前言

SystemServer进程是zygote进程启动后，主动“分裂”的第一个进程。它负责启动大量的Android系统核心服务，其重要性不言而喻。一旦该进程崩溃，整个Android系统将重新启动。

开篇

核心源码

概述

       SystemServer是什么？它是Android Java的两大支柱之一。另外一个支柱是专门负责孵化Java进程的Zygote（Zygote进程是整个android系统的根进程）。这两个支柱倒了任何一个，都会导致Android Java的崩溃（所有由Zygote孵化的Java进程都会被销毁，而SystemServer就是由Zygote孵化而来）。
       若Android Java真的崩溃了，那么Linux系统中的进程init会重新启动“两大支柱”以重建Android Java。
       SystemServer和系统服务有着重要的关系，Android系统中几乎所有的核心服务都是在这个进程中，如：ActivityManagerService、PowerManagerService和WindowManagerService等。当我们的应用需要使用各种系统服务的时候，其实也是通过与SystemServer进程通讯获取各种服务对象的句柄，进而执行相应的操作。

Read The Fucking Code

       SystemServer是由Zygote孵化而来的一个进程，通过ps命令，我们发现其进程名为：system_server。

启动SystemServer进程

在分析zygote进程时，我们知道当zygote进程进入到java世界后，在ZygoteInit.java中，将调用startSystemServer函数启动SystemServer进程，其关键代码是：

            if (startSystemServer) {
                Runnable r = forkSystemServer(abiList, socketName, zygoteServer);                 // fork出system server

                // {@code r == null} in the parent (zygote) process, and {@code r != null} in the
                // child (system_server) process.
                if (r != null) {
                    r.run();
                    return;
                }
            }

我们看下forkSystemServer()：

// 我们这边挑出重要代码讲解，详细代码分析请参考Zygote篇
/* Request to fork the system server process */
pid = Zygote.forkSystemServer(
        parsedArgs.uid, parsedArgs.gid,
        parsedArgs.gids,
        parsedArgs.debugFlags,
        null,
        parsedArgs.permittedCapabilities,
        parsedArgs.effectiveCapabilities);

老样子，源码继续跟下去：

    public static int forkSystemServer(int uid, int gid, int[] gids, int debugFlags,
            int[][] rlimits, long permittedCapabilities, long effectiveCapabilities) {
        VM_HOOKS.preFork();
        // Resets nice priority for zygote process.
        resetNicePriority();
        int pid = nativeForkSystemServer(
                uid, gid, gids, debugFlags, rlimits, permittedCapabilities, effectiveCapabilities);
        // Enable tracing as soon as we enter the system_server.
        if (pid == 0) {
            Trace.setTracingEnabled(true, debugFlags);
        }
        VM_HOOKS.postForkCommon();
        return pid;
    }

容易看出，该函数通过调用native方法，完成实际的创建操作。该Native方法定义于frameworks/base/core/jni/com_android_internal_os_Zygote.cpp中。 我们来看看对应的native函数。

static jint com_android_internal_os_Zygote_nativeForkSystemServer(
        JNIEnv* env, jclass, uid_t uid, gid_t gid, jintArray gids,
        jint debug_flags, jobjectArray rlimits, jlong permittedCapabilities,
        jlong effectiveCapabilities) {
        
  // 进行实际的“分裂”工作
  pid_t pid = ForkAndSpecializeCommon(env, uid, gid, gids,
                                      debug_flags, rlimits,
                                      permittedCapabilities, effectiveCapabilities,
                                      MOUNT_EXTERNAL_DEFAULT, NULL, NULL, true, NULL,
                                      NULL, NULL, NULL);
  if (pid > 0) {
      // The zygote process checks whether the child process has died or not.
      ALOGI("System server process %d has been created", pid);
      
      // 这里SystemServer进程已经创建出来，pid > 0 说明在父进程中
      // 将子进程SystemServer的pid存在zygote进程的全局变量中
      gSystemServerPid = pid;
      
      // There is a slight window that the system server process has crashed
      // but it went unnoticed because we haven't published its pid yet. So
      // we recheck here just to make sure that all is well.
      int status;
      if (waitpid(pid, &status, WNOHANG) == pid) {
          // 小概率，SystemServer进程刚创建，就crash；此时需要重启zygote
          ALOGE("System server process %d has died. Restarting Zygote!", pid);
          RuntimeAbort(env, __LINE__, "System server process has died. Restarting Zygote!");
      }

      // Assign system_server to the correct memory cgroup.
      if (!WriteStringToFile(StringPrintf("%d", pid), "/dev/memcg/system/tasks")) {
        ALOGE("couldn't write %d to /dev/memcg/system/tasks", pid);
      }
  }
  return pid;
}

上述代码中，实际的“分裂”工作，由函数ForAndSpecializeCommon完成。

// Utility routine to fork zygote and specialize the child process.
static pid_t ForkAndSpecializeCommon(JNIEnv* env, uid_t uid, gid_t gid, jintArray javaGids,
                                     jint debug_flags, jobjectArray javaRlimits,
                                     jlong permittedCapabilities, jlong effectiveCapabilities,
                                     jint mount_external,
                                     jstring java_se_info, jstring java_se_name,
                                     bool is_system_server, jintArray fdsToClose,
                                     jintArray fdsToIgnore,
                                     jstring instructionSet, jstring dataDir) {
  SetSigChldHandler();               // 注册信号监听器
  ... ...

  pid_t pid = fork();

  if (pid == 0) {
    ... ...                          // 根据传入参数进行对应的处理，例如设置进程名，设置各种id（用户id，组id）等

    UnsetSigChldHandler();           // 反注册掉信号监听器
    ... ...
    
  } else if (pid > 0) { 
    ... ...
    
  }
  return pid;
}

从上面的代码可以看出，ForkAndSpecializeCommon最终是通过fork的方式，分裂出子进程。
这里需要关注一下的是，在zygote进程fork之前，调用SetSigChldHandler函数注册了一个子进程信号监听器。由于子进程共享父进程中的堆及栈信息，因此在子进程中也会有相应的信号处理器。
为了避免该信号监听器对子进程的影响，可以看到在子进程中进行了UnsetSigChldHandler的操作。

分别看一下SetSigChldHandler和UnsetSigChldHandler所作工作！

SetSigChldHandler

我们看看SetSigChldHandler进行了哪些操作。

// Configures the SIGCHLD handler for the zygote process. This is configured
// very late, because earlier in the runtime we may fork() and exec()
// other processes, and we want to waitpid() for those rather than
// have them be harvested immediately.
//
// This ends up being called repeatedly before each fork(), but there's
// no real harm in that.
static void SetSigChldHandler() {
  struct sigaction sa;
  memset(&sa, 0, sizeof(sa));
  sa.sa_handler = SigChldHandler;

  // 该信号监听器关注子进程结束，对应的处理函数为SigChldHandler
  int err = sigaction(SIGCHLD, &sa, NULL);
  if (err < 0) {
    ALOGW("Error setting SIGCHLD handler: %s", strerror(errno));
  }
}

从上面的代码可以看出，SetSigChldHandler函数将注册一个信号处理器，来监听子进程的死亡。当子进程死亡后，利用SigChldHandler进行操作。需要注意的是，zygote的信号监听器，关注的是zygote所有的子进程，而不只是SystemServer进程（每次创建一个新的进程时，zygote都会注册对应的监听器）。

那就继续分析一下SigChldHandler吧：

static void SigChldHandler(int /*signal_number*/) {
  pid_t pid;
  int status;
  ... ...
  
  while ((pid = waitpid(-1, &status, WNOHANG)) > 0) {
    // 通过status判断子进程结束的原因，并打印相应的log
    ... ...
  
    // If the just-crashed process is the system_server, bring down zygote
    // so that it is restarted by init and system server will be restarted
    // from there.
    if (pid == gSystemServerPid) {     // 上文已经介绍过，gSystemServerPid中记录了SystemServer的pid
      ... ...
      
      kill(getpid(), SIGKILL);         // 如果结束的子进程为SystemServer，Zygote也将结束自己
    }
  }
  ... ...
  
}

发现没？所有zygote的子进程中，zygote只关心了SystemServer的死活。当其它子进程crash时，zygote只打印了log信息。

UnsetSigChldHandler

最后看看UnsetSigChldHandler函数：

// Sets the SIGCHLD handler back to default behavior in zygote children.
static void UnsetSigChldHandler() {
  struct sigaction sa;
  memset(&sa, 0, sizeof(sa));
  sa.sa_handler = SIG_DFL;

  int err = sigaction(SIGCHLD, &sa, NULL);
  if (err < 0) {
    ALOGW("Error unsetting SIGCHLD handler: %s", strerror(errno));
  }
}

SystemServer工作流程

在分析zygote进程时，我们知道当ZygoteInit.java的startSystemServer函数，通过fork创建出SystemServer进程后，SystemServer进程调用handleSystemServerProcess函数，开始执行自己的工作。

handleSystemServerProcess

        /* For child process */
        if (pid == 0) {
            if (hasSecondZygote(abiList)) {
                waitForSecondaryZygote(socketName);
            }

            zygoteServer.closeServerSocket();               // 关闭从zygote进程那里继承下来server socket
            return handleSystemServerProcess(parsedArgs);
        }

接下来，我们来看看handleSystemServerProcess函数的主要内容。

    /**
     * Finish remaining work for the newly forked system server process.
     */
    private static Runnable handleSystemServerProcess(ZygoteConnection.Arguments parsedArgs) {
        // set umask to 0077 so new files and directories will default to owner-only permissions.
        Os.umask(S_IRWXG | S_IRWXO);

        if (parsedArgs.niceName != null) {
            Process.setArgV0(parsedArgs.niceName);
        }

        // 加载SystemServer对应的文件
        final String systemServerClasspath = Os.getenv("SYSTEMSERVERCLASSPATH");
        if (systemServerClasspath != null) {
            performSystemServerDexOpt(systemServerClasspath);
            ... ...

        if (parsedArgs.invokeWith != null) {
            ... ...
        } else {
            // 利用systemServerClass对应的路径构建对应的ClassLoader
            ClassLoader cl = null;
            if (systemServerClasspath != null) {
                cl = createPathClassLoader(systemServerClasspath, parsedArgs.targetSdkVersion);

                Thread.currentThread().setContextClassLoader(cl);
            }

            /*
             * Pass the remaining arguments to SystemServer.
             */
            // 将剩余参数及classLoader递交给ZygoteInit的zygoteInit函数
            return ZygoteInit.zygoteInit(parsedArgs.targetSdkVersion, parsedArgs.remainingArgs, cl);
        }

        /* should never reach here */
    }

从上面的代码可以看出，接下来的流程进入到ZygoteInit的zygoteInit函数。zygoteInit函数将根据classLoader和参数，完成不同进程所需要的初始化工作（SystemServer进程与zygote的其它子进程均将使用zygoteInit函数）。

zygoteInit

    public static final Runnable zygoteInit(int targetSdkVersion, String[] argv, ClassLoader classLoader) {
        if (RuntimeInit.DEBUG) {
            Slog.d(RuntimeInit.TAG, "RuntimeInit: Starting application from zygote");
        }

        Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "ZygoteInit");
        RuntimeInit.redirectLogStreams();

        RuntimeInit.commonInit();
        ZygoteInit.nativeZygoteInit();
        return RuntimeInit.applicationInit(targetSdkVersion, argv, classLoader);
    }

commonInit

commonInit主要进行一些常规初始化。由于自己是做通信的，所以比较关注的是创建UA(user agent): frameworks/base/core/java/com/android/internal/os/RuntimeInit.java

    protected static final void commonInit() {

        ... ...

        /*
         * Sets the default HTTP User-Agent used by HttpURLConnection.
         */
        String userAgent = getDefaultUserAgent();
        System.setProperty("http.agent", userAgent);

        ... ...
    }

User-Agent是Http协议中的一部分，属于头域的组成部分，是一种向访问网站提供你所使用的浏览器类型、操作系统、浏览器内核等信息的标识。通过这个标识，用户所访问的网站可以显示不同的排版，从而为用户提供更好的体验或者进行信息统计。

nativeZygoteInit

函数nativeZyoteInit实现在frameworks/base/core/jni/AndroidRuntime.cpp中，主要用于为Binder通信打下基础。

static void com_android_internal_os_ZygoteInit_nativeZygoteInit(JNIEnv* env, jobject clazz)
{
    gCurRuntime->onZygoteInit();
}

这里需要关注的是，SystemServer进程中的gCurRuntime指的是什么呢？

实际上在zygote进程启动时，在app_main.cpp的main函数中，创建出了AppRuntime：

int main(int argc, char* const argv[])
{
    ........
    AppRuntime runtime(argv[0], computeArgBlockSize(argc, argv));
    ........

AppRuntime定义如下：

class AppRuntime : public AndroidRuntime
{
public:
    AppRuntime(char* argBlockStart, const size_t argBlockLength)
        : AndroidRuntime(argBlockStart, argBlockLength)
        , mClass(NULL)
    {
    }
    ... ...
}

看看AppRuntime的父类AndroidRuntime：

AndroidRuntime::AndroidRuntime(char* argBlockStart, const size_t argBlockLength) :
        mExitWithoutCleanup(false),
        mArgBlockStart(argBlockStart),
        mArgBlockLength(argBlockLength)
{
    SkGraphics::Init();

    // Pre-allocate enough space to hold a fair number of options.
    mOptions.setCapacity(20);

    assert(gCurRuntime == NULL);        // one per process
    gCurRuntime = this;
}

从代码可以看出，AndroidRuntime初始化时定义了gCurRuntime。gCurRuntime指向对象自身，也就是说gCurRuntime指向的是AppRuntime对象。

由于SystemServer进程由zygote进程fork出来，于是system server进程中也存在gCurRuntime对象，类型为AppRuntime。至此我们知道，Native函数中gCurRuntime->onZygoteInit将调用AppRuntime中的onZygoteInit。

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

onZygoteInit的用途是启动一个线程，用于binder通信。

applicationInit

    protected static Runnable applicationInit(int targetSdkVersion, String[] argv,
            ClassLoader classLoader) {
        // 设置一些进程退出的处理策略，可用堆栈上限等
        ... ...
            
        // Remaining arguments are passed to the start class's static main
        return findStaticMain(args.startClass, args.startArgs, classLoader);
    }

继续分析findStaticMain函数：

    private static Runnable findStaticMain(String className, String[] argv,
            ClassLoader classLoader) {
        // className为进行初始化工作的进程类名
        // 在SystemServer初始化时，为com.android.server.SystemServer
        Class<?> cl;

        try {
            // 下面就是通过反射得到对应类的main方法
            cl = Class.forName(className, true, classLoader);
        } catch (ClassNotFoundException ex) {
            throw new RuntimeException(
                    "Missing class when invoking static main " + className,
                    ex);
        }

        ... ...

        /*
         * 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.
         */
        return new MethodAndArgsCaller(m, argv);    // 捕获MethodAndArgsCaller异常
    }

main函数

接下来就进入了SystemServer.java的main函数，其代码如下：

    /**
     * The main entry point from zygote.
     */
    public static void main(String[] args) {
        new SystemServer().run();        // 创建并运行，简单粗暴！
    }

这里比较简单，只是new出一个SystemServer对象并执行其run方法：

    private void run() {
        try {
            traceBeginAndSlog("InitBeforeStartServices");
            // 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.
            // 如何系统时钟早于1970年，则设置系统始终从1970年开始
            if (System.currentTimeMillis() < EARLIEST_SUPPORTED_TIME) {
                Slog.w(TAG, "System clock is before 1970; setting to 1970.");
                // private static final long EARLIEST_SUPPORTED_TIME = 86400 * 1000;
                SystemClock.setCurrentTimeMillis(EARLIEST_SUPPORTED_TIME);
            }
            
            ... ...
            
            // If the system has "persist.sys.language" and friends set, replace them with
            // "persist.sys.locale". Note that the default locale at this point is calculated
            // using the "-Duser.locale" command line flag. That flag is usually populated by
            // AndroidRuntime using the same set of system properties, but only the system_server
            // and system apps are allowed to set them.
            //
            // NOTE: Most changes made here will need an equivalent change to
            // core/jni/AndroidRuntime.cpp
            if (!SystemProperties.get("persist.sys.language").isEmpty()) {
                // 设置区域，语言等选项            
                final String languageTag = Locale.getDefault().toLanguageTag();

                SystemProperties.set("persist.sys.locale", languageTag);
                SystemProperties.set("persist.sys.language", "");
                SystemProperties.set("persist.sys.country", "");
                SystemProperties.set("persist.sys.localevar", "");
            }
            
            // Mmmmmm... more memory!
            // 清除vm内存增长上限，由于启动过程需要较多的虚拟机内存空间
            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.
            // 设置堆栈利用率，GC后会重新计算堆栈空间大小
            VMRuntime.getRuntime().setTargetHeapUtilization(0.8f);                         
            
            // Some devices rely on runtime fingerprint generation, so make sure
            // we've defined it before booting further.
            // 针对部分设备依赖于运行时就产生指纹信息，因此需要在开机完成前已经定义
            Build.ensureFingerprintProperty();                                             
            
            // Within the system server, it is an error to access Environment paths without
            // explicitly specifying a user.
            // 访问环境变量前，需要明确地指定用户
            Environment.setUserRequired(true);                                             
            
            ... ...
            
            // Initialize native services.
            // 加载动态库libandroid_services.so
            System.loadLibrary("android_servers");                                         
            
            // Check whether we failed to shut down last time we tried.
            // This call may not return.
            // 检测上次关机过程是否失败，该方法可能不会返回
            performPendingShutdown();                                                        
            
            // Initialize the system context.
            // 在SystemServer进程中也需要创建Context对象，初始化系统上下文
            createSystemContext();                                                         
            
            // Create the system service manager.
            // 通过SystemServiceManager的构造方法创建了一个新的SystemServiceManager对象
            mSystemServiceManager = new SystemServiceManager(mSystemContext);  
            
            // SystemServer进程主要是用来构建系统各种service服务，而SystemServiceManager就是这些服务的管理对象            
            mSystemServiceManager.setRuntimeRestarted(mRuntimeRestart);           
            
            // 将SystemServiceManager对象保存到SystemServer进程中的一个数据结构中            
            LocalServices.addService(SystemServiceManager.class, mSystemServiceManager);   
            
            // Prepare the thread pool for init tasks that can be parallelized
            SystemServerInitThreadPool.get();
        } finally {
            traceEnd();  // InitBeforeStartServices
        }

        // Start services.
        try {                              // 分种类启动不同的system service
            // 主要用于启动系统Boot级服务
            startBootstrapServices(); 
            
            // 主要用于启动系统核心的服务            
            startCoreServices();    
            
            // 主要用于启动一些非紧要或者非需要及时启动的服务            
            startOtherServices();     
            
            SystemServerInitThreadPool.shutdown();
        } catch (Throwable ex) {
            Slog.e("System", "******************************************");
            Slog.e("System", "************ Failure starting system services", ex);
            throw ex;
        } finally {
            traceEnd();
        }
        
        ... ...
        
        // Loop forever.
        // //启动looper，以处理到来的消息，一直循环执行
        Looper.loop();                                                                       
        throw new RuntimeException("Main thread loop unexpectedly exited");
    }

       以上就是SystemServer的run函数整个流程，我们做个简化，如下：

    private void run() {
        try {
            // Initialize the system context.
            createSystemContext();                                            // 🍁🍁🍁 01.初始化系统上下文 🍁🍁🍁
            
            // Create the system service manager.                                          
            mSystemServiceManager = new SystemServiceManager(mSystemContext); // 🍁🍁🍁 02.创建系统服务管理 🍁🍁🍁
            mSystemServiceManager.setRuntimeRestarted(mRuntimeRestart);                  
            LocalServices.addService(SystemServiceManager.class, mSystemServiceManager);   
        } finally {
            traceEnd();  // InitBeforeStartServices
        }

        // Start services.                                                    // 🍁🍁🍁 03.启动系统各种服务 🍁🍁🍁
        try {
            startBootstrapServices();                                         // 启动引导服务
            startCoreServices();                                              // 启动核心服务
            startOtherServices();                                             // 启动其他服务
            SystemServerInitThreadPool.shutdown();
        } catch (Throwable ex) {
            Slog.e("System", "******************************************");
            Slog.e("System", "************ Failure starting system services", ex);
            throw ex;
        } finally {
            traceEnd();
        }
        
        ... ...
        
        // Loop forever.
        Looper.loop();                                                        // 一直循环执行  
        throw new RuntimeException("Main thread loop unexpectedly exited");
    }

       OK，接下来我们针对SystemServer所做的三部分工作，进行逐个分析！

初始化上下文

    private void run() {
        try {
            // Initialize the system context.
            createSystemContext();                                            // 🍁🍁🍁 01.初始化系统上下文 🍁🍁🍁

       跟踪createSystemContext函数：

    private void createSystemContext() {
        ActivityThread activityThread = ActivityThread.systemMain();
        mSystemContext = activityThread.getSystemContext();
        mSystemContext.setTheme(DEFAULT_SYSTEM_THEME);

        final Context systemUiContext = activityThread.getSystemUiContext();
        systemUiContext.setTheme(DEFAULT_SYSTEM_THEME);
    }

       跟踪systemMain函数：

    public static ActivityThread systemMain() {
        // The system process on low-memory devices do not get to use hardware
        // accelerated drawing, since this can add too much overhead to the
        // process.
        if (!ActivityManager.isHighEndGfx()) {
            ThreadedRenderer.disable(true);                                   // 对于低内存的设备，禁用硬件加速
        } else {
            ThreadedRenderer.enableForegroundTrimming();
        }
        ActivityThread thread = new ActivityThread();
        thread.attach(true);
        return thread;
    }
    
    ActivityThread() {
        mResourcesManager = ResourcesManager.getInstance();                   // 使用单例模式获得一个ResourcesManager实例
    }

       继续跟踪attach函数：

    private void attach(boolean system) {
        sCurrentActivityThread = this;
        mSystemThread = system;
        if (!system) {
            ... ...
        } else {
            // Don't set application object here -- if the system crashes,
            // we can't display an alert, we just want to die die die.
            // 设置SystemServer进程在DDMS中显示的名字为"system_process" 
            android.ddm.DdmHandleAppName.setAppName("system_process",                      
                    UserHandle.myUserId());         // 如不设置，则显示"?"，无法调试该进程
            try {
                mInstrumentation = new Instrumentation();
                // 首先通过getSystemContext()创建系统上下文，然后创建应用上下文
                ContextImpl context = ContextImpl.createAppContext(                        
                        this, getSystemContext().mPackageInfo);
                // 创建Application
                mInitialApplication = context.mPackageInfo.makeApplication(true, null);
                // 调用Application的onCreate()                
                mInitialApplication.onCreate();                                            
            } catch (Exception e) {
                throw new RuntimeException(
                        "Unable to instantiate Application():" + e.toString(), e);
            }
        }

        // add dropbox logging to libcore
        DropBox.setReporter(new DropBoxReporter());

        ViewRootImpl.ConfigChangedCallback configChangedCallback
                = (Configuration globalConfig) -> {
            synchronized (mResourcesManager) {
                // We need to apply this change to the resources immediately, because upon returning
                // the view hierarchy will be informed about it.
                if (mResourcesManager.applyConfigurationToResourcesLocked(globalConfig,
                        null /* compat */)) {
                    updateLocaleListFromAppContext(mInitialApplication.getApplicationContext(),
                            mResourcesManager.getConfiguration().getLocales());

                    // This actually changed the resources! Tell everyone about it.
                    if (mPendingConfiguration == null
                            || mPendingConfiguration.isOtherSeqNewer(globalConfig)) {
                        mPendingConfiguration = globalConfig;
                        sendMessage(H.CONFIGURATION_CHANGED, globalConfig);
                    }
                }
            }
        };
        // 添加回调
        ViewRootImpl.addConfigCallback(configChangedCallback);                             
    }

可以看出，attach主要做了三件事：
       （1）创建系统上下文：getSystemContext() –> createSystemContext() –> new ContextImpl()
       （2）创建应用上下文 ContextImpl.createAppContext() –> new ContextImpl()
       （3）添加回调configChangedCallback到ViewRootImpl

创建系统上下文

getSystemContext():

    public ContextImpl getSystemContext() {
        synchronized (this) {
            if (mSystemContext == null) {
                mSystemContext = ContextImpl.createSystemContext(this);
            }
            return mSystemContext;
        }
    }

跟踪createSystemContext函数：

    static ContextImpl createSystemContext(ActivityThread mainThread) {
        // 这边new出来的LoadedApk将作为创建应用上下文的参数packageInfo
        LoadedApk packageInfo = new LoadedApk(mainThread);             
        // ContextImpl()创建系统上下文         
        ContextImpl context = new ContextImpl(null, mainThread, packageInfo, null, null, null, 0,                 
                null);
        context.setResources(packageInfo.getResources());
        context.mResources.updateConfiguration(context.mResourcesManager.getConfiguration(),
                context.mResourcesManager.getDisplayMetrics());
        return context;
    }

创建应用上下文

ContextImpl.createAppContext()：

    static ContextImpl createAppContext(ActivityThread mainThread, LoadedApk packageInfo) {
        if (packageInfo == null) throw new IllegalArgumentException("packageInfo");
        // ContextImpl()创建应用上下文
        ContextImpl context = new ContextImpl(null, mainThread, packageInfo, null, null, null, 0,                 
                null);
        context.setResources(packageInfo.getResources());
        return context;
    }

我们可以看出：new ContextImpl时，系统上下文和应用上下文的参数是一样的，createAppContext()中的参数packageInfo，就是createSystemContext()中new的LoadedApk。
创建完成之后，系统上下文赋值给了ActivityThread的成员变量mSystemContext，而应用上下文只是作为函数中的局部变量临时使用。
我们回顾一下创建上下文的代码：

            try {
                mInstrumentation = new Instrumentation();
                // 首先通过getSystemContext()创建系统上下文，然后创建应用上下文
                ContextImpl context = ContextImpl.createAppContext(                        
                        this, getSystemContext().mPackageInfo);
                // 创建Application
                mInitialApplication = context.mPackageInfo.makeApplication(true, null);    
                // 调用Application的onCreate()
                mInitialApplication.onCreate();                                            

接下来就继续看下创建Application的流程：

创建Application

    public Application makeApplication(boolean forceDefaultAppClass,
            Instrumentation instrumentation) {
        if (mApplication != null) {
            return mApplication;
        }

        Application app = null;

        String appClass = mApplicationInfo.className;
        // 参数forceDefaultAppClass为true 
        if (forceDefaultAppClass || (appClass == null)) {                                  
            appClass = "android.app.Application";
        }

        try {
            java.lang.ClassLoader cl = getClassLoader();
            // 此LoadedApk对象是createSystemContext时new的，mPackageName="android"
            if (!mPackageName.equals("android")) {                                         
                initializeJavaContextClassLoader();
            }
            // 又创建了一个局部应用上下文
            ContextImpl appContext = ContextImpl.createAppContext(mActivityThread, this);  
            // 创建Application 
            app = mActivityThread.mInstrumentation.newApplication(                         
                    cl, appClass, appContext);
            appContext.setOuterContext(app);
        } catch (Exception e) {
            ... ...
        }

        // 将前面创建的app添加到应用列表
        mActivityThread.mAllApplications.add(app);
        mApplication = app;
        
        ... ...

        return app;
    }

这一步主要创建了一个ActivityThread对象，然后执行了该对象的attach()方法，attach()方法中创建了系统上下文mSystemContext（类型为ContextImpl），并创建Application对象。
系统上下文中，new了一个LoadedApk的成员变量，并将ActivityThread对象传给LoadedApk成员，后面的Application对象就是LoadedApk使用ActivityThread创建的，LoadedApk创建了Application对象后，将Application添加到ActivityThread的应用列表中。

创建系统服务管理

我们回顾下相关代码：

            // Create the system service manager.
            mSystemServiceManager = new SystemServiceManager(mSystemContext);
            mSystemServiceManager.setRuntimeRestarted(mRuntimeRestart);
            LocalServices.addService(SystemServiceManager.class, mSystemServiceManager);

这一步比较简单，只是new了一个SystemServiceManager，并将其添加到本地服务列表中。mSystemContext为第一步中创建的系统上下文。本地服务列表是以类为key保存的一个列表，即列表中某种类型的对象最多只能有一个。

public class SystemServiceManager {
    ... ...

    // Services that should receive lifecycle events.
    // 系统服务列表，系统服务必须继承SystemService
    private final ArrayList<SystemService> mServices = new ArrayList<SystemService>();

    // 当前处于开机过程的哪个阶段，SystemService.PHASE_XXXXX
    private int mCurrentPhase = -1;

    SystemServiceManager(Context context) {
        mContext = context;
    }

    /**
     * Starts a service by class name.
     *
     * @return The service instance.
     */
    @SuppressWarnings("unchecked")
    // 通过类名启动系统服务，可能会找不到类而抛异常
    public SystemService startService(String className) {
        final Class<SystemService> serviceClass;
        try {
            serviceClass = (Class<SystemService>)Class.forName(className);
        } catch (ClassNotFoundException ex) {
            Slog.i(TAG, "Starting " + className);
            throw new RuntimeException("Failed to create service " + className
                    + ": service class not found, usually indicates that the caller should "
                    + "have called PackageManager.hasSystemFeature() to check whether the "
                    + "feature is available on this device before trying to start the "
                    + "services that implement it", ex);
        }
        return startService(serviceClass);
    }

    /**
     * Creates and starts a system service. The class must be a subclass of
     * {@link com.android.server.SystemService}.
     *
     * @param serviceClass A Java class that implements the SystemService interface.
     * @return The service instance, never null.
     * @throws RuntimeException if the service fails to start.
     */
    @SuppressWarnings("unchecked")
    // 创建并启动系统服务，系统服务类必须继承SystemService
    public <T extends SystemService> T startService(Class<T> serviceClass) {
        try {
            final String name = serviceClass.getName();
            Slog.i(TAG, "Starting " + name);
            Trace.traceBegin(Trace.TRACE_TAG_SYSTEM_SERVER, "StartService " + name);

            // Create the service.
            if (!SystemService.class.isAssignableFrom(serviceClass)) {
                throw new RuntimeException("Failed to create " + name
                        + ": service must extend " + SystemService.class.getName());
            }
            final T service;
            try {
                Constructor<T> constructor = serviceClass.getConstructor(Context.class);
                service = constructor.newInstance(mContext);
            } 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);
            }

            startService(service);
            return service;
        } finally {
            Trace.traceEnd(Trace.TRACE_TAG_SYSTEM_SERVER);
        }
    }

    public void startService(@NonNull final SystemService service) {
        // Register it.
        mServices.add(service);
        // Start it.
        long time = SystemClock.elapsedRealtime();
        try {
            service.onStart();
        } catch (RuntimeException ex) {
            throw new RuntimeException("Failed to start service " + service.getClass().getName()
                    + ": onStart threw an exception", ex);
        }
        warnIfTooLong(SystemClock.elapsedRealtime() - time, service, "onStart");
    }

    /**
     * Starts the specified boot phase for all system services that have been started up to
     * this point.
     *
     * @param phase The boot phase to start.
     */
    // 通知系统服务到了开机的哪个阶段，会遍历调用所有系统服务的onBootPhase()函数
    public void startBootPhase(final int phase) {
        if (phase <= mCurrentPhase) {
            throw new IllegalArgumentException("Next phase must be larger than previous");
        }
        mCurrentPhase = phase;

        Slog.i(TAG, "Starting phase " + mCurrentPhase);
        try {
            Trace.traceBegin(Trace.TRACE_TAG_SYSTEM_SERVER, "OnBootPhase " + phase);
            final int serviceLen = mServices.size();
            for (int i = 0; i < serviceLen; i++) {
                final SystemService service = mServices.get(i);
                long time = SystemClock.elapsedRealtime();
                Trace.traceBegin(Trace.TRACE_TAG_SYSTEM_SERVER, service.getClass().getName());
                try {
                    service.onBootPhase(mCurrentPhase);
                } catch (Exception ex) {
                    throw new RuntimeException("Failed to boot service "
                            + service.getClass().getName()
                            + ": onBootPhase threw an exception during phase "
                            + mCurrentPhase, ex);
                }
                warnIfTooLong(SystemClock.elapsedRealtime() - time, service, "onBootPhase");
                Trace.traceEnd(Trace.TRACE_TAG_SYSTEM_SERVER);
            }
        } finally {
            Trace.traceEnd(Trace.TRACE_TAG_SYSTEM_SERVER);
        }
    }

启动系统各种服务

    try {
        startBootstrapServices();                 // 启动引导服务
        startCoreServices();                      // 启动核心服务
        startOtherServices();                     // 启动其他服务

启动引导服务

首先来看下startBootstrapServices()：

    private void startBootstrapServices() {
        ... ...

        // 启动Installer服务，阻塞等待与installd建立socket通道
        Installer installer = mSystemServiceManager.startService(Installer.class);

        mSystemServiceManager.startService(DeviceIdentifiersPolicyService.class);

        // 启动ActivityManagerService（AMS），关于AMS我们后面会详细讲解
        mActivityManagerService = mSystemServiceManager.startService(
                ActivityManagerService.Lifecycle.class).getService();
        mActivityManagerService.setSystemServiceManager(mSystemServiceManager);
        mActivityManagerService.setInstaller(installer);

        // 启动PowerManagerService
        mPowerManagerService = mSystemServiceManager.startService(PowerManagerService.class);

        // PowerManagerService就绪，AMS初始化电源管理
        mActivityManagerService.initPowerManagement();

        // Bring up recovery system in case a rescue party needs a reboot
        if (!SystemProperties.getBoolean("config.disable_noncore", false)) {
            traceBeginAndSlog("StartRecoverySystemService");
            mSystemServiceManager.startService(RecoverySystemService.class);
            traceEnd();
        }

        RescueParty.noteBoot(mSystemContext);

        // 启动LightsService
        mSystemServiceManager.startService(LightsService.class);

        // 启动DisplayManagerService（before package manager）
        mDisplayManagerService = mSystemServiceManager.startService(DisplayManagerService.class);

        // 初始化package manager之前，需要默认显示。阻塞，10s超时，see DisplayManagerService.onBootPhase()
        mSystemServiceManager.startBootPhase(SystemService.PHASE_WAIT_FOR_DEFAULT_DISPLAY);

        // Only run "core" apps if we're encrypting the device.
        // 当设备正在加密时，仅运行核心应用
        String cryptState = SystemProperties.get("vold.decrypt");
        if (ENCRYPTING_STATE.equals(cryptState)) {
            Slog.w(TAG, "Detected encryption in progress - only parsing core apps");
            mOnlyCore = true;
        } else if (ENCRYPTED_STATE.equals(cryptState)) {
            Slog.w(TAG, "Device encrypted - only parsing core apps");
            mOnlyCore = true;
        }

        // Start the package manager.
        if (!mRuntimeRestart) {
            MetricsLogger.histogram(null, "boot_package_manager_init_start",
                    (int) SystemClock.elapsedRealtime());
        }

        // 启动PackageManagerService
        mPackageManagerService = PackageManagerService.main(mSystemContext, installer,
                mFactoryTestMode != FactoryTest.FACTORY_TEST_OFF, mOnlyCore);
        mFirstBoot = mPackageManagerService.isFirstBoot();
        mPackageManager = mSystemContext.getPackageManager();

        ... ...

        // 将UserManagerService添加到服务列表，该服务是在PackageManagerService中初始化的        
        mSystemServiceManager.startService(UserManagerService.LifeCycle.class);


        // Initialize attribute cache used to cache resources from packages.
        // 初始化用来缓存包资源的属性缓存
        AttributeCache.init(mSystemContext);

        // Set up the Application instance for the system process and get started.
        // 设置AMS
        mActivityManagerService.setSystemProcess();

        ... ...

        // The sensor service needs access to package manager service, app ops
        // service, and permissions service, therefore we start it after them.
        // Start sensor service in a separate thread. Completion should be checked
        // before using it.
        mSensorServiceStart = SystemServerInitThreadPool.get().submit(() -> {
            TimingsTraceLog traceLog = new TimingsTraceLog(
                    SYSTEM_SERVER_TIMING_ASYNC_TAG, Trace.TRACE_TAG_SYSTEM_SERVER);
            traceLog.traceBegin(START_SENSOR_SERVICE);
            // 启动传感器服务（native 服务，依赖PackageManagerService、AppOpsService、permissions service）
            startSensorService();
            traceLog.traceEnd();
        }, START_SENSOR_SERVICE);
    }

这步首先等待installd启动完成，然后启动一些相互依赖的关键服务。所创建的服务：ActivityManagerService，PowerManagerService，LightsService，DisplayManagerService，PackageManagerService，UserManagerService，sensor服务。

启动核心服务

接下来继续看下核心服务：

    /**
     * Starts some essential services that are not tangled up in the bootstrap process.
     */
    private void startCoreServices() {
        mSystemServiceManager.startService(DropBoxManagerService.class);

        // 启动BatteryService，用于统计电池电量，需要LightService
        mSystemServiceManager.startService(BatteryService.class);

        // 启动UsageStatsService，用于统计应用使用情况
        mSystemServiceManager.startService(UsageStatsService.class);
        mActivityManagerService.setUsageStatsManager(
                LocalServices.getService(UsageStatsManagerInternal.class));

        // 启动WebViewUpdateService
        mWebViewUpdateService = mSystemServiceManager.startService(WebViewUpdateService.class);
    }

启动服务DropBoxManagerService、BatteryService，UsageStatsService，WebViewUpdateService。

启动其他服务

代码很长（1200多行…），但是逻辑简单，主要是启动各种服务。

    /**
     * Starts a miscellaneous grab bag of stuff that has yet to be refactored
     * and organized.
     */
    private void startOtherServices() {
        ... ...

        try {

            ... ...
            traceBeginAndSlog("StartSchedulingPolicyService");   // 调度策略
            ServiceManager.addService("scheduling_policy", new SchedulingPolicyService());
            traceEnd();

            traceBeginAndSlog("StartTelecomLoaderService");
            mSystemServiceManager.startService(TelecomLoaderService.class);
            traceEnd();

            traceBeginAndSlog("StartTelephonyRegistry");         // 提供电话注册、管理服务，可以获取电话的链接状态、信号强度等
            telephonyRegistry = new TelephonyRegistry(context);
            ServiceManager.addService("telephony.registry", telephonyRegistry);
            traceEnd();

            traceBeginAndSlog("StartEntropyMixer");              // 随机数相关，原名EntropyService
            mEntropyMixer = new EntropyMixer(context);
            traceEnd();
            
            mContentResolver = context.getContentResolver();

            // The AccountManager must come before the ContentService
            traceBeginAndSlog("StartAccountManagerService");     // 提供所有账号、密码、认证管理等等的服务
            mSystemServiceManager.startService(ACCOUNT_SERVICE_CLASS);
            traceEnd();

            traceBeginAndSlog("StartContentService");            // ContentProvider服务，提供跨进程数据交换
            mSystemServiceManager.startService(CONTENT_SERVICE_CLASS);
            traceEnd();

            traceBeginAndSlog("InstallSystemProviders");
            mActivityManagerService.installSystemProviders();
            traceEnd();

            traceBeginAndSlog("StartVibratorService");           // 振动器服务
            vibrator = new VibratorService(context);
            ServiceManager.addService("vibrator", vibrator);
            traceEnd();

            ... ...
            
            traceBeginAndSlog("InitWatchdog");                   // 初始化 Watchdog
            final Watchdog watchdog = Watchdog.getInstance();
            watchdog.init(context, mActivityManagerService);
            traceEnd();

            traceBeginAndSlog("StartInputManagerService");       // 事件传递分发服务
            inputManager = new InputManagerService(context);
            traceEnd();

            traceBeginAndSlog("StartWindowManagerService");      // 窗口管理服务
            // WMS needs sensor service ready
            ConcurrentUtils.waitForFutureNoInterrupt(mSensorServiceStart, START_SENSOR_SERVICE);
            mSensorServiceStart = null;
            wm = WindowManagerService.main(context, inputManager,
                    mFactoryTestMode != FactoryTest.FACTORY_TEST_LOW_LEVEL,
                    !mFirstBoot, mOnlyCore, new PhoneWindowManager());
            ServiceManager.addService(Context.WINDOW_SERVICE, wm);
            ServiceManager.addService(Context.INPUT_SERVICE, inputManager);
            traceEnd();
            ... ...
        }
        
        ... ...
    
        LockSettingsService               // 屏幕锁定服务，管理每个用户的相关锁屏信息
        DeviceIdleController              // Doze模式的主要驱动，参考“深入Android 'M' Doze”
        DevicePolicyManagerService        // 提供一些系统级别的设置及属性
        StatusBarManagerService           // 状态栏管理服务
        ClipboardService                  // 系统剪切板服务
        NetworkManagementService          // 网络管理服务
        TextServicesManagerService        // 文本服务，例如文本检查等
        NetworkScoreService               // 网络评分服务
        NetworkStatsService               // 网络状态服务
        NetworkPolicyManagerService       // 网络策略服务
        WifiP2pService                    // Wifi Direct服务
        WifiService                       // Wifi服务
        WifiScanningService               // Wifi扫描服务
        RttService                        // Wifi相关
        EthernetService                   // 以太网服务
        ConnectivityService               // 网络连接管理服务
        NsdService                        // 网络发现服务
        ... ...

        NotificationManagerService        // 通知栏管理服务
        DeviceStorageMonitorService       // 磁盘空间状态检测服务
        LocationManagerService            // 位置服务，GPS、定位等
        CountryDetectorService            // 检测用户国家
        SearchManagerService              // 搜索管理服务
        DropBoxManagerService             // 用于系统运行时日志的存储于管理
        WallpaperManagerService           // 壁纸管理服务
        AudioService                      // AudioFlinger的上层管理封装，主要是音量、音效、声道及铃声等的管理
        DockObserver                      // 如果系统有个座子，当手机装上或拔出这个座子的话，就得靠他来管理了
        WiredAccessoryManager             // 监视手机和底座上的耳机
        UsbService                        // USB服务
        SerialService                     // 串口服务
        TwilightService                   // 指出用户当前所在位置是否为晚上，被UiModeManager等用来调整夜间模式。
        BackupManagerService              // 备份服务
        AppWidgetService                  // 提供Widget的管理和相关服务
        VoiceInteractionManagerService    // 语音交互管理服务
        DiskStatsService                  // 磁盘统计服务，供dumpsys使用
        SamplingProfilerService           // 用于耗时统计等
        NetworkTimeUpdateService          // 监视网络时间，当网络时间变化时更新本地时间。
        CommonTimeManagementService       // 管理本地常见的时间服务的配置，在网络配置变化时重新配置本地服务。
        CertBlacklister                   // 提供一种机制更新SSL certificate blacklist
        DreamManagerService               // 屏幕保护
        AssetAtlasService                 // 负责将预加载的bitmap组装成纹理贴图，生成的纹理贴图可以被用来跨进程使用，以减少内存。
        PrintManagerService               // 打印服务
        HdmiControlService                // HDMI控制服务
        FingerprintService                // 指纹服务
        ... ...
    }

以上代码仅按顺序列出启动的服务，有些服务根据条件，如是否是工厂模式，或系统属性配置，选择性启动，这里不考虑条件判断和异常处理。

自此，SystemServer相关源码分析完毕。

参考Blog

1. https://blog.csdn.net/kingodc…
2. https://blog.csdn.net/qq_2354…
3. https://blog.csdn.net/gaugame…