3. SystemServer进程的启动_Android framework系列学习笔记

在 2. Android系统进程启动流程中我们已经知道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();  //1
        if (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);
    //1
    if (!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);//1
        return 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 exit
            return;
        }
        // Remaining arguments are passed to the start class's static main
        invokeStaticMain(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(); //3
        if (! (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 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 }); //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);//2
           LocalServices.addService(SystemServiceManager.class, mSystemServiceManager);
       ...  
        try {
           Trace.traceBegin(Trace.TRACE_TAG_SYSTEM_SERVER, "StartServices");
           startBootstrapServices();//3
           startCoreServices();//4
           startOtherServices();//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);  //1
        m.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); //2
        return 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：

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