Android 系统（14）---SystemServer进程启动过程

SystemServer进程的启动

在上一篇文章Framework学习（二）Zygote进程启动过程中，我们已经知道Zygote进程会启动SystemServer进程，但具体启动流程还没有涉及，本文我们就来看看SystemServer进程具体启动过程。

首先回顾下ZygoteInit#startSystemServer()函数：

frameworks/base/core/java/com/android/internal/os/ZygoteInit.java

ZygoteInit#startSystemServer()

private static boolean startSystemServer(String abiList, String socketName) throws MethodAndArgsCaller, RuntimeException {...int pid;try {parsedArgs = new ZygoteConnection.Arguments(args); ZygoteConnection.applyDebuggerSystemProperty(parsedArgs);ZygoteConnection.applyInvokeWithSystemProperty(parsedArgs);/* 1 */// 请求fork SystemServer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);}// pid为0表示子进程，即SystemServer进程，从此SystemServer进程与Zygote进程分道扬镳if (pid == 0) {if (hasSecondZygote(abiList)) {waitForSecondaryZygote(socketName);}handleSystemServerProcess(parsedArgs); //2}return true;}

注释1处调用Zygote的forkSystemServer，主要通过fork函数在当前进程创建一个子进程(也就是SystemServer进程)，如果返回的pid 为0，也就是表示在新创建的子进程中执行的，则执行注释2处的handleSystemServerProcess，来看看handleSystemServerProcess是如何启动SystemServer进程的。

ZygoteInit#handleSystemServerProcess()

private static void handleSystemServerProcess(ZygoteConnection.Arguments parsedArgs)throws ZygoteInit.MethodAndArgsCaller {closeServerSocket();  //1if (parsedArgs.niceName != null) {Process.setArgV0(parsedArgs.niceName);  //2}...if (parsedArgs.invokeWith != null) {String[] args = parsedArgs.remainingArgs;// If we have a non-null system server class path, we'll have to duplicate the// existing arguments and append the classpath to it. ART will handle the classpath// correctly when we exec a new process.if (systemServerClasspath != null) {String[] amendedArgs = new String[args.length + 2];amendedArgs[0] = "-cp";amendedArgs[1] = systemServerClasspath;System.arraycopy(parsedArgs.remainingArgs, 0, amendedArgs, 2, parsedArgs.remainingArgs.length);}WrapperInit.execApplication(parsedArgs.invokeWith,parsedArgs.niceName, parsedArgs.targetSdkVersion,VMRuntime.getCurrentInstructionSet(), null, args);} else {ClassLoader cl = null;if (systemServerClasspath != null) {cl = createSystemServerClassLoader(systemServerClasspath,parsedArgs.targetSdkVersion);Thread.currentThread().setContextClassLoader(cl);}/** Pass the remaining arguments to SystemServer.*/RuntimeInit.zygoteInit(parsedArgs.targetSdkVersion, parsedArgs.remainingArgs, cl); //3}/* should never reach here */}

注释1处SyetemServer进程是复制了Zygote进程的地址空间，因此也会得到Zygote进程创建的Socket，这个Socket对于SyetemServer进程没有用处，所以调用closeServerSocket()关闭它。
注释2如果你看Arguments封装函数，会发现parsedArgs.niceName=system_server，在这里调用Process.setArgV0()设置进程名为：system_server。
注释3由于parsedArgs.invokeWith属性默认为null，最后调用RuntimeInit.zygoteInit来进一步启动SystemServer。

frameworks/base/core/java/com/android/internal/os/RuntimeInit.java

RuntimeInit#zygoteInit()

 public static final void zygoteInit(int targetSdkVersion, String[] argv, ClassLoader classLoader)throws ZygoteInit.MethodAndArgsCaller {if (DEBUG) Slog.d(TAG, "RuntimeInit: Starting application from zygote");Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "RuntimeInit");// 重定向Log输出redirectLogStreams();//初始化运行环境commonInit();//启动Binder线程池nativeZygoteInit(); //1//调用程序入口函数applicationInit(targetSdkVersion, argv, classLoader); //2}

注释1处调用nativeZygoteInit函数，一看函数的名称就知道调用Native层的代码，它是用来启动Binder线程池的。
注释2处调用了applicationInit函数启动应用程序。

Binder线程池启动过程

先看看启动Binder线程池。来查看nativeZygoteInit函数对应的JNI文件，如下所示。

frameworks/base/core/jni/AndroidRuntime.cpp

static const JNINativeMethod gMethods[] = {{ "nativeFinishInit", "()V",(void*) com_android_internal_os_RuntimeInit_nativeFinishInit },{ "nativeZygoteInit", "()V",(void*) com_android_internal_os_RuntimeInit_nativeZygoteInit },{ "nativeSetExitWithoutCleanup", "(Z)V",(void*) com_android_internal_os_RuntimeInit_nativeSetExitWithoutCleanup },
};

通过JNI的gMethods数组，可以看出nativeZygoteInit函数对应的是JNI文件AndroidRuntime.cpp的com_android_internal_os_RuntimeInit_nativeZygoteInit函数：

AndroidRuntime#com_android_internal_os_RuntimeInit_nativeZygoteInit()

...
static AndroidRuntime* gCurRuntime = NULL;AndroidRuntime::AndroidRuntime(char* argBlockStart, const size_t argBlockLength) :mExitWithoutCleanup(false),mArgBlockStart(argBlockStart),mArgBlockLength(argBlockLength)
{...gCurRuntime = this;
}...static void com_android_internal_os_RuntimeInit_nativeZygoteInit(JNIEnv* env, jobject clazz)
{gCurRuntime->onZygoteInit();
}

这里gCurRuntime是AndroidRuntime类型的指针，AndroidRuntime的子类AppRuntime在app_main.cpp中定义，我们来查看AppRuntime的onZygoteInit函数

frameworks/base/cmds/app_process/app_main.cpp

AppRuntime#onZygoteInit()

virtual void onZygoteInit(){sp<ProcessState> proc = ProcessState::self();ALOGV("App process: starting thread pool.\n");proc->startThreadPool();//1}

注释1处会调用ProcessState的startThreadPool函数。

frameworks/native/libs/binder/ProcessState.cpp

ProcessState#startThreadPool()

void ProcessState::startThreadPool()
{AutoMutex _l(mLock);//1if (!mThreadPoolStarted) {  mThreadPoolStarted = true;spawnPooledThread(true);}
}

支持Binder通信的进程中都有一个ProcessState类，它里面有一个mThreadPoolStarted 变量，来表示Binder线程池是否已经被启动过，默认值为false。在每次调用这个函数时都会先去检查这个标记，从而确保Binder线程池只会被启动一次。
注释1如果Binder线程池未被启动则设置mThreadPoolStarted为true，最后调用spawnPooledThread函数来创建线程池中的第一个线程，也就是线程池的main线程。

ProcessState#spawnPooledThread()

void ProcessState::spawnPooledThread(bool isMain)
{if (mThreadPoolStarted) {String8 name = makeBinderThreadName();ALOGV("Spawning new pooled thread, name=%s\n", name.string());sp<Thread> t = new PoolThread(isMain);t->run(name.string());//1}
}

可以看到Binder线程为一个PoolThread。注释1调用PoolThread的run函数来启动一个新的线程。

PoolThread

class PoolThread : public Thread
{
..
protected:virtual bool threadLoop(){IPCThreadState::self()->joinThreadPool(mIsMain);//1return false;}const bool mIsMain;
};

PoolThread类继承了Thread类。注释1处会将调用IPCThreadState的joinThreadPool函数，将当前线程注册到Binder驱动程序中，这样我们创建的线程就加入了Binder线程池中，这样新创建的SyetemServer进程就支持Binder进程间通信了。

回到RuntimeInit#zygoteInit()的注释2。

RuntimeInit#applicationInit()

 private static void applicationInit(int targetSdkVersion, String[] argv, ClassLoader classLoader)throws ZygoteInit.MethodAndArgsCaller {...// 初始化虚拟机环境VMRuntime.getRuntime().setTargetHeapUtilization(0.75f);VMRuntime.getRuntime().setTargetSdkVersion(targetSdkVersion);final Arguments args;try {args = new Arguments(argv);} catch (IllegalArgumentException ex) {Slog.e(TAG, ex.getMessage());// let the process exitreturn;}// Remaining arguments are passed to the start class's static maininvokeStaticMain(args.startClass, args.startArgs, classLoader); //1}

注释1处applicationInit函数中主要调用了invokeStaticMain函数。

RuntimeInit#invokeStaticMain()

private static void invokeStaticMain(String className, String[] argv, ClassLoader classLoader) throws ZygoteInit.MethodAndArgsCaller {Class<?> cl;try {cl = Class.forName(className, true, classLoader); //1} catch (ClassNotFoundException ex) {throw new RuntimeException("Missing class when invoking static main " + className, ex);}Method m;try {// 获取main方法m = cl.getMethod("main", new Class[] { String[].class }); //2} catch (NoSuchMethodException ex) {throw new RuntimeException("Missing static main on " + className, ex);} catch (SecurityException ex) {throw new RuntimeException("Problem getting static main on " + className, ex);}// 判断修饰符int modifiers = m.getModifiers(); //3if (! (Modifier.isStatic(modifiers) && Modifier.isPublic(modifiers))) {throw new RuntimeException("Main method is not public and static on " + className);}/** This throw gets caught in ZygoteInit.main(), which responds* by invoking the exception's run() method. This arrangement* clears up all the stack frames that were required in setting* up the process.*/throw new ZygoteInit.MethodAndArgsCaller(m, argv); //4}

注释1处传入的className就是com.android.server.SystemServer ，因此通过反射返回的cl为SystemServer类。
注释2获取main方法。
注释3判断修饰符，必须是static而且必须是public类型。
注释4有点意思，做完这一切之后，将找到的main函数传入到MethodAndArgsCaller异常中并抛出该异常。辛苦辛苦各种初始化，各种变着法的调用，最后你居然给我抛个异常！先别急，这个异常在ZygoteInit#main()方法中捕获。这么做的作用是清除应用程序进程创建过程的调用栈。

ZygoteInit#main()函数见上一篇文章，我们这里再看一下框架：

public static void main(String argv[]) {try {...startSystemServer(abiList, socketName);...} catch (MethodAndArgsCaller caller) {caller.run(); //1}
}

在注释1处调用了MethodAndArgsCaller的run函数。

MethodAndArgsCaller

public static class MethodAndArgsCaller extends Exceptionimplements 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 }); //1} 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);}}}

注释1处通过反射调用了com.android.server.SystemServer#main(String[] args)。至此，Zygote进程fork出SystemServer进程，并成功调用SystemServer#main()。

解析SyetemServer进程

我们先来查看SystemServer的main函数：

frameworks/base/services/java/com/android/server/SystemServer.java

SystemServer#main()

    public static void main(String[] args) {new SystemServer().run(); //1}

注释1处main函数中只调用了SystemServer的run函数。

SystemServer#run()

private void run() {...System.loadLibrary("android_servers");//1...mSystemServiceManager = new SystemServiceManager(mSystemContext);//2LocalServices.addService(SystemServiceManager.class, mSystemServiceManager);...    try {Trace.traceBegin(Trace.TRACE_TAG_SYSTEM_SERVER, "StartServices");startBootstrapServices();//3startCoreServices();//4startOtherServices();//5} catch (Throwable ex) {Slog.e("System", "******************************************");Slog.e("System", "************ Failure starting system services", ex);throw ex;} finally {Trace.traceEnd(Trace.TRACE_TAG_SYSTEM_SERVER);}...}

注释1处加载了libandroid_servers.so。
注释2处创建SystemServiceManager，它会对系统的服务进行创建、启动和生命周期管理。启动系统的各种服务。
注释3中的startBootstrapServices函数中用SystemServiceManager启动了ActivityManagerService、PowerManagerService、PackageManagerService等服务。
注释4处的函数中则启动了BatteryService、UsageStatsService和WebViewUpdateService。
注释5处的startOtherServices函数中则启动了CameraService、AlarmManagerService、VrManagerService等服务，这些服务的父类为SystemService。

从注释3、4、5的函数可以看出，官方把系统服务分为了三种类型，分别是引导服务、核心服务和其他服务，其中其他服务为一些非紧要和一些不需要立即启动的服务。系统服务大约有80多个，这里列出部分系统服务以及它们的作用：

继续往下看，我们这里只看看注释3如何启动引导服务，注释4、5类似。

SystemServer#startBootstrapServices()

private void startBootstrapServices() {...mPowerManagerService = mSystemServiceManager.startService(PowerManagerService.class); ...   mPackageManagerService = PackageManagerService.main(mSystemContext, installer, mFactoryTestMode != FactoryTest.FACTORY_TEST_OFF, mOnlyCore); ...
}

这里面要启动的引导服务很多，我们来看看PowerManagerService和PackageManagerService吧。

首先是PowerManagerService，它是通过SystemServiceManager.startService()来启动的：

frameworks/base/services/core/java/com/android/server/SystemServiceManager.java

SystemServiceManager#startService()

    public <T extends SystemService> T startService(Class<T> serviceClass) {try {final String name = serviceClass.getName();...final T service;try {Constructor<T> constructor = serviceClass.getConstructor(Context.class);service = constructor.newInstance(mContext);  //1} catch (InstantiationException ex) {throw new RuntimeException("Failed to create service " + name + ": service could not be instantiated", ex);} catch (IllegalAccessException ex) {throw new RuntimeException("Failed to create service " + name + ": service must have a public constructor with a Context argument", ex);} catch (NoSuchMethodException ex) {throw new RuntimeException("Failed to create service " + name + ": service must have a public constructor with a Context argument", ex);} catch (InvocationTargetException ex) {throw new RuntimeException("Failed to create service " + name + ": service constructor threw an exception", ex);}// Register it.mServices.add(service); //2// Start it.try {service.onStart();  //3} catch (RuntimeException ex) {throw new RuntimeException("Failed to start service " + name  + ": onStart threw an exception", ex);}return service;} finally {Trace.traceEnd(Trace.TRACE_TAG_SYSTEM_SERVER);}}

注释1处通过构造器创建SystemService，这里的SystemService就是PowerManagerService。
注释2处将PowerManagerService添加到mServices中，这里mServices是一个存储SystemService类型的ArrayList。
注释3处接着调用PowerManagerService的onStart函数启动PowerManagerService并返回，这样就完成了PowerManagerService启动的过程。

接下来再来看看PackageManagerService，它是通过另外一种方式启动的，直接调用了PackageManagerService的main函数：

frameworks/base/services/core/java/com/android/server/pm/PackageManagerService.java

PackageManagerService#main()

    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);  //1m.enableSystemUserPackages();// Disable any carrier apps. We do this very early in boot to prevent the apps from being// disabled after already being started.CarrierAppUtils.disableCarrierAppsUntilPrivileged(context.getOpPackageName(), m, UserHandle.USER_SYSTEM);ServiceManager.addService("package", m); //2return m;}

注释1创建PackageManagerService实例。
注释2将PackageManagerService实例注册到ServiceManager中。ServiceManager用来管理系统中的各种Service，用系统C/S架构中的Binder机制通信，Client端要使用某个Service，则需要先到ServiceManager查询Service的相关信息，然后根据Service的相关信息与Service所在的Server进程建立通讯通路，这样Client端就可以使用Service了。

总结SyetemServer进程

SyetemServer在启动时做了如下工作：

1.启动Binder线程池，这样就可以与其他进程进行通信。
2.创建SystemServiceManager用于对系统的服务进行创建、启动和生命周期管理。
3.启动各种系统服务。