SystemServer是Android系统的核心，APK应用中能够直接交互的大部分系统服务都在该进程中运行，常见的比如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。

startSystemServer

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",//最终的进程名            "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，并传入了一些参数来设置新进程的一些信息，如uid、gid等，函数返回后，如果是子进程pid=0，进入handleSystemServerProcess函数，如下：

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可以看到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 注意不应该运行到这里*/    }

在此函数中首先调用 closeServerSocket()关闭从父进程zygote复制来的socket服务，因为只要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，并执行run()函数，通过反射启动SystemServer的main函数。
下面是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);        //加载动态库libandroid_servers.so        System.loadLibrary("android_servers");        //init1()函数是一个native函数，通过JNI最终调用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。该函数执行完毕后，其内部会调用Java端的init2()函数，这就是为什么Java源码中没有引用init2()的地方.        init1(args);    }    //主要的系统服 务都是在init2()函数中完成的。    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()方法内部开始真正启动各种服务线程。        Thread thr = new ServerThread();        thr.setName("android.server.ServerThread");        thr.start();    }

main()函数首先会执行init1()函数,init1()函数是一个native函数，通过JNI最终调用system_init.cpp中的system_init()函数：

extern "C" status_t system_init(){    ALOGI("Entered system_init()");    sp<ProcessState> proc(ProcessState::self());    sp<IServiceManager> sm = defaultServiceManager();    ALOGI("ServiceManager: %p\n", sm.get());    sp<GrimReaper> grim = new GrimReaper();    sm->asBinder()->linkToDeath(grim, grim.get(), 0);    char propBuf[PROPERTY_VALUE_MAX];    property_get("system_init.startsurfaceflinger", propBuf, "1");    if (strcmp(propBuf, "1") == 0) {        //启动SurfaceFlinger        SurfaceFlinger::instantiate();    }    property_get("system_init.startsensorservice", propBuf, "1");    if (strcmp(propBuf, "1") == 0) {        //启动sensor service        SensorService::instantiate();    }    // And now start the Android runtime.  We have to do this bit    // of nastiness because the Android runtime initialization requires    // some of the core system services to already be started.    // All other servers should just start the Android runtime at    // the beginning of their processes's main(), before calling    // the init function.    ALOGI("System server: starting Android runtime.\n");    AndroidRuntime* runtime = AndroidRuntime::getRuntime();    ALOGI("System server: starting Android services.\n");    JNIEnv* env = runtime->getJNIEnv();    if (env == NULL) {        return UNKNOWN_ERROR;    }    //通过“com/android/server/SystemServer”找到SystemServer.java的类    jclass clazz = env->FindClass("com/android/server/SystemServer");    if (clazz == NULL) {        return UNKNOWN_ERROR;    }    //找到SystemServer.java的类的init2方法    jmethodID methodId = env->GetStaticMethodID(clazz, "init2", "()V");    if (methodId == NULL) {        return UNKNOWN_ERROR;    }    //运行init2()函数    env->CallStaticVoidMethod(clazz, methodId);    ALOGI("System server: entering thread pool.\n");    ProcessState::self()->startThreadPool();    IPCThreadState::self()->joinThreadPool();    ALOGI("System server: exiting thread pool.\n");    return NO_ERROR;}

system_init()函数回调init2()函数，init2()函数启动ServerThread线程来添加各种服务以及其他的一些初始化工作，ServerThread是一个内部类，大体代码如下；

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，增加随机数的不可预测性            Slog.i(TAG, "Entropy Mixer");            ServiceManager.addService("entropy", new EntropyMixer());//用Context.getSystemService (String name) 才获取到相应的服务    //向ServiceManager添加电源管理服务            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.");        }//开始处理消息的循环        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

android启动之SystemServer启动

startSystemServer

SystemServer.java

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