SystemServer的在Android体系中所处的地位，SystemServer由Zygote fork生成的，进程名为system_server，该进程承载着framework的核心服务。startSystemServer()函数是system_server启动流程的起点， 启动流程图如下：
上图前4步骤（即颜色为紫色的流程）运行在是Zygote进程，从第5步（即颜色为蓝色的流程）ZygoteInit.handleSystemServerProcess开始是运行在新创建的system_server，这是fork机制实现的。
[–>ZygoteInit.java]
private static boolean startSystemServer(String abiList, String socketName) throws MethodAndArgsCaller, RuntimeException {
...
//参数准备
String args[] = {
"--setuid=1000",
"--setgid=1000",
"--setgroups=1001,1002,1003,1004,1005,1006,1007,1008,1009,1010,1018,1021,1032,3001,3002,3003,3006,3007",
"--capabilities=" + capabilities + "," + capabilities,
"--nice-name=system_server",
"--runtime-args",
"com.android.server.SystemServer",
};
ZygoteConnection.Arguments parsedArgs = null;
int pid;
try {
//用于解析参数，生成目标格式
parsedArgs = new ZygoteConnection.Arguments(args);
ZygoteConnection.applyDebuggerSystemProperty(parsedArgs);
ZygoteConnection.applyInvokeWithSystemProperty(parsedArgs);
// fork子进程，该进程是system_server进程
pid = Zygote.forkSystemServer(
parsedArgs.uid, parsedArgs.gid,
parsedArgs.gids,
parsedArgs.debugFlags,
null,
parsedArgs.permittedCapabilities,
parsedArgs.effectiveCapabilities);
} catch (IllegalArgumentException ex) {
throw new RuntimeException(ex);
}
//进入子进程system_server
if (pid == 0) {
if (hasSecondZygote(abiList)) {
waitForSecondaryZygote(socketName);
}
// 完成system_server进程剩余的工作
handleSystemServerProcess(parsedArgs);
}
return true;
}准备参数并fork新进程，从上面可以看出system server进程参数信息为uid=1000,gid=1000,进程名为sytem_server，从zygote进程fork新进程后，需要关闭zygote原有的socket。
[–>Zygote.java]
public static int forkSystemServer(int uid, int gid, int[] gids, int debugFlags, int[][] rlimits, long permittedCapabilities, long effectiveCapabilities) {
VM_HOOKS.preFork();
// 调用native方法fork system_server进程
int pid = nativeForkSystemServer(
uid, gid, gids, debugFlags, rlimits, permittedCapabilities, effectiveCapabilities);
if (pid == 0) {
Trace.setTracingEnabled(true);
}
VM_HOOKS.postForkCommon();
return pid;
}[–>ZygoteInit.java]
private static void handleSystemServerProcess( ZygoteConnection.Arguments parsedArgs) throws ZygoteInit.MethodAndArgsCaller {
closeServerSocket(); //关闭父进程zygote复制而来的Socket//system_server故进入此分支
RuntimeInit.zygoteInit(parsedArgs.targetSdkVersion, parsedArgs.remainingArgs, cl);
}zygoteInit
[–>RuntimeInit.java]
public static final void zygoteInit(int targetSdkVersion, String[] argv, ClassLoader classLoader) throws ZygoteInit.MethodAndArgsCaller {
nativeZygoteInit(); // zygote初始化
applicationInit(targetSdkVersion, argv, classLoader); // 应用初始化
}[–>RuntimeInit.java]
private static void applicationInit(int targetSdkVersion, String[] argv, ClassLoader classLoader) throws ZygoteInit.MethodAndArgsCaller {//调用startClass的static方法 main()
invokeStaticMain(args.startClass, args.startArgs, classLoader);
}此处args.startClass为”com.android.server.SystemServer”。
[–>RuntimeInit.java]
private static void invokeStaticMain(String className, String[] argv, ClassLoader classLoader) throws ZygoteInit.MethodAndArgsCaller {
Class<?> cl = Class.forName(className, true, classLoader);
...
Method m;
try {
m = cl.getMethod("main", new Class[] { String[].class });
} catch (NoSuchMethodException ex) {
...
} catch (SecurityException ex) {
...
}
int modifiers = m.getModifiers();
if (! (Modifier.isStatic(modifiers) && Modifier.isPublic(modifiers))) {
...
}
//通过抛出异常，回到ZygoteInit.main()。这样做好处是能清空栈帧，提高栈帧利用率。
throw new ZygoteInit.MethodAndArgsCaller(m, argv);
}[–>ZygoteInit.java]
public static void main(String argv[]) {
try {
startSystemServer(abiList, socketName);//启动system_server
....
} catch (MethodAndArgsCaller caller) {
caller.run();
} catch (RuntimeException ex) {
closeServerSocket();
throw ex;
}
}
invokeStaticMain()方法中抛出的异常MethodAndArgsCaller，从而进入caller.run()方法。
[–>ZygoteInit.java]
public static class MethodAndArgsCaller extends Exception implements Runnable {
public void run() {
try {
//根据传递过来的参数，可知此处通过反射机制调用的是SystemServer.main()方法
mMethod.invoke(null, new Object[] { mArgs });
} catch (IllegalAccessException ex) {
throw new RuntimeException(ex);
} catch (InvocationTargetException ex) {
Throwable cause = ex.getCause();
if (cause instanceof RuntimeException) {
throw (RuntimeException) cause;
} else if (cause instanceof Error) {
throw (Error) cause;
}
throw new RuntimeException(ex);}
}
} 在RuntimeInit.java中invokeStaticMain方法通过创建并抛出异常ZygoteInit.MethodAndArgsCaller，在ZygoteInit.java中的main()方法会捕捉该异常，并调用caller.run()，再通过反射便会调用到SystemServer.main()方法，该方法主要执行流程：
SystemServer.main
SystemServer.run
createSystemContext
startBootstrapServices();
startCoreServices();
startOtherServices();
Looper.loop();public final class SystemServer {
...
public static void main(String[] args) {
//先初始化SystemServer对象，再调用对象的run()方法
new SystemServer().run();
}
}
private void run() {
//当系统时间比1970年更早，就设置当前系统时间为1970年
if (System.currentTimeMillis() < EARLIEST_SUPPORTED_TIME) {
SystemClock.setCurrentTimeMillis(EARLIEST_SUPPORTED_TIME);
}//加载android_servers.so库，该库包含的源码在frameworks/base/services/目录下
System.loadLibrary("android_servers");
//检测上次关机过程是否失败，该方法可能不会返回
performPendingShutdown();
//初始化系统上下文
createSystemContext();
//创建系统服务管理
mSystemServiceManager = new SystemServiceManager(mSystemContext);
//将mSystemServiceManager添加到本地服务的成员sLocalServiceObjects
LocalServices.addService(SystemServiceManager.class, mSystemServiceManager);
//启动各种系统服务
try {
startBootstrapServices(); // 启动引导服务
startCoreServices(); // 启动核心服务
startOtherServices(); // 启动其他服务
} catch (Throwable ex) {
Slog.e("System", "************ Failure starting system services", ex);
throw ex;
}
//一直循环执行
Looper.loop();
throw new RuntimeException("Main thread loop unexpectedly exited");
}
[–>SystemServer.java]
private void createSystemContext() {
//创建system_server进程的上下文信息
ActivityThread activityThread = ActivityThread.systemMain();
mSystemContext = activityThread.getSystemContext();
//设置主题
mSystemContext.setTheme(android.R.style.Theme_DeviceDefault_Light_DarkActionBar);
}
createSystemContext()过程会创建对象有ActivityThread，Instrumentation, ContextImpl，LoadedApk，Application。
[–>SystemServer.java]
private void startBootstrapServices() {
//阻塞等待与installd建立socket通道
Installer installer = mSystemServiceManager.startService(Installer.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);
//初始化power management
mActivityManagerService.initPowerManagement();
//启动服务LightsService
mSystemServiceManager.startService(LightsService.class);
//启动服务DisplayManagerService
mDisplayManagerService = mSystemServiceManager.startService(DisplayManagerService.class);
//Phase100: 在初始化package manager之前，需要默认的显示.
mSystemServiceManager.startBootPhase(SystemService.PHASE_WAIT_FOR_DEFAULT_DISPLAY);
//当设备正在加密时，仅运行核心
String cryptState = SystemProperties.get("vold.decrypt");
if (ENCRYPTING_STATE.equals(cryptState)) {
mOnlyCore = true;
} else if (ENCRYPTED_STATE.equals(cryptState)) {
mOnlyCore = true;
}
//启动服务PackageManagerService
mPackageManagerService = PackageManagerService.main(mSystemContext, installer,
mFactoryTestMode != FactoryTest.FACTORY_TEST_OFF, mOnlyCore);
mFirstBoot = mPackageManagerService.isFirstBoot();
mPackageManager = mSystemContext.getPackageManager();
//启动服务UserManagerService，新建目录/data/user/
ServiceManager.addService(Context.USER_SERVICE, UserManagerService.getInstance());
AttributeCache.init(mSystemContext);
//设置AMS
mActivityManagerService.setSystemProcess();
//启动传感器服务
startSensorService();
}
该方法所创建的服务：ActivityManagerService, PowerManagerService, LightsService, DisplayManagerService， PackageManagerService， UserManagerService， sensor服务.
private void startCoreServices() {
//启动服务BatteryService，用于统计电池电量，需要LightService.
mSystemServiceManager.startService(BatteryService.class);
//启动服务UsageStatsService，用于统计应用使用情况
mSystemServiceManager.startService(UsageStatsService.class);
mActivityManagerService.setUsageStatsManager(
LocalServices.getService(UsageStatsManagerInternal.class));
mPackageManagerService.getUsageStatsIfNoPackageUsageInfo();
//启动服务WebViewUpdateService
mSystemServiceManager.startService(WebViewUpdateService.class);
}
启动服务BatteryService，UsageStatsService，WebViewUpdateService。
private void startOtherServices() {
...
SystemConfig.getInstance();
mContentResolver = context.getContentResolver(); // resolver
...
mActivityManagerService.installSystemProviders(); //provider
mSystemServiceManager.startService(AlarmManagerService.class); // alarm
// watchdog
watchdog.init(context, mActivityManagerService);
inputManager = new InputManagerService(context); // input
wm = WindowManagerService.main(...); // window
inputManager.start(); //启动input
mDisplayManagerService.windowManagerAndInputReady();
...
mSystemServiceManager.startService(MOUNT_SERVICE_CLASS); // mount
mPackageManagerService.performBootDexOpt(); // dexopt操作
ActivityManagerNative.getDefault().showBootMessage(...); //显示启动界面
...
statusBar = new StatusBarManagerService(context, wm); //statusBar
//dropbox
ServiceManager.addService(Context.DROPBOX_SERVICE,
new DropBoxManagerService(context, new File("/data/system/dropbox")));
mSystemServiceManager.startService(JobSchedulerService.class); //JobScheduler
lockSettings.systemReady(); //lockSettings
//phase480 和phase500
mSystemServiceManager.startBootPhase(SystemService.PHASE_LOCK_SETTINGS_READY);
mSystemServiceManager.startBootPhase(SystemService.PHASE_SYSTEM_SERVICES_READY);
...
// 准备好window, power, package, display服务
wm.systemReady();
mPowerManagerService.systemReady(...);
mPackageManagerService.systemReady();
mDisplayManagerService.systemReady(...);
mActivityManagerService.systemReady(new Runnable() {
public void run() {
...
}
});
}
SystemServer启动各种服务中最后的一个环节便是AMS.systemReady()，ActivityManagerService启动过程。
到此, System_server主线程的启动工作完成, 进入Looper.loop()状态,等待其他线程通过handler发送消息到主线再处理.