一 什么是SystemServer？
简单来说，systemServer就是系统用来启动各种service的入口，安卓系统在启动的时候，会初始化两个重要的部分，一个是zygote进程，另一个是由zygote进程fork出来的SystemServer进程，SystemServer会启动我们系统中所需要的一系列service，下面会做分析。
二 从源码出发，分析Systemserver
2.1 SystemServer调用的入口
public static void main(String[] args) {
new SystemServer().run();
}
SystemServer的初始化入口在main（）方法中，我们进入run（）方法中，可以看到以下代码：
private void run() {
try {
......
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", "");
}
// Here we go!
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());
VMRuntime.getRuntime().clearGrowthLimit();
VMRuntime.getRuntime().setTargetHeapUtilization(0.8f);
Build.ensureFingerprintProperty();
Environment.setUserRequired(true);
BaseBundle.setShouldDefuse(true);
BinderInternal.disableBackgroundScheduling(true);
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);
} finally {
Trace.traceEnd(Trace.TRACE_TAG_SYSTEM_SERVER);
}
// Start services.
try {
Trace.traceBegin(Trace.TRACE_TAG_SYSTEM_SERVER, "StartServices");
startBootstrapServices();
startCoreServices();
startOtherServices();
} catch (Throwable ex) {
Slog.e("System", "******************************************");
Slog.e("System", "************ Failure starting system services", ex);
throw ex;
} finally {
Trace.traceEnd(Trace.TRACE_TAG_SYSTEM_SERVER);
}
// Loop forever.
Looper.loop();
throw new RuntimeException("Main thread loop unexpectedly exited");
}
以上代码已经去除了部分无用代码，我们下面来一步一步的对上面的代码进行切割分析
if (System.currentTimeMillis() < EARLIEST_SUPPORTED_TIME) {
Slog.w(TAG, "System clock is before 1970; setting to 1970.");
SystemClock.setCurrentTimeMillis(EARLIEST_SUPPORTED_TIME);
}
首先，SystemServer会去判断系统当前的时间，如果当前时间小于1970年那么，系统就会把当前的时间设置成1970年。
接下来，
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", "");
}
这代代码主要是初始化一些基本环境设置，比如说当前的语言，国家这些。
接着，
SystemProperties.set("persist.sys.dalvik.vm.lib.2", VMRuntime.getRuntime().vmLibrary());
VMRuntime.getRuntime().clearGrowthLimit();
VMRuntime.getRuntime().setTargetHeapUtilization(0.8f);
Build.ensureFingerprintProperty();
Environment.setUserRequired(true);
BaseBundle.setShouldDefuse(true);
BinderInternal.disableBackgroundScheduling(true);
android.os.Process.setThreadPriority(
android.os.Process.THREAD_PRIORITY_FOREGROUND);
android.os.Process.setCanSelfBackground(false);
Looper.prepareMainLooper();
// Initialize native services.
System.loadLibrary("android_servers");
这一部分主要是加载一些虚拟机环境和一些运行库，这些都不是本文所要分析的重点，接下来还有一个方法：
// Initialize the system context.
createSystemContext();
private void createSystemContext() {
ActivityThread activityThread = ActivityThread.systemMain();
mSystemContext = activityThread.getSystemContext();
mSystemContext.setTheme(DEFAULT_SYSTEM_THEME);
}
这个方法表示SystemServer需要获取当前系统的上下文
接下来，就会初始化系统的system Service Manager
mSystemServiceManager = new SystemServiceManager(mSystemContext);
LocalServices.addService(SystemServiceManager.class, mSystemServiceManager);
systemManagerService的作用是用来管理service的创建、开始或者SystemService下的其他生命周期事件。好了，接下来就是重头戏来了
try {
startBootstrapServices();
startCoreServices();
startOtherServices();
} catch (Throwable ex) {
......
throw ex;
} finally {
......
}
从谷歌给我们的注释我们给出以上三个方法简单的解释
1 startBootstrapServices() 主要是用来开启一些系统级别的service，这些service具有高度的复杂的相互依赖关系，所以我们需要把他们的初始化放在同一个地方
2 startCoreServices() 主要是启动一些核心的Service，但是这些service跟我们的bootservice没有相互依赖关系的，是相对独立的服务
2 startOtherServices() 正如英文所示，这是一些费关键非核心的service
Step 1 分析startBootstrapService()方法
private void startBootstrapServices() {
Installer installer = mSystemServiceManager.startService(Installer.class);
traceBeginAndSlog("DeviceIdentifiersPolicyService");
mSystemServiceManager.startService(DeviceIdentifiersPolicyService.class);
traceEnd();
// 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.
Trace.traceBegin(Trace.TRACE_TAG_SYSTEM_SERVER, "InitPowerManagement");
mActivityManagerService.initPowerManagement();
Trace.traceEnd(Trace.TRACE_TAG_SYSTEM_SERVER);
// 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.
traceBeginAndSlog("StartPackageManagerService");
mPackageManagerService = PackageManagerService.main(mSystemContext, installer,
mFactoryTestMode != FactoryTest.FACTORY_TEST_OFF, mOnlyCore);
mFirstBoot = mPackageManagerService.isFirstBoot();
mPackageManager = mSystemContext.getPackageManager();
Trace.traceEnd(Trace.TRACE_TAG_SYSTEM_SERVER);
// Manages A/B OTA dexopting. This is a bootstrap service as we need it to rename
// A/B artifacts after boot, before anything else might touch/need them.
// Note: this isn't needed during decryption (we don't have /data anyways).
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 {
Trace.traceEnd(Trace.TRACE_TAG_SYSTEM_SERVER);
}
}
}
traceBeginAndSlog("StartUserManagerService");
mSystemServiceManager.startService(UserManagerService.LifeCycle.class);
Trace.traceEnd(Trace.TRACE_TAG_SYSTEM_SERVER);
// 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();
}
在这段代码中，我们先分析核心的代码：
traceBeginAndSlog("StartInstaller");
Installer installer = mSystemServiceManager.startService(Installer.class);
traceEnd();
// In some cases after launching an app we need to access device identifiers,
// therefore register the device identifier policy before the activity manager.
traceBeginAndSlog("DeviceIdentifiersPolicyService");
mSystemServiceManager.startService(DeviceIdentifiersPolicyService.class);
traceEnd();
这段代码的执行，就是为了先启动installer，这样android才有机会去创建一些关键的路径（data/user）,这些都需要在其他Service启动前完成。其次，通过mSystemServiceManager.startService(DeviceIdentifiersPolicyService.class);这段代码，android需要注册当前的设备表示，以防有一些特殊的时候需要用到。
我们进入install类的onStart()方法一看
@Override
public void onStart() {
if (mIsolated) {
mInstalld = null;
} else {
connect();
}
}
private void connect() {
IBinder binder = ServiceManager.getService("installd");
if (binder != null) {
try {
binder.linkToDeath(new DeathRecipient() {
@Override
public void binderDied() {
Slog.w(TAG, "installd died; reconnecting");
connect();
}
}, 0);
} catch (RemoteException e) {
binder = null;
}
}
if (binder != null) {
mInstalld = IInstalld.Stub.asInterface(binder);
try {
invalidateMounts();
} catch (InstallerException ignored) {
}
} else {
Slog.w(TAG, "installd not found; trying again");
BackgroundThread.getHandler().postDelayed(() -> {
connect();
}, DateUtils.SECOND_IN_MILLIS);
}
}
我们可以看到，其实这就是一个递归。install通过调用connect，去判断installer是否被初始化。只有installer被初始化了，才会继续往下掉用，初始化其他的服务。
接下来，启动ActivityManagerService。我们通过代码分析：
mActivityManagerService = mSystemServiceManager.startService(
ActivityManagerService.Lifecycle.class).getService();
mActivityManagerService.setSystemServiceManager(mSystemServiceManager);
mActivityManagerService.setInstaller(installer);
我们开启ams传入的是 ActivityManagerService.Lifecycle.class，同时把installer传入给ams。我们进入AMS查看代码，可以看到
final void finishBooting() {
...
for (String abi : Build.SUPPORTED_ABIS) {
zygoteProcess.establishZygoteConnectionForAbi(abi);
final String instructionSet = VMRuntime.getInstructionSet(abi);
if (!completedIsas.contains(instructionSet)) {
try {
mInstaller.markBootComplete(VMRuntime.getInstructionSet(abi));
} catch (InstallerException e) {
Slog.w(TAG, "Unable to mark boot complete for abi: " + abi + " (" +
e.getMessage() +")");
}
completedIsas.add(instructionSet);
}
}
...
}
所以finishBooting是ams和zygote进行通讯的入口。
startBootstrapServices()方法中，还启动了很多其他的services，包括PowerManagerService、RecoverySystemService、LightsService、DisplayManagerService等等，这里就不重复做出分析了。
Step 2 分析startCoreServices()方法
private void startCoreServices() {
// Records errors and logs, for example wtf()
traceBeginAndSlog("StartDropBoxManager");
mSystemServiceManager.startService(DropBoxManagerService.class);
traceEnd();
traceBeginAndSlog("StartBatteryService");
// Tracks the battery level. Requires LightService.
mSystemServiceManager.startService(BatteryService.class);
traceEnd();
// Tracks application usage stats.
traceBeginAndSlog("StartUsageService");
mSystemServiceManager.startService(UsageStatsService.class);
mActivityManagerService.setUsageStatsManager(
LocalServices.getService(UsageStatsManagerInternal.class));
traceEnd();
// Tracks whether the updatable WebView is in a ready state and watches for update installs.
traceBeginAndSlog("StartWebViewUpdateService");
mWebViewUpdateService = mSystemServiceManager.startService(WebViewUpdateService.class);
traceEnd();
}
这个方法主要启动一些跟bootstrap进程无关的service，这里比较好玩的是，android会在这里去检测WebView是处于就绪状态和手动更新安装。
Step 3 分析startOtherServices()方法
这个方法过长，就不贴上源码的，这个方法主要是为了整理或者重构一些杂七杂八的包，不太重要，不做分析。
总结
至此，systemServer启动流程分析完毕，可能篇幅太长，大家需要自己好好整理，没事多复习。
（1）SystemServer进程是android中一个很重要的进程由Zygote进程启动；
（2）SystemServer进程主要用于启动系统中的服务；
（3）SystemServer进程启动服务的启动函数为main函数；
（4）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；
（5）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
（6）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；
（7）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；
（8）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；
（备注）以上8点的总结，参考的是http://blog.csdn.net/qq_23547831/article/details/51105171 在此感谢作者
