherServices(); // 6. 启动其他服务
SystemServerInitThreadPool.shutdown();
} catch (Throwable ex) {
Slog.e("System", "******************************************");
Slog.e("System", "************ Failure starting system services", ex);
throw ex;
} finally {
traceEnd();
}
StrictMode.initVmDefaults(null);
if (!mRuntimeRestart && !isFirstBootOrUpgrade()) {
int uptimeMillis = (int) SystemClock.elapsedRealtime();
MetricsLogger.histogram(null, "boot_system_server_ready", uptimeMillis);
final int MAX_UPTIME_MILLIS = 60 * 1000;
if (uptimeMillis > MAX_UPTIME_MILLIS) {
Slog.wtf(SYSTEM_SERVER_TIMING_TAG,
"SystemServer init took too long. uptimeMillis=" + uptimeMillis);
}
}
// 7. Loop forever.
Looper.loop();
throw new RuntimeException("Main thread loop unexpectedly exited");
}
代码虽然比较长，但是逻辑很清晰。我在注释里标记了比较重要的 7 个步骤，逐一分析。
Looper.prepareMainLooper()
初始化 Looper。关于 Handler 消息机制，可以阅读我的另一篇文章 深入理解 Handler 消息机制 。最后会调用 Looper.loop() 开启消息循环，开始处理消息。
createSystemContext()
private void createSystemContext() {
// 创建 system_server 上下文信息
ActivityThread activityThread = ActivityThread.systemMain();
mSystemContext = activityThread.getSystemContext();
mSystemContext.setTheme(DEFAULT_SYSTEM_THEME);
final Context systemUiContext = activityThread.getSystemUiContext();
// 设置主题，用于系统 dialog 等
systemUiContext.setTheme(DEFAULT_SYSTEM_THEME);
}
创建系统上下文。首先调用 ActivityThread.systemMain() 方法获取 ActivityThread 对象，然后再获取上下文。
public static ActivityThread systemMain() {
// 判断是否是大内存设备,在低内存设备上不启用硬件加速
if (!ActivityManager.isHighEndGfx()) {
ThreadedRenderer.disable(true);
} else {
ThreadedRenderer.enableForegroundTrimming();
}
ActivityThread thread = new ActivityThread();
thread.attach(true, 0);
return thread;
}
关于 ActivityThread.attach() 方法这里不做具体分析了，后面文章说到应用启动时再来详细解析。
创建完系统上下文，接下来就是启动各种系统服务了。源码中把服务大致分为了三类，再来回顾一下：
startBootstrapServices(); // 4. 启动系统引导服务
startCoreServices(); // 5. 启动系统核心服务
startOtherServices(); // 6. 启动其他服务
逐一进行分析。
startBootstrapServices()
private void startBootstrapServices() {
final String TAG_SYSTEM_CONFIG = "ReadingSystemConfig";
SystemServerInitThreadPool.get().submit(SystemConfig::getInstance, TAG_SYSTEM_CONFIG);
// 阻塞等待与 installd 建立 socket 通道
Installer installer = mSystemServiceManager.startService(Installer.class);
// 启动 DeviceIdentifiersPolicyService，在 ActivityManagerService 之前
mSystemServiceManager.startService(DeviceIdentifiersPolicyService.class);
// 启动服务 ActivityManagerService
mActivityManagerService = mSystemServiceManager.startService(
ActivityManagerService.Lifecycle.class).getService();
mActivityManagerService.setSystemServiceManager(mSystemServiceManager);
mActivityManagerService.setInstaller(installer);
// 启动服务 PowerManagerService
mPowerManagerService = mSystemServiceManager.startService(PowerManagerService.class);
// Now that the power manager has been started, let the activity manager
// initialize power management features.
mActivityManagerService.initPowerManagement();
// 启动服务 RecoverySystemService
mSystemServiceManager.startService(RecoverySystemService.class);
// Now that we have the bare essentials of the OS up and running, take
// note that we just booted, which might send out a rescue party if
// we're stuck in a runtime restart loop.
RescueParty.noteBoot(mSystemContext);
// 启动服务 LightsService
mSystemServiceManager.startService(LightsService.class);
// Package manager isn't started yet; need to use SysProp not hardware feature
if (SystemProperties.getBoolean("config.enable_sidekick_graphics", false)) {
mSystemServiceManager.startService(WEAR_SIDEKICK_SERVICE_CLASS);
}
// 启动 DisplayManagerService，在 PackageManagerService 之前
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);
// 正在加密设备时只运行核心 app
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;
}
// 启动服务 PackageManagerService
if (!mRuntimeRestart) {
MetricsLogger.histogram(null, "boot_package_manager_init_start",
(int) SystemClock.elapsedRealtime());
}
mPackageManagerService = PackageManagerService.main(mSystemContext, installer,
mFactoryTestMode != FactoryTest.FACTORY_TEST_OFF, mOnlyCore);
mFirstBoot = mPackageManagerService.isFirstBoot();
mPackageManager = mSystemContext.getPackageManager();
if (!mRuntimeRestart && !isFirstBootOrUpgrade()) {
MetricsLogger.histogram(null, "boot_package_manager_init_ready",
(int) SystemClock.elapsedRealtime());
}
if (!mOnlyCore) {
boolean disableOtaDexopt = SystemProperties.getBoolean("config.disable_otadexopt",
false);
if (!disableOtaDexopt) {
traceBeginAndSlog("StartOtaDexOptService");
try {
OtaDexoptService.main(mSystemContext, mPackageManagerService);
} catch (Throwable e) {
reportWtf("starting OtaDexOptService", e);
} finally {
traceEnd();
}
}
}
// 启动服务 UserManagerService
mSystemServiceManager.startService(UserManagerService.LifeCycle.class);
// 初始化属性 cache 以缓存包资源
AttributeCache.init(mSystemContext);
// 设置 AMS
mActivityManagerService.setSystemProcess();
// DisplayManagerService needs to setup android.display scheduling related policies
// since setSystemProcess() would have overridden policies due to setProcessGroup
mDisplayManagerService.setupSchedulerPolicies();
// 启动服务 OverlayManagerService
OverlayManagerService overlayManagerService = new OverlayManagerService(
mSystemContext, installer);
mSystemServiceManager.startService(overlayManagerService);
if (SystemProperties.getInt("persist.sys.displayinset.top", 0) > 0) {
// DisplayManager needs the overlay immediately.
overlayManagerService.updateSystemUiContext();
LocalServices.getService(DisplayManagerInternal.class).onOverlayChanged();
}
// 在单独的线程中启动 SensorService
mSensorServiceStart = SystemServerInitThreadPool.get().submit(() -> {
TimingsTraceLog traceLog = new TimingsTraceLog(
SYSTEM_SERVER_TIMING_ASYNC_TAG, Trace.TRACE_TAG_SYSTEM_SERVER);
startSensorService();
}, START_SENSOR_SERVICE);
}
startBootstrapServices() 方法中的都是系统启动过程中的关键服务，且相互依赖，主要下列服务 ：
InstallerDeviceIdentifiersPolicyServiceActivityManagerServicePowerManagerServiceRecoverySystemServiceLightsServiceStartSidekickServiceDisplayManagerServiceSystemService.PHASE_WAIT_FOR_DEFAULT_DISPLAY (100)
PackageManagerServiceUserManagerServiceOverlayManagerServiceSensorService
一共启动了十二个核心服务。注意中间的 SystemService.PHASE_WAIT_FOR_DEFAULT_DISPLAY，它并不是代表什么系统服务，而是一个 int 值 100,类似的 int 值还有一些，定义在 SystemService 类中，它的作用是给服务启动过程划分阶段，每个阶段都有特定的含义，可以做不同的事情。这里先混个脸熟，等介绍完所有的服务，再回过头来总结一下有哪些阶段。
startCoreServices()
private void startCoreServices() {
// 启动服务 BatteryService，需要 LightService
mSystemServiceManager.startService(BatteryService.class);
// 启动服务 UsageStatsService，统计应用使用情况
mSystemServiceManager.startService(UsageStatsService.class);
mActivityManagerService.setUsageStatsManager(
LocalServices.getService(UsageStatsManagerInternal.class));
// 检查是否存在可更新的 WebView。存在就启动服务 WebViewUpdateService
if (mPackageManager.hasSystemFeature(PackageManager.FEATURE_WEBVIEW)) {
mWebViewUpdateService = mSystemServiceManager.startService(WebViewUpdateService.class);
}
// 启动服务 BinderCallsStatsService，跟踪 Binder 调用的 cpu 时间消耗
BinderCallsStatsService.start();
}
启动了四个服务，BatteryService UsageStatsService WebViewUpdateService 和 BinderCallsStatsService 。
startOtherServices()
startOtherServices() 源码有一千多行，就像一个杂货铺，启动了一系列的服务。下面尽量精简一下代码：
KeyAttestationApplicationIdProviderService/KeyChainSystemService
SchedulingPolicyService/TelecomLoaderService/TelephonyRegistry
mContentResolver = context.getContentResolver();
AccountManagerService/ContentService
mActivityManagerService.installSystemProviders();
DropBoxManagerService/VibratorService/ConsumerIrService/AlarmManagerService
final Watchdog watchdog = Watchdog.getInstance();
watchdog.init(context, mActivityManagerService);
InputManagerService/WindowManagerService/VrManagerService/BluetoothService
IpConnectivityMetrics/NetworkWatchlistService/PinnerService
InputMethodManagerService/AccessibilityManagerService/StorageManagerService
StorageStatsService/UiModeManagerService/LockSettingsService
PersistentDataBlockService/OemLockService/DeviceIdleController
DevicePolicyManagerService/StatusBarManagerService/ClipboardService
NetworkManagementService/IpSecService/TextServicesManagerService
TextClassificationManagerService/NetworkScoreService/NetworkStatsService
NetworkPolicyManagerService/WifiScanningService/RttService
WifiAware/WifiP2P/Lowpan/Ethernet/ConnectivityService/NsdService
SystemUpdateManagerService/UpdateLockService/NotificationManagerService
DeviceStorageMonitorService/LocationManagerService/CountryDetectorService
SearchManagerService/WallpaperManagerService/AudioService/BroadcastRadioService
DockObserver/ThermalObserver/WiredAccessoryManager/MidiManager/UsbService
SerialService/HardwarePropertiesManagerService/TwilightService
ColorDisplayService/JobSchedulerService/SoundTriggerService/TrustManagerService
BackupManager/AppWidgerService/VoiceRecognitionManager/GestureLauncherService
SensorNotificationService/ContextHubSystemService/DiskStatsService
TimeZoneRulesManagerService/NetworkTimeUpdateService/CommonTimeManagementService
CertBlacklister/EmergencyAffordanceService/DreamManagerService/GraphicsStatsService
CoverageService/PrintManager/CompanionDeviceManager/RestrictionsManagerService
MediaSessionService/MediaUpdateService/HdmiControlService/TvInputManagerService
MediaResourceMonitorService/TvRemoteService/MediaRouterService/FingerprintService
BackgroundDexOptService/PruneInstantAppsJobService/ShortcutService
LauncherAppsService/CrossProfileAppsService/MediaProjectionManagerService
WearConfigService/WearConnectivityService/WearTimeService/WearLeftyService
WearGlobalActionsService/SliceManagerService/CameraServiceProxy/IoTSystemService
MmsServiceBroker/AutoFillService
// It is now time to start up the app processes...
vibrator.systemReady();
lockSettings.systemReady();
// 480
mSystemServiceManager.startBootPhase(SystemService.PHASE_LOCK_SETTINGS_READY);
// 500
mSystemServiceManager.startBootPhase(SystemService.PHASE_SYSTEM_SERVICES_READY);
wm.systemReady();
mPowerManagerService.systemReady(mActivityManagerService.getAppOpsService());
mPackageManagerService.systemReady();
mDisplayManagerService.systemReady(safeMode, mOnlyCore);
// Start device specific services
final String[] classes = mSystemContext.getResources().getStringArray(
R.array.config_deviceSpecificSystemServices);
for (final String className : classes) {
try {
mSystemServiceManager.startService(className);
} catch (Throwable e) {
reportWtf("starting " + className, e);
}
}
// 520
mSystemServiceManager.startBootPhase(SystemService.PHASE_DEVICE_SPECIFIC_SERVICES_READY);
mActivityManagerService.systemReady(() -> {
// 550
mSystemServiceManager.startBootPhase(SystemService.PHASE_ACTIVITY_MANAGER_READY);
startSystemUi(context, windowManagerF);
networkManagementF.systemReady();
ipSecServiceF.systemReady();
networkStatsF.systemReady();
connectivityF.systemReady();
Watchdog.getInstance().start
mPackageManagerService.waitForAppDataPrepared();
// 600
mSystemServiceManager.startBootPhase(SystemService.PHASE_THIRD_PARTY_APPS_CAN_START);
locationF.systemRunning();
countryDetectorF.systemRunning();
networkTimeUpdaterF.systemRunning();
commonTimeMgmtServiceF.systemRunning();
inputManagerF.systemRunning();
telephonyRegistryF.systemRunning();
mediaRouterF.systemRunning();
mmsServiceF.systemRunning();
incident.systemRunning();
}
通过上面的代码可以看到启动了相当多的系统服务。startOtherServices() 方法共经历了五个启动阶段，如下所示：
SystemService.PHASE_LOCK_SETTINGS_READY // 480
SystemService.PHASE_SYSTEM_SERVICES_READY // 500
SystemService.PHASE_DEVICE_SPECIFIC_SERVICES_READY // 520
SystemService.PHASE_ACTIVITY_MANAGER_READY // 550
SystemService.PHASE_THIRD_PARTY_APPS_CAN_START // 600
最后调用的 mActivityManagerService.systemReady() 方法。该方法中会调用 startHomeActivityLocked 来启动桌面 Activity，这样桌面应用就启动了。
Looper.loop()
至此，system_server 进程的主要工作就算完成了，进入 Looper.loop() 状态,等待其他线程通过 Handler 发送消息到主线程并处理。
SystemServer 启动阶段分类
回过头再来看看前面提到的启动阶段分类，定义在 com.android.server.SystemService 类中：
/*
* Boot Phases
*
* 启动阶段
*/
public static final int PHASE_WAIT_FOR_DEFAULT_DISPLAY = 100; // maybe should be a dependency?
/**
* After receiving this boot phase, services can obtain lock settings data.
*/
public static final int PHASE_LOCK_SETTINGS_READY = 480;
/**
* After receiving this boot phase, services can safely call into core system services
* such as the PowerManager or PackageManager.
*
* 在这个阶段之后，可以安全的调用系统核心服务，如 PowerManager 和 PackageManager
*/
public static final int PHASE_SYSTEM_SERVICES_READY = 500;
/**
* After receiving this boot phase, services can safely call into device specific services.
*
* 在这个阶段之后，可以安全调用设备特定的服务
*/
public static final int PHASE_DEVICE_SPECIFIC_SERVICES_READY = 520;
/**
* After receiving this boot phase, services can broadcast Intents.
*
* 在这个阶段之后，服务可以广播
*/
public static final int PHASE_ACTIVITY_MANAGER_READY = 550;
/**
* After receiving this boot phase, services can start/bind to third party apps.
* Apps will be able to make Binder calls into services at this point.
*
* 在这个阶段之后，服务可以启动/绑定第三方应用
* 应用此时可以进行 Binder 调用
*/
public static final int PHASE_THIRD_PARTY_APPS_CAN_START = 600;
/**
* After receiving this boot phase, services can allow user interaction with the device.
* This phase occurs when boot has completed and the home application has started.
* System services may prefer to listen to this phase rather than registering a
* broadcast receiver for ACTION_BOOT_COMPLETED to reduce overall latency.
*
* 在这个阶段之后，允许用户和设备交互。
* 这个阶段发生在启动完成，home 应用已经开始。
* 系统服务更倾向于监听这个阶段，而不是监听启动广播 ACTION_BOOT_COMPLETED，以降低延迟
*/
public static final int PHASE_BOOT_COMPLETED = 1000;
在 system_server 启动过程中各个阶段的位置大致如下：
private void startBootstrapServices() {
...
// 100
mSystemServiceManager.startBootPhase(SystemService.PHASE_WAIT_FOR_DEFAULT_DISPLAY);
...
}
private void startOtherServices() {
...
// 480
mSystemServiceManager.startBootPhase(SystemService.PHASE_LOCK_SETTINGS_READY);
// 500
mSystemServiceManager.startBootPhase(SystemService.PHASE_SYSTEM_SERVICES_READY);
...
// 520
mSystemServiceManager.startBootPhase(SystemService.PHASE_DEVICE_SPECIFIC_SERVICES_READY);
mActivityManagerService.systemReady(() -> {
mSystemServiceManager.startBootPhase(
SystemService.PHASE_ACTIVITY_MANAGER_READY); // 550
...
mSystemServiceManager.startBootPhase(
SystemService.PHASE_THIRD_PARTY_APPS_CAN_START); // 600
}
}
最后的 SystemService.PHASE_BOOT_COMPLETED（1000） 在 AMS 的 finishBooting() 方法中调用。另外注意 480 和 500 两个阶段是连在一起的，中间没有发生任何事情。
那么，划分阶段的具体作用是什么呢？答案就在 startBootPhase() 方法中：
public void startBootPhase(final int phase) {
if (phase <= mCurrentPhase) {
throw new IllegalArgumentException("Next phase must be larger than previous");
}
mCurrentPhase = phase;
try {
final int serviceLen = mServices.size();
for (int i = 0; i < serviceLen; i++) {
final SystemService service = mServices.get(i);
long time = SystemClock.elapsedRealtime();
try {
// 回调系统服务的 onBootPhase() 方法
service.onBootPhase(mCurrentPhase);
} catch (Exception ex) {
throw new RuntimeException("Failed to boot service "
+ service.getClass().getName()
+ ": onBootPhase threw an exception during phase "
+ mCurrentPhase, ex);
}
warnIfTooLong(SystemClock.elapsedRealtime() - time, service, "onBootPhase");
}
} finally {
Trace.traceEnd(Trace.TRACE_TAG_SYSTEM_SERVER);
}
}
核心就在于 service.onBootPhase(mCurrentPhase);。所有系统服务都是继承于 SystemService 的，startBootPhase() 方法会回调当前阶段已经加入 mServices 的所有系统服务的 onBootPhase() 方法，在合适的阶段做一些合适的事情。以 AMS 为例：
@Override
public void onBootPhase(int phase) {
mService.mBootPhase = phase;
if (phase == PHASE_SYSTEM_SERVICES_READY) {
mService.mBatteryStatsService.systemServicesReady();
mService.mServices.systemServicesReady();
}
}
SystemServer 是如何启动服务的 ？
看完 SystemServer 的源码，它最重要的工作就是创建和启动各种系统服务。那么服务一般是如何创建的呢？下面以 startBootstrapServices() 中创建的第一个服务 Installer 为例来看一下：
Installer installer = mSystemServiceManager.startService(Installer.class);
进入 SystemServiceManager 的 startService() 方法：
public <T extends SystemService> T startService(Class<T> serviceClass) {
try {
// 获取服务名称
final String name = serviceClass.getName();
// 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 {
// 获取服务类的构造器
Constructor<T> constructor = serviceClass.getConstructor(Context.class);
// 反射创建 service
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);
}
startService(service);
return service;
} finally {
Trace.traceEnd(Trace.TRACE_TAG_SYSTEM_SERVER);
}
}
创建并启动一个系统服务。这个系统服务必须是 com.android.server.SystemService 的子类。根据参数传入的 Class 对象反射创建其实例，再调用重载方法 startService() ：
public void startService(@NonNull final SystemService service) {
// Register it.
mServices.add(service);
// Start it.
long time = SystemClock.elapsedRealtime();
try {
// 回调系统服务的 onStart() 方法
service.onStart();
} catch (RuntimeException ex) {
throw new RuntimeException("Failed to start service " + service.getClass().getName()
+ ": onStart threw an exception", ex);
}
warnIfTooLong(SystemClock.elapsedRealtime() - time, service, "onStart");
}
就两步。第一步，注册服务，mServices 是一个 ArrayList<SystemService> 对象，用来保存已经创建的系统服务。第二步，回调服务的 onStart() 方法，还是以 Installer 为例：
@Override
public void onStart() {
if (mIsolated) {
mInstalld = null;
} else {
connect();
}
}
这样一个服务就启动完成了。这是一种比较普遍的启动方式，当然还有一些系统服务具有不一样的启动方式，这里就不一一分析了，后面有机会解析具体服务的时候再来分析。
总结
SystemServer 的启动流程比较耿直，没有那么多弯弯绕，下面简单总结一下：
- 语言、时区、地区等设置
- 虚拟机内存设置
- 指纹信息，Binder 调用设置
Looper.prepareMainLooper()，创建主线程 Looper- 初始化 native 服务，加载
libandroid_servers.so createSystemContext()，初始化系统上下文- 创建系统服务管理
SystemServiceManager startBootstrapServices，启动系统引导服务startCoreServices，启动系统核心服务startOtherServices，启动其他服务Looper.loop()，开启消息循环
另外，在 startOtherServices 的最后会调用 AMS 的 onSystemReady() 方法启动桌面 Activity。
预告
还记得 Zygote 进程的 runSelectLoop() 方法吗？Zygote 在创建完 system_server 进程之后，就开始默默的等待客户端请求创建应用进程。下一篇，我们将从源码角度来捋一遍客户端是如何发送请求，Zygote 是如何处理请求，应用进程是如何创建的，敬请期待！
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