版本:Android O
SystemServer和Zygote是Android java世界的两大支柱,SystemServer是Zygote孵化出来的进程,进程名为system_server,几乎所有的系统服务都在该进程中,eg:AMS,PMS,WMS .etc
1 分析SystemServer代码
1.1 从SystemServer.java分析
源码位置:frameworks\base\services\java\com\android\server\SystemServer.java,从main()函数开始分析:/**
* The main entry point from zygote.
*/
public static void main(String[] args) {
new SystemServer().run();
}
new出一个SystemServer类,执行其run()方法.public SystemServer() {
// Check for factory test mode.
mFactoryTestMode = FactoryTest.getMode();
// Remember if it's runtime restart(when sys.boot_completed is already set) or reboot
mRuntimeRestart = "1".equals(SystemProperties.get("sys.boot_completed"));
}
这是SystemServer的构造方法,具体没有研究,有blog说SystemServer.java是final的,不能被继承,但是在O中不是(也没有具体研究,研究了再补充).接下来看run()方法:private void run() {
try {
traceBeginAndSlog("InitBeforeStartServices");
// If a device's clock is before 1970 (before 0), a lot of
// APIs crash dealing with negative numbers, notably
// java.io.File#setLastModified, so instead we fake it and
// hope that time from cell towers or NTP fixes it shortly.
if (System.currentTimeMillis() < EARLIEST_SUPPORTED_TIME) {
Slog.w(TAG, "System clock is before 1970; setting to 1970.");
SystemClock.setCurrentTimeMillis(EARLIEST_SUPPORTED_TIME);
}
//
// Default the timezone property to GMT if not set.
//
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");
}
// If the system has "persist.sys.language" and friends set, replace them with
// "persist.sys.locale". Note that the default locale at this point is calculated
// using the "-Duser.locale" command line flag. That flag is usually populated by
// AndroidRuntime using the same set of system properties, but only the system_server
// and system apps are allowed to set them.
//
// NOTE: Most changes made here will need an equivalent change to
// core/jni/AndroidRuntime.cpp
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);
// 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);
}
/// M: BOOTPROF
addBootEvent("Android:SysServerInit_START");
// In case the runtime switched since last boot (such as when
// the old runtime was removed in an OTA), set the system
// property so that it is in sync. We can | xq oqi't do this in
// libnativehelper's JniInvocation::Init code where we already
// had to fallback to a different runtime because it is
// running as root and we need to be the system user to set
// the property. http://b/11463182
SystemProperties.set("persist.sys.dalvik.vm.lib.2", VMRuntime.getRuntime().vmLibrary());
// Enable the sampling profiler.
if (SamplingProfilerIntegration.isEnabled()) {
SamplingProfilerIntegration.start();
mProfilerSnapshotTimer = new Timer();
mProfilerSnapshotTimer.schedule(new TimerTask() {
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);
// Within the system server, any incoming Bundles should be defused
// to avoid throwing BadParcelableException.
BaseBundle.setShouldDefuse(true);
// Ensure binder calls into the system always run at foreground priority.
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);
//开启消息循环
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);
mSystemServiceManager.setRuntimeRestarted(mRuntimeRestart);
LocalServices.addService(SystemServiceManager.class, mSystemServiceManager);
// Prepare the thread pool for init tasks that can be parallelized
SystemServerInitThreadPool.get();
} finally {
traceEnd(); // InitBeforeStartServices
}
//创建服务
// Start services.
try {
traceBeginAndSlog("StartServices");
startBootstrapServices();
startCoreServices();
startOtherServices();
SystemServerInitThreadPool.shutdown();
} catch (Throwable ex) {
Slog.e("System", "******************************************");
Slog.e("System", "************ Failure starting system services", ex);
throw ex;
} finally {
traceEnd();
}
// For debug builds, log event loop stalls to dropbox for analysis.
if (StrictMode.conditionallyEnableDebugLogging()) {
Slog.i(TAG, "Enabled StrictMode for system server main thread.");
}
/// M: open wtf when load is user or userdebug.
if (!"eng".equals(Build.TYPE) && !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);
}
}
/// M: BOOTPROF
addBootEvent("Android:SysServerInit_END");
// Loop forever.
Looper.loop();
throw new RuntimeException("Main thread loop unexpectedly exited");
}
最终的实现实在run方法里面,接下来分块分析run方法:if (System.currentTimeMillis() < EARLIEST_SUPPORTED_TIME) {
Slog.w(TAG, "System clock is before 1970; setting to 1970.");
SystemClock.setCurrentTimeMillis(EARLIEST_SUPPORTED_TIME);
}
为了判断系统时钟是否早于1970/01/01/00:00,如果早于这个时间,系统后面的处理可能会出问题,所以如果早于该时间,统一设置为1970/01/01/00:00String 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");
}
如果没有设置时区,统一设置为GMTif (!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", "");
}
设置系统语言环境;// 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);
}
/// M: BOOTPROF
addBootEvent("Android:SysServerInit_START");
// In case the runtime switched since last boot (such as when
// the old runtime was removed in an OTA), set the system
// property so that it is in sync. We can | xq oqi't do this in
// libnativehelper's JniInvocation::Init code where we already
// had to fallback to a different runtime because it is
// running as root and we need to be the system user to set
// the property. http://b/11463182
SystemProperties.set("persist.sys.dalvik.vm.lib.2", VMRuntime.getRuntime().vmLibrary());
// Enable the sampling profiler.
if (SamplingProfilerIntegration.isEnabled()) {
SamplingProfilerIntegration.start();
mProfilerSnapshotTimer = new Timer();
mProfilerSnapshotTimer.schedule(new TimerTask() {
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);
// Within the system server, any incoming Bundles should be defused
// to avoid throwing BadParcelableException.
BaseBundle.setShouldDefuse(true);
// Ensure binder calls into the system always run at foreground priority.
BinderInternal.disableBackgroundScheduling(true);
// Increase the number of binder threads in system_server
BinderInternal.setMaxThreads(sMaxBinderThreads);
这段代码的主要作用是设置虚拟机(VMRuntime)运行内存,相关的操作,(没有细致的研究过).// Prepare the main looper thread (this thread).
android.os.Process.setThreadPriority(
android.os.Process.THREAD_PRIORITY_FOREGROUND);
android.os.Process.setCanSelfBackground(false);
Looper.prepareMainLooper();
创建消息循环,和ActivityThread.java里的main()函数里建立异步消息循环一样.// Initialize native services.
System.loadLibrary("android_servers");
加载了libandroid_servers.so文件// Check whether we failed to shut down last time we tried.
// This call may not return.
performPendingShutdown();
performPendingShutdown()函数代码如下:private void performPendingShutdown() {
final String shutdownAction = SystemProperties.get(
ShutdownThread.SHUTDOWN_ACTION_PROPERTY, "");
if (shutdownAction != null && shutdownAction.length() > 0) {
boolean reboot = (shutdownAction.charAt(0) == '1');
final String reason;
if (shutdownAction.length() > 1) {
reason = shutdownAction.substring(1, shutdownAction.length());
} else {
reason = null;
}
// If it's a pending reboot into recovery to apply an update,
// always make sure uncrypt gets executed properly when needed.
// If '/cache/recovery/block.map' hasn't been created, stop the
// reboot which will fail for sure, and get a chance to capture a
// bugreport when that's still feasible. (Bug: 26444951)
if (reason != null && reason.startsWith(PowerManager.REBOOT_RECOVERY_UPDATE)) {
File packageFile = new File(UNCRYPT_PACKAGE_FILE);
if (packageFile.exists()) {
String filename = null;
try {
filename = FileUtils.readTextFile(packageFile, 0, null);
} catch (IOException e) {
Slog.e(TAG, "Error reading uncrypt package file", e);
}
if (filename != null && filename.startsWith("/data")) {
if (!new File(BLOCK_MAP_FILE).exists()) {
Slog.e(TAG, "Can't find block map file, uncrypt failed or " +
"unexpected runtime restart?");
return;
}
}
}
}
ShutdownThread.rebootOrShutdown(null, reboot, reason);
}
}
依据注释,和代码名,最近一次关机操作(启动SystemServer是在开机过程中),是非正常状态,该代码将shutdown操作悬挂.(想要明白此处还要明白android的关机流程,还要努力!!!).
// Initialize the system context. |
看看createSystemContext()做了什么private void createSystemContext() {
ActivityThread activityThread = ActivityThread.systemMain();
mSystemContext = activityThread.getSystemContext();
mSystemContext.setTheme(DEFAULT_SYSTEM_THEME);
final Context systemUiContext = activityThread.getSystemUiContext();
systemUiContext.setTheme(DEFAULT_SYSTEM_THEME);
}
这里还是通过ActivityThread.java来操作,和启动一个应用进程类似,只不过应用进程是使用ActivityThread.java->main(),这里是使用ActivityThread.java->systemMain();这两者一定有区别,因为普通进程是要attach到AMS的,这里AMS还没有出生…可以对比ActivityThread.java里面的代码进行分析,对比.为了不偏离主线,这里就不深入分析了.接着往下看:
// Create the system service manager. |
启动AMS\PMS\WMS等等这些系统级的服务前,要把管理他们的服务SystemServiceManager启动起来吧,老铁,这没有问题吧~~~LocalServices.addService(SystemServiceManager.class, mSystemServiceManager);将SystemServiceManager对象保存SystemServer进程中的一个数据结构中.SystemServerInitThreadPool.get();初始化线程池,为了初始化任务能够并行处理.这些启动系统级服务的准备工作都做好了,是不是该启动服务了:// Start services.
try {
traceBeginAndSlog("StartServices");
startBootstrapServices();
startCoreServices();
startOtherServices();
SystemServerInitThreadPool.shutdown();
} catch (Throwable ex) {
Slog.e("System", "******************************************");
Slog.e("System", "************ Failure starting system services", ex);
throw ex;
} finally {
traceEnd();
}
终于到启动这些重量级的系统级的服务了,通过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.启动完成之后使用SystemServerInitThreadPool.shutdown()讲线程池关闭.接着我们注意分析startBootstrapServices(),startCoreServices(),startOtherServices()这三个函数.
首先看startBootstrapServices():/**
* 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() {
......
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();
// Activity manager runs the show.
traceBeginAndSlog("StartActivityManager");
mActivityManagerService = mSystemServiceManager.startService(
ActivityManagerService.Lifecycle.class).getService();
mActivityManagerService.setSystemServiceManager(mSystemServiceManager);
mActivityManagerService.setInstaller(installer);
traceEnd();
// 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).
traceBeginAndSlog("StartPowerManager");
mPowerManagerService = mSystemServiceManager.startService(PowerManagerService.class);
traceEnd();
// Now that the power manager has been started, let the activity manager
// initialize power management features.
traceBeginAndSlog("InitPowerManagement");
mActivityManagerService.initPowerManagement();
traceEnd();
// Bring up recovery system in case a rescue party needs a reboot
if (!SystemProperties.getBoolean("config.disable_noncore", false)) {
traceBeginAndSlog("StartRecoverySystemService");
mSystemServiceManager.startService(RecoverySystemService.class);
traceEnd();
}
// 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);
// Manages LEDs and display backlight so we need it to bring up the display.
traceBeginAndSlog("StartLightsService");
mSystemServiceManager.startService(LightsService.class);
traceEnd();
// Display manager is needed to provide display metrics before package manager
// starts up.
traceBeginAndSlog("StartDisplayManager");
mDisplayManagerService = mSystemServiceManager.startService(DisplayManagerService.class);
traceEnd();
// We need the default display before we can initialize the package manager.
traceBeginAndSlog("WaitForDisplay");
mSystemServiceManager.startBootPhase(SystemService.PHASE_WAIT_FOR_DEFAULT_DISPLAY);
traceEnd();
// 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.
if (!mRuntimeRestart) {
MetricsLogger.histogram(null, "boot_package_manager_init_start",
(int) SystemClock.elapsedRealtime());
}
traceBeginAndSlog("StartPackageManagerService");
mPackageManagerService = PackageManagerService.main(mSystemContext, installer,
mFactoryTestMode != FactoryTest.FACTORY_TEST_OFF, mOnlyCore);
mFirstBoot = mPackageManagerService.isFirstBoot();
mPackageManager = mSystemContext.getPackageManager();
traceEnd();
if (!mRuntimeRestart && !isFirstBootOrUpgrade()) {
MetricsLogger.histogram(null, "boot_package_manager_init_ready",
(int) SystemClock.elapsedRealtime());
}
// 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 {
traceEnd();
}
}
}
traceBeginAndSlog("StartUserManagerService");
mSystemServiceManager.startService(UserManagerService.LifeCycle.class);
traceEnd();
// Initialize attribute cache used to cache resources from packages.
traceBeginAndSlog("InitAttributerCache");
AttributeCache.init(mSystemContext);
traceEnd();
// Set up the Application instance for the system process and get started.
traceBeginAndSlog("SetSystemProcess");
mActivityManagerService.setSystemProcess();
traceEnd();
// DisplayManagerService needs to setup android.display scheduling related policies
// since setSystemProcess() would have overridden policies due to setProcessGroup
mDisplayManagerService.setupSchedulerPolicies();
/// M: CTA requirement - permission control @{
/// M: MOTA for CTA permissions handling
/*
* This function is used for granting CTA permissions after OTA upgrade.
* This should be placed after AMS is added to ServiceManager and before
* starting other services since granting permissions needs AMS instance
* to do permission checking.
*/
mPackageManagerService.onAmsAddedtoServiceMgr();
/// @}
// Manages Overlay packages
traceBeginAndSlog("StartOverlayManagerService");
mSystemServiceManager.startService(new OverlayManagerService(mSystemContext, installer));
traceEnd();
// The sensor service needs access to package manager service, app ops
// service, and permissions service, therefore we start it after them.
// Start sensor service in a separate thread. Completion should be checked
// before using it.
mSensorServiceStart = SystemServerInitThreadPool.get().submit(() -> {
BootTimingsTraceLog traceLog = new BootTimingsTraceLog(
SYSTEM_SERVER_TIMING_ASYNC_TAG, Trace.TRACE_TAG_SYSTEM_SERVER);
traceLog.traceBegin(START_SENSOR_SERVICE);
startSensorService();
traceLog.traceEnd();
}, START_SENSOR_SERVICE);
}
首先分析Installer installer = mSystemServiceManager.startService(Installer.class);
mSystemServiceManager是系统服务管理对象,在前面介绍的run()里面已经实例化了.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），我们需要在启动完成Installer服务之后才能启动其他的系统服务.接着可以看到ActivityManagerService,PowerManagerService,RecoverySystemService,LightsService,DisplayManagerService,UserManagerService这些服务都是使用mSystemServiceManager.startService()方法将服务启动起来.
发现PackageManagerService服务和其他的有点不一样,他是直接调用了静态方法main()方法实现了// Start the package manager.
if (!mRuntimeRestart) {
MetricsLogger.histogram(null, "boot_package_manager_init_start",
(int) SystemClock.elapsedRealtime());
}
traceBeginAndSlog("StartPackageManagerService");
mPackageManagerService = PackageManagerService.main(mSystemContext, installer,
mFactoryTestMode != FactoryTest.FACTORY_TEST_OFF, mOnlyCore);
//mFirstBoot是判断是否第一次开机的标示,isFirstBoot()是对应的方法,原理就是判断packages.xml是否存在
mFirstBoot = mPackageManagerService.isFirstBoot();
mPackageManager = mSystemContext.getPackageManager();
traceEnd();
if (!mRuntimeRestart && !isFirstBootOrUpgrade()) {
MetricsLogger.histogram(null, "boot_package_manager_init_ready",
(int) SystemClock.elapsedRealtime());
}
分析main()函数:public static PackageManagerService main(Context context, Installer installer,
boolean factoryTest, boolean onlyCore) {
// Self-check for initial settings.
PackageManagerServiceCompilerMapping.checkProperties();
PackageManagerService m = new PackageManagerService(context, installer,
factoryTest, onlyCore);
m.enableSystemUserPackages();
ServiceManager.addService("package", m);
return m;
}
这里是直接newPackageManagerService,然后调用ServiceManager.addService,通过了binder(以后再仔细研究).
这里有一段涉及vold服务的:// 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;
}
android 4.0新增的一个功能，即设备加密（encrypting the device）,该功能由系统属性vold.decrypt指定.涉及设备安全和加密的,vold(Volume Daemon)用于管理和控制Android平台外部存储设备的后台进程，这些管理和控制，包括SD卡的插拔事件检测、SD卡挂载、卸载、格式化等。这里的设置是当我们的设备处于加密状态,只启动核心服务,通过设置mOnlyCore来进行标示.
接着分析startCoreServices()函数:/**
* Starts some essential services that are not tangled up in the bootstrap process.
*/
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();
}
启动了DropBoxManagerService(系统出问题的调用栈信息),BatteryService(电池相关的服务),UsageStatsService,WebViewUpdateService.
startOtherServices方法，主要用于启动系统中其他的服务,包括Luncher的启动
总结
- SystemServer进程是android中一个很重要的进程由Zygote进程启动,是Zygote的嫡长子,如果该进程崩溃,Zygote会调用方法kill掉自己；
- 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,其实真正干活的还是在run()方法里面；
- 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；
参考文章:
http://blog.csdn.net/qq_23547831/article/details/51105171