SystemServer是Android系统的核心，APK应用中可以直接交互的大部分系统服务都在该进程中执行，常见的比方WindowManagerServer（Wms）、ActivityManagerSystemService（AmS）、 PackageManagerServer（PmS）等，这些系统服务都是以一个线程的方式存在于SystemServer进程中。
以下是ZygoteInit.java类的main函数捕获异常的代码：
systemServer是通过zygote启动的时候fork启动的，我们先看回到ZygoteInit.java类中看一下SystemServer的启动。在zygote启动过程中，通过调用startSystemServer启动systemServer：
private static boolean startSystemServer()
throws MethodAndArgsCaller, RuntimeException {
/* Hardcoded command line to start the system server */
String args[] = {
"--setuid=1000",
"--setgid=1000",
"--setgroups=1001,1002,1003,1004,1005,1006,1007,1008,1009,1010,1018,3001,3002,3003,3006,3007",
"--capabilities=130104352,130104352",
"--runtime-init",
"--nice-name=system_server",//终于的进程名
"com.android.server.SystemServer",
};
ZygoteConnection.Arguments parsedArgs = null;
int pid;
try {
parsedArgs = new ZygoteConnection.Arguments(args);
ZygoteConnection.applyDebuggerSystemProperty(parsedArgs);
ZygoteConnection.applyInvokeWithSystemProperty(parsedArgs);
/* Request to fork the system server process */
pid = Zygote.forkSystemServer(
parsedArgs.uid, parsedArgs.gid,
parsedArgs.gids,
parsedArgs.debugFlags,
null,
parsedArgs.permittedCapabilities,
parsedArgs.effectiveCapabilities);
} catch (IllegalArgumentException ex) {
throw new RuntimeException(ex);
}
/* For child process */
if (pid == 0) {
handleSystemServerProcess(parsedArgs);
}
return true;
}首先通过Zygote.forkSystemServer fork出一个子进程来，并传入了一些參数来设置新进程的一些信息，如uid、gid等，函数返回后，假设是子进程pid=0，进入handleSystemServerProcess函数，例如以下：private static void handleSystemServerProcess(
ZygoteConnection.Arguments parsedArgs)
throws ZygoteInit.MethodAndArgsCaller {
closeServerSocket();
// set umask to 0077 so new files and directories will default to owner-only permissions.
Libcore.os.umask(S_IRWXG | S_IRWXO);
//ps能够看到system 261 98 391508 49008 ffffffff 4005c880 S system_server，此处system_server就是那个parsedArgs.niceName
if (parsedArgs.niceName != null) {
Process.setArgV0(parsedArgs.niceName);
}
if (parsedArgs.invokeWith != null) {
WrapperInit.execApplication(parsedArgs.invokeWith,
parsedArgs.niceName, parsedArgs.targetSdkVersion,
null, parsedArgs.remainingArgs);
} else {
/*
* Pass the remaining arguments to SystemServer.
*/
RuntimeInit.zygoteInit(parsedArgs.targetSdkVersion, parsedArgs.remainingArgs);
}
/* should never reach here 注意不应该执行到这里*/
}在此函数中首先调用 closeServerSocket()关闭从父进程zygote复制来的socket服务，由于仅仅要zygote去监听处理socket请求就够了。然后继续执行，RuntimeInit.zygoteInit（）会通过JNI调用app_main.cpp中的onZygoteInit,启动线程池处理Binder事件 。最后通过invokeStaticMain()函数抛出异常MethodAndArgsCaller。public static class MethodAndArgsCaller extends Exception implements Runnable {
/** method to call */
private final Method mMethod;
/** argument array */
private final String[] mArgs;
public MethodAndArgsCaller(Method method, String[] args) {
mMethod = method;
mArgs = args;
}
public void run() {
try {
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);
}
}
}
这个类继承自Exception，实现了Runnable。该异常被ZygoteInit.java类的main函数catch，并运行run()函数，通过反射启动SystemServer的main函数。以下是ZygoteInit.java类的main函数捕获异常的代码：
} catch (MethodAndArgsCaller caller) {
caller.run();
}
这样我们就启动了SystemServer.java的main函数了，源代码在 (base\services\java\com\android\server)。
SystemServer.java
让我们来看看这个类中的部分代码：
/**
* This method is called from Zygote to initialize the system. This will cause the native
* services (SurfaceFlinger, AudioFlinger, etc..) to be started. After that it will call back
* up into init2() to start the Android services.
*/
native public static void init1(String[] args);
public static void main(String[] args) {
//首先检查系统时间设置和SamplingProfiler
if (System.currentTimeMillis() < EARLIEST_SUPPORTED_TIME) {
// 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.
Slog.w(TAG, "System clock is before 1970; setting to 1970.");
SystemClock.setCurrentTimeMillis(EARLIEST_SUPPORTED_TIME);
}
if (SamplingProfilerIntegration.isEnabled()) {
SamplingProfilerIntegration.start();
timer = new Timer();
timer.schedule(new TimerTask() {
@Override
public void run() {
SamplingProfilerIntegration.writeSnapshot("system_server", null);
}
}, SNAPSHOT_INTERVAL, SNAPSHOT_INTERVAL);
}
// Mmmmmm... more memory!
dalvik.system.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);
//载入动态库libandroid_servers.so
System.loadLibrary("android_servers");
//init1()函数是一个native函数，通过JNI终于调用system_init.cpp中的system_init()函数，内部会进行一些与Dalvik虚拟机相关的初始化工作。该函数运行完毕后，其内部会调用Java端的init2()函数，这就是为什么Java源代码中没有引用init2()的地方.
init1(args);
}
//基本的系统服 务都是在init2()函数中完毕的。
public static final void init2() {
Slog.i(TAG, "Entered the Android system server!");
//该函数首先创建了一个ServerThread对象，该对象是一个线程，然后直接运行该线程,从ServerThread的run()方法内部開始真正启动各种服务线程。
Thread thr = new ServerThread();
thr.setName("android.server.ServerThread");
thr.start();
}
main()函数首先会运行init1()函数,init1()函数是一个native函数，通过JNI终于调用system_init.cpp中的system_init()函数：system_init()函数回调init2()函数，init2()函数启动ServerThread线程来加入各种服务以及其它的一些初始化工作，ServerThread是一个内部类，大体代码例如以下；class ServerThread extends Thread { ... @Override public void run() { EventLog.writeEvent(EventLogTags.BOOT_PROGRESS_SYSTEM_RUN, SystemClock.uptimeMillis()); Looper.prepareMainLooper(); ... String factoryTestStr = SystemProperties.get("ro.factorytest"); int factoryTest = "".equals(factoryTestStr) ? SystemServer.FACTORY_TEST_OFF : Integer.parseInt(factoryTestStr); final boolean headless = "1".equals(SystemProperties.get("ro.config.headless", "0")); //定义变量,初始化服务，创建各种服务实例，如：电源、网络、Wifi、蓝牙，USB等 ... Context context = null; ... // Create a shared handler thread for UI within the system server. // This thread is used by at least the following components: // - WindowManagerPolicy // - KeyguardViewManager // - DisplayManagerService HandlerThread uiHandlerThread = new HandlerThread("UI"); uiHandlerThread.start(); Handler uiHandler = new Handler(uiHandlerThread.getLooper()); uiHandler.post(new Runnable() { @Override public void run() { //Looper.myLooper().setMessageLogging(new LogPrinter( // Log.VERBOSE, "WindowManagerPolicy", Log.LOG_ID_SYSTEM)); android.os.Process.setThreadPriority( android.os.Process.THREAD_PRIORITY_FOREGROUND); android.os.Process.setCanSelfBackground(false); // For debug builds, log event loop stalls to dropbox for analysis. if (StrictMode.conditionallyEnableDebugLogging()) { Slog.i(TAG, "Enabled StrictMode logging for UI Looper"); } } }); // Create a handler thread just for the window manager to enjoy. HandlerThread wmHandlerThread = new HandlerThread("WindowManager"); wmHandlerThread.start(); Handler wmHandler = new Handler(wmHandlerThread.getLooper()); wmHandler.post(new Runnable() { @Override public void run() { //Looper.myLooper().setMessageLogging(new LogPrinter( // android.util.Log.DEBUG, TAG, android.util.Log.LOG_ID_SYSTEM)); android.os.Process.setThreadPriority( android.os.Process.THREAD_PRIORITY_DISPLAY); android.os.Process.setCanSelfBackground(false); // For debug builds, log event loop stalls to dropbox for analysis. if (StrictMode.conditionallyEnableDebugLogging()) { Slog.i(TAG, "Enabled StrictMode logging for WM Looper"); } } }); // Critical services... boolean onlyCore = false; try { // Wait for installd to finished starting up so that it has a chance to // create critical directories such as /data/user with the appropriate // permissions. We need this to complete before we initialize other services. Slog.i(TAG, "Waiting for installd to be ready."); installer = new Installer(); installer.ping(); //原来的EntropyService，熵混淆，添加随机数的不可预測性 Slog.i(TAG, "Entropy Mixer"); ServiceManager.addService("entropy", new EntropyMixer());//用Context.getSystemService (String name) 才获取到对应的服务 //向ServiceManager加入电源管理服务 Slog.i(TAG, "Power Manager"); power = new PowerManagerService(); ServiceManager.addService(Context.POWER_SERVICE, power); //启动ActivityManagerService，注意返回了一个context Slog.i(TAG, "Activity Manager"); context = ActivityManagerService.main(factoryTest); //使用uiHandler、wmHandler Slog.i(TAG, "Display Manager"); display = new DisplayManagerService(context, wmHandler, uiHandler); ServiceManager.addService(Context.DISPLAY_SERVICE, display, true); ... //使用wmHandler Slog.i(TAG, "Input Manager"); inputManager = new InputManagerService(context, wmHandler); //使用uiHandler、wmHandler Slog.i(TAG, "Window Manager"); wm = WindowManagerService.main(context, power, display, inputManager, uiHandler, wmHandler, factoryTest != SystemServer.FACTORY_TEST_LOW_LEVEL, !firstBoot, onlyCore); ServiceManager.addService(Context.WINDOW_SERVICE, wm); ServiceManager.addService(Context.INPUT_SERVICE, inputManager); ActivityManagerService.setSystemProcess(); ... Slog.i(TAG, "System Content Providers"); ActivityManagerService.installSystemProviders(); ... // only initialize the power service after we have started the lights service, content providers and the battery service. power.init(context, lights, ActivityManagerService.self(), battery, BatteryStatsService.getService(), display); ... Slog.i(TAG, "Init Watchdog"); Watchdog.getInstance().init(context, battery, power, alarm, ActivityManagerService.self()); ... ActivityManagerService.self().setWindowManager(wm); ... //使用wmHandler if (context.getResources().getBoolean( com.android.internal.R.bool.config_dreamsSupported)) { try { Slog.i(TAG, "Dreams Service"); // Dreams (interactive idle-time views, a/k/a screen savers) dreamy = new DreamManagerService(context, wmHandler); ServiceManager.addService(DreamService.DREAM_SERVICE, dreamy); } catch (Throwable e) { reportWtf("starting DreamManagerService", e); } } } catch (RuntimeException e) { Slog.e("System", "******************************************"); Slog.e("System", "************ Failure starting core service", e); } ... try { Slog.i(TAG, "NetworkPolicy Service"); networkPolicy = new NetworkPolicyManagerService( context, ActivityManagerService.self(), power, networkStats, networkManagement); ServiceManager.addService(Context.NETWORK_POLICY_SERVICE, networkPolicy); } catch (Throwable e) { reportWtf("starting NetworkPolicy Service", e); } ... } ... if (safeMode) { ActivityManagerService.self().showSafeModeOverlay(); } ... //系统服务初始化准备就绪，通知各个模块 ActivityManagerService.self().systemReady(new Runnable() { public void run() { Slog.i(TAG, "Making services ready"); if (!headless) startSystemUi(contextF); try { if (mountServiceF != null) mountServiceF.systemReady(); } catch (Throwable e) { reportWtf("making Mount Service ready", e); } ... try { if (telephonyRegistryF != null) telephonyRegistryF.systemReady(); } catch (Throwable e) { reportWtf("making TelephonyRegistry ready", e); } } }); // For debug builds, log event loop stalls to dropbox for analysis. if (StrictMode.conditionallyEnableDebugLogging()) { Slog.i(TAG, "Enabled StrictMode for system server main thread."); } //開始处理消息的循环 Looper.loop(); Slog.d(TAG, "System ServerThread is exiting!"); } static final void startSystemUi(Context context) { Intent intent = new Intent(); intent.setComponent(new ComponentName("com.android.systemui", "com.android.systemui.SystemUIService")); //Slog.d(TAG, "Starting service: " + intent); context.startServiceAsUser(intent, UserHandle.OWNER); } }到这里系统ApplicationFramework层的XxxServiceManager准备就绪