System Server是android基本服务的提供者，是android系统运行的最基本需求，所有server运行在一个叫system_process的进程中。system_process进程是android java虚拟机启动的第一个进程。可以说，整个android系统的业务都是围绕system server而展开。所以，当system_process死掉后，系统必须重启。
1.System Server启动的入口
在System Server中，首先调用main()。然后调用了SystemServer().run()。
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);
// The system server should always load safe labels
PackageItemInfo.setForceSafeLabels(true);
// 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);
}
// 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());
// 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);
// Within the system server, when parceling exceptions, include the stack trace
Parcel.setStackTraceParceling(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);
Looper.prepareMainLooper();
Looper.getMainLooper().setSlowLogThresholdMs(
SLOW_DISPATCH_THRESHOLD_MS, SLOW_DELIVERY_THRESHOLD_MS);
// 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.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();
startCoreServices();
startOtherServices();
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);
}
}
// Loop forever.
Looper.loop();
throw new RuntimeException("Main thread loop unexpectedly exited");
}
在上述代码中，首先创建了一个system context，这个对象在整个system server中使用非常频繁。接着创建了一个SystemServiceManager实例。顾名思义，SystemServiceManager就是SystemService的管理者，管理所有的SystemService。代码中把接着SystemServiceManager的实例通过LocalServices.addService()语句设置到LocalServices中。LocalService的功能类似于SystemServiceManager，不同点在于LocalService专门用来管理system_process
进程中的SystemService，也就没有真正的跨进程通信，所以也就没有Binder 对象。
2.System Server的启动过程
接着就是启动各种系统服务，在这里分三种类型的SystemService依次启动，对应三种方法。
startBootstrapServices();
startCoreServices();
startOtherServices();
这三种方法的顺序不能混乱，一定要按照上述顺序依次执行。通过查看代码，我们发现在startBootstrapServices()中启动的Service为：
Installer
DeviceIdentifiersPolicyService
ActivityManagerService.Lifecycle.class
PowerManagerService
RecoverySystemService
LightsService
DisplayManagerService
UserManagerService
OverlayManagerService
启动这些SystemService都是通过SystemServiceManager.startService()的方式启动。启动完成后会调用SystemService的onStart()方法。
在startCoreServices()中启动的Service为：
BatteryService
UsageStatsService
WebViewUpdateService
接着就是调用startOtherServices()。在startOtherServices()中启动的service非常多。这些service不一定全部都启动，系统会依照一些配置，选择性启动某些服务。
3.三种不同的启动服务的方式
在以上的启动服务中，我们发现了三种启动方式。
第一种：
LocalServices.addService(SystemServiceManager.class, mSystemServiceManager);
第二种：
mSystemServiceManager.startService(LightsService.class);
第三种：
ServiceManager.addService("vibrator", vibrator);
这三种方式有什么不同呢？
1.类型不同
SystemServiceManager.startService()和LocalServices.addService()启动的系统服务是SystemService的子类，启动这些服务后会回调SystemService的onStart()方法。ServiceManager.addService()启动的系统服务是实现了Android IPC 的Binder的子类，这些服务启动后会调用systemReady()或systemRunning()方法。
2.SystemServiceManager.startService()和ServiceManager.addService()中启动的服务需要Binder对象，而LocalServices.addService()却没有Binder对象
3.SystemServiceManager.startService()中启动的是需要关心lifecycle events的服务，而ServiceManager.addService()和LocalServices.addService()不关心lifecycle events
4.ServiceManager.addService()启动的服务本身是实现Binder通信的子类，而一般SystemServiceManager.startService()启动的服务是某个服务的内部类且这个内部类是SystemService的子类，如BackupManagerService$Lifecycle，在启动这个Lifecycle的内部类服务时，当回调onStart()方法时通过SystemService的publishBinderService()方法应用某个服务的Binder对象，且这个Binder的实现类也是某个服务的内部类。也就是说需要启动服务，而这个服务需要关心lifecycle events，所以不能通过ServiceManager.addService()的方式启动，然后在这个服务中声明一个实现了SystemService的子类Lifecycle，来接受lifecycle events，通过SystemServiceManager.startService()的方式启动这个服务，在SystemService的回调方法onStart()中应用这个服务的Binder对象。所以通过SystemServiceManager.startService()启动的服务，实际是启动了一个即需要关心lifecycle events，也需要像ServiceManager.addService()那样需要Binder对象的服务。
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