Zygote及System进程启动

1.  init 根据init.rc 运行 app_process, 并携带‘--zygote' 和 ’--startSystemServer' 参数。

2.  AndroidRuntime.cpp::start() 里将启动JavaVM，并且注册所有framework相关的系统JNI接口。

3.  第一次进入Java世界，运行ZygoteInit.java::main() 函数初始化Zygote. Zygote 并创建Socket的server 端。

4.  然后fork一个新的进程并在新进程里初始化SystemServer. Fork之前，Zygote是preload常用的Java类库，以及系统的resources，同时GC（）清理内存空间，为子进程省去重复的工作。

5.  SystemServer 里将所有的系统Service初始化，包括ActivityManager 和 WindowManager, 他们是应用程序运行起来的前提。

6.  与此同时，Zygote监听服务端Socket，等待新的应用启动请求。

7.  ActivityManager ready 之后寻找系统的“Startup” Application, 将请求发给Zygote。

8.  Zygote收到请求后，fork出一个新的进程。

9.  Zygote监听并处理SystemServer 的 SIGCHID 信号，一旦System Server崩溃，立即将自己杀死。init会重启Zygote.

以下内容主要针对上面的介绍展开，附上两张图，这两张图片包含了我们今天要讲解的所有内容

zygote启动流程图

zygote相关的所有类的结构图

　　这里要讲的是zygote启动流程所以主要针对第一张图来讲 ，关于这部分内容，个人觉得，对于想自己开发系统的人来说是很有帮助，每个系统首先启动的便是孵化器进程，以及server监听进程。

一 概念

　　在Android系统中，所有的应用程序和系统服务进程，都是Zygote负责创建的，因此，Zygote也就被成为进程孵化器。Zygote进程是通过复制自身的方式来创建System进程和应用程序进程的。 Zygote会在系统启动时创建一个虚拟机实例，因此，通过复制Zygote进程而得到的System进程和应用程序进程可以快速第在内部获得一个虚拟机实例拷贝。　　

　　Zygote进程启动完成之后，会将system进程启动起来，以便它可以将系统的关键服务启动起来。例如AMS ContentService 和WMS及PMS等。

二 Zygote启动　　

　　Zygote进程在init进程启动过程中被以service服务的形式启动，代码如下：./system/core/rootdir/init.zygote32.rc的service_start()

　　　 service zygote /system/bin/app_process -Xzygote /system/bin --zygote --start-system-server
    　　...
    　socket zygote stream  root system

　　　　onrestart write /sys/android_power/request_state wake
　　　　onrestart write /sys/power/state on
　　　　onrestart restart media
　　　　onrestart restart netd

　　 前面的关键字service告诉init进程创建一个名为"zygote"的进程，这个zygote进程要执行的程序是/system/bin/app_process，后面是要传给app_process的参数。

接下来的socket关键字表示这个zygote进程需要一个名称为"zygote"的socket资源，这样，系统启动后，我们就可以在/dev/socket目录下看到有一个名为zygote的文件。这里定义的socket的类型为unix domain socket，它是用来作本地进程间通信用的。前面我们说到的ActivityManagerService就是通这个socket来和zygote进程通信请求fork一个应用程序进程的了。

最后的一系列onrestart关键字表示这个zygote进程重启时需要执行的命令。

关于init.rc文件的更多信息，请参考system/core/init/readme.txt文件。

了解了这个信息之后，我们就知道Zygote进程要执行的程序便是system/bin/app_process了，它的源代码位于frameworks/base/cmds/app_process/app_main.cpp文件中，入口函数是main。

　在继续分析Zygote进程启动的过程之前，我们先来看看它的启动序列图：　

下面我们就详细分析每一个步骤。

        Step 1. app_process.main

这个函数定义在frameworks/base/cmds/app_process/app_main.cpp文件中：

int main(int argc, char* const argv[])
{
    if (prctl(PR_SET_NO_NEW_PRIVS, , , , ) < ) {
        // Older kernels don't understand PR_SET_NO_NEW_PRIVS and return
        // EINVAL. Don't die on such kernels.
        if (errno != EINVAL) {
            LOG_ALWAYS_FATAL("PR_SET_NO_NEW_PRIVS failed: %s", strerror(errno));
            return ;
        }
    }

    AppRuntime runtime(argv[], computeArgBlockSize(argc, argv));
    // Process command line arguments
    // ignore argv[0]
    argc--;
    argv++;

    // Everything up to '--' or first non '-' arg goes to the vm.
    //
    // The first argument after the VM args is the "parent dir", which
    // is currently unused.
    //
    // After the parent dir, we expect one or more the following internal
    // arguments :
    //
    // --zygote : Start in zygote mode
    // --start-system-server : Start the system server.
    // --application : Start in application (stand alone, non zygote) mode.
    // --nice-name : The nice name for this process.
    //
    // For non zygote starts, these arguments will be followed by
    // the main class name. All remaining arguments are passed to
    // the main method of this class.
    //
    // For zygote starts, all remaining arguments are passed to the zygote.
    // main function.
    //
    // Note that we must copy argument string values since we will rewrite the
    // entire argument block when we apply the nice name to argv0.

    int i;
    for (i = ; i < argc; i++) {
        if (argv[i][] != '-') {
            break;
        }
        if (argv[i][] == '-' && argv[i][] == ) {
            ++i; // Skip --.
            break;
        }
        runtime.addOption(strdup(argv[i]));
    }

    // Parse runtime arguments.  Stop at first unrecognized option.
    bool zygote = false;
    bool startSystemServer = false;
    bool application = false;
    String8 niceName;
    String8 className;

    ++i;  // Skip unused "parent dir" argument.
    while (i < argc) {
        const char* arg = argv[i++];
        if (strcmp(arg, "--zygote") == ) {
            zygote = true;
            niceName = ZYGOTE_NICE_NAME;
        } else if (strcmp(arg, "--start-system-server") == ) {
            startSystemServer = true;
        } else if (strcmp(arg, "--application") == ) {
            application = true;
        } else if (strncmp(arg, "--nice-name=", ) == ) {
            niceName.setTo(arg + );
        } else if (strncmp(arg, "--", ) != ) {
            className.setTo(arg);
            break;
        } else {
            --i;
            break;
        }
    }

    Vector<String8> args;
    if (!className.isEmpty()) {
        // We're not in zygote mode, the only argument we need to pass
        // to RuntimeInit is the application argument.
        //
        // The Remainder of args get passed to startup class main(). Make
        // copies of them before we overwrite them with the process name.
        args.add(application ? String8("application") : String8("tool"));
        runtime.setClassNameAndArgs(className, argc - i, argv + i);
    } else {
        // We're in zygote mode.
        maybeCreateDalvikCache();

        if (startSystemServer) {
            args.add(String8("start-system-server"));
        }

        char prop[PROP_VALUE_MAX];
        if (property_get(ABI_LIST_PROPERTY, prop, NULL) == ) {
            LOG_ALWAYS_FATAL("app_process: Unable to determine ABI list from property %s.",
                ABI_LIST_PROPERTY);
            return ;
        }

        String8 abiFlag("--abi-list=");
        abiFlag.append(prop);
        args.add(abiFlag);

        // In zygote mode, pass all remaining arguments to the zygote
        // main() method.
        for (; i < argc; ++i) {
            args.add(String8(argv[i]));
        }
    }

    if (!niceName.isEmpty()) {
        runtime.setArgv0(niceName.string());
        set_process_name(niceName.string());
    }

    if (zygote) {
        runtime.start("com.android.internal.os.ZygoteInit", args, zygote);
    } else if (className) {
        runtime.start("com.android.internal.os.RuntimeInit", args, zygote);
    } else {
        fprintf(stderr, "Error: no class name or --zygote supplied.\n");
        app_usage();
        LOG_ALWAYS_FATAL("app_process: no class name or --zygote supplied.");
        return ;
    }
}

　　这个函数的主要作用就是创建一个AppRuntime变量，然后调用它的start成员函数。它同样是在frameworks/base/cmds/app_process/app_main.cpp文件中定义：

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

    void setClassNameAndArgs(const String8& className, int argc, char * const *argv) {
        mClassName = className;
        for (int i = ; i < argc; ++i) {
             mArgs.add(String8(argv[i]));
        }
    }

    virtual void onVmCreated(JNIEnv* env)
    {
        if (mClassName.isEmpty()) {
            return; // Zygote. Nothing to do here.
        }

        /*
         * This is a little awkward because the JNI FindClass call uses the
         * class loader associated with the native method we're executing in.
         * If called in onStarted (from RuntimeInit.finishInit because we're
         * launching "am", for example), FindClass would see that we're calling
         * from a boot class' native method, and so wouldn't look for the class
         * we're trying to look up in CLASSPATH. Unfortunately it needs to,
         * because the "am" classes are not boot classes.
         *
         * The easiest fix is to call FindClass here, early on before we start
         * executing boot class Java code and thereby deny ourselves access to
         * non-boot classes.
         */
        char* slashClassName = toSlashClassName(mClassName.string());
        mClass = env->FindClass(slashClassName);
        if (mClass == NULL) {
            ALOGE("ERROR: could not find class '%s'\n", mClassName.string());
        }
        free(slashClassName);

        mClass = reinterpret_cast<jclass>(env->NewGlobalRef(mClass));
    }

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

        AndroidRuntime* ar = AndroidRuntime::getRuntime();
        ar->callMain(mClassName, mClass, mArgs);

        IPCThreadState::self()->stopProcess();
    }

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

    virtual void onExit(int code)
    {
        if (mClassName.isEmpty()) {
            // if zygote
            IPCThreadState::self()->stopProcess();
        }

        AndroidRuntime::onExit(code);
    }

    String8 mClassName;
    Vector<String8> mArgs;
    jclass mClass;
};

}

　　它继承于AndroidRuntime类， AndroidRuntime类定义在frameworks/base/core/jni/AndroidRuntime.cpp文件中：

static AndroidRuntime* gCurRuntime = NULL;
...

AndroidRuntime::AndroidRuntime(char* argBlockStart, const size_t argBlockLength) :
        mExitWithoutCleanup(false),
        mArgBlockStart(argBlockStart),
        mArgBlockLength(argBlockLength)
{
    SkGraphics::Init();
    // There is also a global font cache, but its budget is specified by
    // SK_DEFAULT_FONT_CACHE_COUNT_LIMIT and SK_DEFAULT_FONT_CACHE_LIMIT.

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

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

　　当AppRuntime对象创建时，会调用其父类AndroidRuntime的构造函数，而在AndroidRuntime类的构造函数里面，会将this指针保存在静态全局变量gCurRuntime中，这样，当其它地方需要使用这个AppRuntime对象时，就可以通过同一个文件中的这个函数来获取这个对象的指针：

AndroidRuntime* AndroidRuntime::getRuntime()
{
    return gCurRuntime;
}

　　回到上面的main函数中，由于我们在init.rc文件中，设置了app_process启动参数--zygote和--start-system-server，因此，在main函数里面，最终会执行下面语句：

runtime.start("com.android.internal.os.ZygoteInit", args, zygote);

　　这里的参数startSystemServer为true，表示要启动SystemServer组件。由于AppRuntime没有实现自己的start函数，它继承了父类AndroidRuntime的start函数，因此，下面会执行AndroidRuntime类的start函数。

        Step 2. AndroidRuntime.start

这个函数定义在frameworks/base/core/jni/AndroidRuntime.cpp文件中：

/*
 * Start the Android runtime.  This involves starting the virtual machine
 * and calling the "static void main(String[] args)" method in the class
 * named by "className".
 *
 * Passes the main function two arguments, the class name and the specified
 * options string.
 */
void AndroidRuntime::start(const char* className, const Vector<String8>& options, bool zygote)
{
    ALOGD(">>>>>> START %s uid %d <<<<<<\n",
            className != NULL ? className : "(unknown)", getuid());

    static const String8 startSystemServer("start-system-server");

    /*
     * 'startSystemServer == true' means runtime is obsolete and not run from
     * init.rc anymore, so we print out the boot start event here.
     */
    for (size_t i = ; i < options.size(); ++i) {
        if (options[i] == startSystemServer) {
           /* track our progress through the boot sequence */
           const int LOG_BOOT_PROGRESS_START = ;
           LOG_EVENT_LONG(LOG_BOOT_PROGRESS_START,  ns2ms(systemTime(SYSTEM_TIME_MONOTONIC)));
        }
    }

    const char* rootDir = getenv("ANDROID_ROOT");
    if (rootDir == NULL) {
        rootDir = "/system";
        if (!hasDir("/system")) {
            LOG_FATAL("No root directory specified, and /android does not exist.");
            return;
        }
        setenv("ANDROID_ROOT", rootDir, );
    }

    //const char* kernelHack = getenv("LD_ASSUME_KERNEL");
    //ALOGD("Found LD_ASSUME_KERNEL='%s'\n", kernelHack);

    /* start the virtual machine */
    JniInvocation jni_invocation;
    jni_invocation.Init(NULL);
    JNIEnv* env;
    if (startVm(&mJavaVM, &env, zygote) != ) {
        return;
    }
    onVmCreated(env);

    /*
     * Register android functions.
     */
    if (startReg(env) < ) {
        ALOGE("Unable to register all android natives\n");
        return;
    }

    /*
     * We want to call main() with a String array with arguments in it.
     * At present we have two arguments, the class name and an option string.
     * Create an array to hold them.
     */
    jclass stringClass;
    jobjectArray strArray;
    jstring classNameStr;

    stringClass = env->FindClass("java/lang/String");
    assert(stringClass != NULL);
    strArray = env->NewObjectArray(options.size() + , stringClass, NULL);
    assert(strArray != NULL);
    classNameStr = env->NewStringUTF(className);
    assert(classNameStr != NULL);
    env->SetObjectArrayElement(strArray, , classNameStr);

    for (size_t i = ; i < options.size(); ++i) {
        jstring optionsStr = env->NewStringUTF(options.itemAt(i).string());
        assert(optionsStr != NULL);
        env->SetObjectArrayElement(strArray, i + , optionsStr);
    }

    /*
     * Start VM.  This thread becomes the main thread of the VM, and will
     * not return until the VM exits.
     */
    char* slashClassName = toSlashClassName(className);
    jclass startClass = env->FindClass(slashClassName);
    if (startClass == NULL) {
        ALOGE("JavaVM unable to locate class '%s'\n", slashClassName);
        /* keep going */
    } else {
        jmethodID startMeth = env->GetStaticMethodID(startClass, "main",
            "([Ljava/lang/String;)V");
        if (startMeth == NULL) {
            ALOGE("JavaVM unable to find main() in '%s'\n", className);
            /* keep going */
        } else {
            env->CallStaticVoidMethod(startClass, startMeth, strArray);

#if 0
            if (env->ExceptionCheck())
                threadExitUncaughtException(env);
#endif
        }
    }
    free(slashClassName);

    ALOGD("Shutting down VM\n");
    if (mJavaVM->DetachCurrentThread() != JNI_OK)
        ALOGW("Warning: unable to detach main thread\n");
    if (mJavaVM->DestroyJavaVM() != )
        ALOGW("Warning: VM did not shut down cleanly\n");
}

　　这个函数的作用是启动Android系统运行时库，它主要做了三件事情，一是调用函数startVM启动虚拟机，二是调用函数startReg注册JNI方法，三是调用了com.android.internal.os.ZygoteInit类的main函数。

 Step 3. ZygoteInit.main

这个函数定义在frameworks/base/core/java/com/android/internal/os/ZygoteInit.java文件中：

    public static void main(String argv[]) {
        try {
            RuntimeInit.enableDdms();
            // Start profiling the zygote initialization.
            SamplingProfilerIntegration.start();

            boolean startSystemServer = false;
            String socketName = "zygote";
            String abiList = null;
            for (int i = 1; i < argv.length; i++) {
                if ("start-system-server".equals(argv[i])) {
                    startSystemServer = true;
                } else if (argv[i].startsWith(ABI_LIST_ARG)) {
                    abiList = argv[i].substring(ABI_LIST_ARG.length());
                } else if (argv[i].startsWith(SOCKET_NAME_ARG)) {
                    socketName = argv[i].substring(SOCKET_NAME_ARG.length());
                } else {
                    throw new RuntimeException("Unknown command line argument: " + argv[i]);
                }
            }

            if (abiList == null) {
                throw new RuntimeException("No ABI list supplied.");
            }

            registerZygoteSocket(socketName);
            EventLog.writeEvent(LOG_BOOT_PROGRESS_PRELOAD_START,
                SystemClock.uptimeMillis());
            preload();
            EventLog.writeEvent(LOG_BOOT_PROGRESS_PRELOAD_END,
                SystemClock.uptimeMillis());

            // Finish profiling the zygote initialization.
            SamplingProfilerIntegration.writeZygoteSnapshot();

            // Do an initial gc to clean up after startup
            gcAndFinalize();

            // Disable tracing so that forked processes do not inherit stale tracing tags from
            // Zygote.
            Trace.setTracingEnabled(false);

            if (startSystemServer) {
                startSystemServer(abiList, socketName);
            }

            Log.i(TAG, "Accepting command socket connections");
            runSelectLoop(abiList);

            closeServerSocket();
        } catch (MethodAndArgsCaller caller) {
            caller.run();
        } catch (RuntimeException ex) {
            Log.e(TAG, "Zygote died with exception", ex);
            closeServerSocket();
            throw ex;
        }
    }

　　它主要作了三件事情，一个调用registerZygoteSocket函数创建了一个socket接口，用来和ActivityManagerService通讯，二是调用startSystemServer函数来启动SystemServer组件，三是调用runSelectLoopMode函数进入一个无限循环在前面创建的socket接口上等待ActivityManagerService请求创建新的应用程序进程。

Step 4. ZygoteInit.registerZygoteSocket

这个函数定义在frameworks/base/core/java/com/android/internal/os/ZygoteInit.java文件中：

　　private static void registerZygoteSocket(String socketName) {
        if (sServerSocket == null) {
            int fileDesc;
            final String fullSocketName = ANDROID_SOCKET_PREFIX + socketName;
            try {
                String env = System.getenv(fullSocketName);
                fileDesc = Integer.parseInt(env);
            } catch (RuntimeException ex) {
                throw new RuntimeException(fullSocketName + " unset or invalid", ex);
            }

            try {
                FileDescriptor fd = new FileDescriptor();
                fd.setInt$(fileDesc);
                sServerSocket = new LocalServerSocket(fd);
            } catch (IOException ex) {
                throw new RuntimeException(
                        "Error binding to local socket '" + fileDesc + "'", ex);
            }
        }
    }

　　这个socket接口是通过文件描述符来创建的，这个文件描符代表的就是我们前面说的/dev/socket/zygote文件了。这个文件描述符是通过环境变量ANDROID_SOCKET_ENV得到的，它定义为：

public class ZygoteInit {
    ......  

    private static final String ANDROID_SOCKET_ENV = "ANDROID_SOCKET_zygote";  

    ......
}  

　　那么，这个环境变量的值又是由谁来设置的呢？我们知道，系统启动脚本文件system/core/rootdir/init.rc是由init进程来解释执行的，而init进程的源代码位于system/core/init目录中，在init.cpp文件中，是由service_start函数来解释init.rc文件中的service命令的：

void service_start(struct service *svc, const char *dynamic_args)
{
    // Starting a service removes it from the disabled or reset state and
    // immediately takes it out of the restarting state if it was in there.
    svc->flags &= (~(SVC_DISABLED|SVC_RESTARTING|SVC_RESET|SVC_RESTART|SVC_DISABLED_START));
    svc->time_started = ;

    // Running processes require no additional work --- if they're in the
    // process of exiting, we've ensured that they will immediately restart
    // on exit, unless they are ONESHOT.
    if (svc->flags & SVC_RUNNING) {
        return;
    }

    bool needs_console = (svc->flags & SVC_CONSOLE);
    if (needs_console && !have_console) {
        ERROR("service '%s' requires console\n", svc->name);
        svc->flags |= SVC_DISABLED;
        return;
    }

    struct stat s;
    if (stat(svc->args[], &s) != ) {
        ERROR("cannot find '%s', disabling '%s'\n", svc->args[], svc->name);
        svc->flags |= SVC_DISABLED;
        return;
    }

    if ((!(svc->flags & SVC_ONESHOT)) && dynamic_args) {
        ERROR("service '%s' must be one-shot to use dynamic args, disabling\n",
               svc->args[]);
        svc->flags |= SVC_DISABLED;
        return;
    }

    char* scon = NULL;
    if (is_selinux_enabled() > ) {
        if (svc->seclabel) {
            scon = strdup(svc->seclabel);
            if (!scon) {
                ERROR("Out of memory while starting '%s'\n", svc->name);
                return;
            }
        } else {
            char *mycon = NULL, *fcon = NULL;

            INFO("computing context for service '%s'\n", svc->args[]);
            int rc = getcon(&mycon);
            if (rc < ) {
                ERROR("could not get context while starting '%s'\n", svc->name);
                return;
            }

            rc = getfilecon(svc->args[], &fcon);
            if (rc < ) {
                ERROR("could not get context while starting '%s'\n", svc->name);
                freecon(mycon);
                return;
            }

            rc = security_compute_create(mycon, fcon, string_to_security_class("process"), &scon);
            if (rc ==  && !strcmp(scon, mycon)) {
                ERROR("Warning!  Service %s needs a SELinux domain defined; please fix!\n", svc->name);
            }
            freecon(mycon);
            freecon(fcon);
            if (rc < ) {
                ERROR("could not get context while starting '%s'\n", svc->name);
                return;
            }
        }
    }

    NOTICE("Starting service '%s'...\n", svc->name);

    pid_t pid = fork();
    if (pid == ) {
        struct socketinfo *si;
        struct svcenvinfo *ei;
        char tmp[];
        int fd, sz;

        umask();
        if (properties_initialized()) {
            get_property_workspace(&fd, &sz);
            snprintf(tmp, sizeof(tmp), "%d,%d", dup(fd), sz);
            add_environment("ANDROID_PROPERTY_WORKSPACE", tmp);
        }

        for (ei = svc->envvars; ei; ei = ei->next)
            add_environment(ei->name, ei->value);

        for (si = svc->sockets; si; si = si->next) {
            int socket_type = (
                    !strcmp(si->type, "stream") ? SOCK_STREAM :
                        (!strcmp(si->type, "dgram") ? SOCK_DGRAM : SOCK_SEQPACKET));
            int s = create_socket(si->name, socket_type,
                                  si->perm, si->uid, si->gid, si->socketcon ?: scon);
            if (s >= ) {
                publish_socket(si->name, s);
            }
        }

        freecon(scon);
        scon = NULL;

        if (svc->writepid_files_) {
            std::string pid_str = android::base::StringPrintf("%d", pid);
            for (auto& file : *svc->writepid_files_) {
                if (!android::base::WriteStringToFile(pid_str, file)) {
                    ERROR("couldn't write %s to %s: %s\n",
                          pid_str.c_str(), file.c_str(), strerror(errno));
                }
            }
        }

        if (svc->ioprio_class != IoSchedClass_NONE) {
            if (android_set_ioprio(getpid(), svc->ioprio_class, svc->ioprio_pri)) {
                ERROR("Failed to set pid %d ioprio = %d,%d: %s\n",
                      getpid(), svc->ioprio_class, svc->ioprio_pri, strerror(errno));
            }
        }

        if (needs_console) {
            setsid();
            open_console();
        } else {
            zap_stdio();
        }

        if (false) {
            for (size_t n = ; svc->args[n]; n++) {
                INFO("args[%zu] = '%s'\n", n, svc->args[n]);
            }
            for (size_t n = ; ENV[n]; n++) {
                INFO("env[%zu] = '%s'\n", n, ENV[n]);
            }
        }

        setpgid(, getpid());

        // As requested, set our gid, supplemental gids, and uid.
        if (svc->gid) {
            if (setgid(svc->gid) != ) {
                ERROR("setgid failed: %s\n", strerror(errno));
                _exit();
            }
        }
        if (svc->nr_supp_gids) {
            if (setgroups(svc->nr_supp_gids, svc->supp_gids) != ) {
                ERROR("setgroups failed: %s\n", strerror(errno));
                _exit();
            }
        }
        if (svc->uid) {
            if (setuid(svc->uid) != ) {
                ERROR("setuid failed: %s\n", strerror(errno));
                _exit();
            }
        }
        if (svc->seclabel) {
            if (is_selinux_enabled() >  && setexeccon(svc->seclabel) < ) {
                ERROR("cannot setexeccon('%s'): %s\n", svc->seclabel, strerror(errno));
                _exit();
            }
        }

        if (!dynamic_args) {
            if (execve(svc->args[], (char**) svc->args, (char**) ENV) < ) {
                ERROR("cannot execve('%s'): %s\n", svc->args[], strerror(errno));
            }
        } else {
            char *arg_ptrs[INIT_PARSER_MAXARGS+];
            int arg_idx = svc->nargs;
            char *tmp = strdup(dynamic_args);
            char *next = tmp;
            char *bword;

            /* Copy the static arguments */
            memcpy(arg_ptrs, svc->args, (svc->nargs * sizeof(char *)));

            while((bword = strsep(&next, " "))) {
                arg_ptrs[arg_idx++] = bword;
                if (arg_idx == INIT_PARSER_MAXARGS)
                    break;
            }
            arg_ptrs[arg_idx] = NULL;
            execve(svc->args[], (char**) arg_ptrs, (char**) ENV);
        }
        _exit();
    }

    freecon(scon);

    if (pid < ) {
        ERROR("failed to start '%s'\n", svc->name);
        svc->pid = ;
        return;
    }

    svc->time_started = gettime();
    svc->pid = pid;
    svc->flags |= SVC_RUNNING;

    if ((svc->flags & SVC_EXEC) != ) {
        INFO("SVC_EXEC pid %d (uid %d gid %d+%zu context %s) started; waiting...\n",
             svc->pid, svc->uid, svc->gid, svc->nr_supp_gids,
             svc->seclabel ? : "default");
        waiting_for_exec = true;
    }

    svc->NotifyStateChange("running");
}

　　每一个service命令都会促使init进程调用fork函数来创建一个新的进程，在新的进程里面，会分析里面的socket选项，对于每一个socket选项，都会通过create_socket函数来在/dev/socket目录下创建一个文件，在这个场景中，这个文件便是zygote了，然后得到的文件描述符通过publish_socket函数写入到环境变量中去：

static void publish_socket(const char *name, int fd)
{
    char key[] = ANDROID_SOCKET_ENV_PREFIX;
    char val[];

    strlcpy(key + sizeof(ANDROID_SOCKET_ENV_PREFIX) - ,
            name,
            sizeof(key) - sizeof(ANDROID_SOCKET_ENV_PREFIX));
    snprintf(val, sizeof(val), "%d", fd);
    add_environment(key, val);

    /* make sure we don't close-on-exec */
    fcntl(fd, F_SETFD, );
}

　　这里传进来的参数name值为"zygote"，而ANDROID_SOCKET_ENV_PREFIX在system/core/include/cutils/sockets.h定义为：

#define ANDROID_SOCKET_ENV_PREFIX       "ANDROID_SOCKET_"
#define ANDROID_SOCKET_DIR              "/dev/socket"

　　因此，这里就把上面得到的文件描述符写入到以"ANDROID_SOCKET_zygote"为key值的环境变量中。又因为上面的ZygoteInit.registerZygoteSocket函数与这里创建socket文件的create_socket函数是运行在同一个进程中，因此，上面的ZygoteInit.registerZygoteSocket函数可以直接使用这个文件描述符来创建一个Java层的LocalServerSocket对象。如果其它进程也需要打开这个/dev/socket/zygote文件来和Zygote进程进行通信，那就必须要通过文件名来连接这个LocalServerSocket了，ActivityManagerService是通过Process.start函数来创建一个新的进程的，而Process.start函数会首先通过Socket连接到Zygote进程中，最终由Zygote进程来完成创建新的应用程序进程，而Process类是通过openZygoteSocketIfNeeded函数来连接到Zygote进程中的Socket的：frameworks/base/core/java/android/os/Process.java

　　public static final String ZYGOTE_SOCKET = "zygote";
　　/**
     * Tries to open socket to Zygote process if not already open. If
     * already open, does nothing.  May block and retry.
     */
    private static ZygoteState openZygoteSocketIfNeeded(String abi) throws ZygoteStartFailedEx {
        if (primaryZygoteState == null || primaryZygoteState.isClosed()) {
            try {
                primaryZygoteState = ZygoteState.connect(ZYGOTE_SOCKET);
            } catch (IOException ioe) {
                throw new ZygoteStartFailedEx("Error connecting to primary zygote", ioe);
            }
        }

        if (primaryZygoteState.matches(abi)) {
            return primaryZygoteState;
        }

        // The primary zygote didn't match. Try the secondary.
        if (secondaryZygoteState == null || secondaryZygoteState.isClosed()) {
            try {
            secondaryZygoteState = ZygoteState.connect(SECONDARY_ZYGOTE_SOCKET);
            } catch (IOException ioe) {
                throw new ZygoteStartFailedEx("Error connecting to secondary zygote", ioe);
            }
        }

        if (secondaryZygoteState.matches(abi)) {
            return secondaryZygoteState;
        }

        throw new ZygoteStartFailedEx("Unsupported zygote ABI: " + abi);
    }

ZYGOTE_SOCKET刚好就是对应/dev/socket目录下的zygote文件了。

Android系统中的socket机制和binder机制一样，都是可以用来进行进程间通信。

Socket对象创建完成之后，回到Step 3中的ZygoteInit.main函数中，startSystemServer函数来启动SystemServer组件。

Step 5. ZygoteInit.startSystemServer
       这个函数定义在frameworks/base/core/java/com/android/internal/os/ZygoteInit.java文件中：

    /**
     * Prepare the arguments and fork for the system server process.
     */
    private static boolean startSystemServer(String abiList, String socketName)
            throws MethodAndArgsCaller, RuntimeException {
        long capabilities = posixCapabilitiesAsBits(
            OsConstants.CAP_BLOCK_SUSPEND,
            OsConstants.CAP_KILL,
            OsConstants.CAP_NET_ADMIN,
            OsConstants.CAP_NET_BIND_SERVICE,
            OsConstants.CAP_NET_BROADCAST,
            OsConstants.CAP_NET_RAW,
            OsConstants.CAP_SYS_MODULE,
            OsConstants.CAP_SYS_NICE,
            OsConstants.CAP_SYS_RESOURCE,
            OsConstants.CAP_SYS_TIME,
            OsConstants.CAP_SYS_TTY_CONFIG
        );
        /* Hardcoded command line to start the system server */
        String args[] = {
            "--setuid=1000",
            "--setgid=1000",
            "--setgroups=1001,1002,1003,1004,1005,1006,1007,1008,1009,1010,1018,1021,1032,3001,3002,3003,3006,3007",
            "--capabilities=" + capabilities + "," + capabilities,
            "--nice-name=system_server",
            "--runtime-args",
            "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) {
            if (hasSecondZygote(abiList)) {
                waitForSecondaryZygote(socketName);
            }

            handleSystemServerProcess(parsedArgs);
        }

        return true;
    }

　　 这里我们可以看到，Zygote进程通过Zygote.forkSystemServer函数来创建一个新的进程来启动SystemServer组件，返回值pid等0的地方就是新的进程要执行的路径，即新创建的进程会执行handleSystemServerProcess函数。

Step 6. ZygoteInit.handleSystemServerProcess
        这个函数定义在frameworks/base/core/java/com/android/internal/os/ZygoteInit.java文件中：

    /**
     * Finish remaining work for the newly forked system server process.
     */
    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.
        Os.umask(S_IRWXG | S_IRWXO);

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

        final String systemServerClasspath = Os.getenv("SYSTEMSERVERCLASSPATH");
        if (systemServerClasspath != null) {
            performSystemServerDexOpt(systemServerClasspath);
        }

        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 = new PathClassLoader(systemServerClasspath, ClassLoader.getSystemClassLoader());
                Thread.currentThread().setContextClassLoader(cl);
            }

            /*
             * Pass the remaining arguments to SystemServer.
             */
            RuntimeInit.zygoteInit(parsedArgs.targetSdkVersion, parsedArgs.remainingArgs, cl);
        }

        /* should never reach here */
    }

　　由于由Zygote进程创建的子进程会继承Zygote进程在前面Step 4中创建的Socket文件描述符，而这里的子进程又不会用到它，因此，这里就调用closeServerSocket函数来关闭它。这个函数接着调用RuntimeInit.zygoteInit函数来进一步执行启动SystemServer组件的操作。

Step 7. RuntimeInit.zygoteInit

这个函数定义在frameworks/base/core/java/com/android/internal/os/RuntimeInit.java文件中：

    /**
     * The main function called when started through the zygote process. This
     * could be unified with main(), if the native code in nativeFinishInit()
     * were rationalized with Zygote startup.<p>
     *
     * Current recognized args:
     * <ul>
     *   <li> <code> [--] &lt;start class name&gt;  &lt;args&gt;
     * </ul>
     *
     * @param targetSdkVersion target SDK version
     * @param argv arg strings
     */
    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");
        redirectLogStreams();

        commonInit();
        nativeZygoteInit();
        applicationInit(targetSdkVersion, argv, classLoader);
    }

　　这个函数会执行两个操作，一个是调用zygoteInitNative函数来执行一个Binder进程间通信机制的初始化工作，这个工作完成之后，这个进程中的Binder对象就可以方便地进行进程间通信了，另一个是调用上面Step 5传进来的com.android.server.SystemServer类的main函数。

Step 8. RuntimeInit.nativeZygoteInit

这个函数定义在frameworks/base/core/java/com/android/internal/os/RuntimeInit.java文件中：

public class RuntimeInit {
    ......    

    public static final native void nativeZygoteInit();    

    ......
}

　　这里可以看出，函数nativeZygoteInit是一个Native函数，实现在frameworks/base/core/jni/AndroidRuntime.cpp文件中，完成这一步后，这个进程的Binder进程间通信机制基础设施就准备好了。

回到Step 7中的RuntimeInit.nativeZygoteInit函数，下一步它就要执行com.android.server.SystemServer类的main函数了。

Step 9. SystemServer.main

这个函数定义在frameworks/base/services/java/com/android/server/SystemServer.java文件中：

    /**
     * The main entry point from zygote.
     */
    public static void main(String[] args) {
        new SystemServer().run();
    }
    public SystemServer() {
        // Check for factory test mode.
        mFactoryTestMode = FactoryTest.getMode();
    }

    private void run() {
        // 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.
        if (System.currentTimeMillis() < EARLIEST_SUPPORTED_TIME) {
            Slog.w(TAG, "System clock is before 1970; setting to 1970.");
            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()) {
            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", "");
        }

        // Here we go!
        Slog.i(TAG, "Entered the Android system server!");
        EventLog.writeEvent(EventLogTags.BOOT_PROGRESS_SYSTEM_RUN, SystemClock.uptimeMillis());

        // In case the runtime switched since last boot (such as when
        // the old runtime was removed in an OTA), set the system
        // property so that it is in sync. We can't do this in
        // libnativehelper's JniInvocation::Init code where we already
        // had to fallback to a different runtime because it is
        // running as root and we need to be the system user to set
        // the property. http://b/11463182
        SystemProperties.set("persist.sys.dalvik.vm.lib.2", VMRuntime.getRuntime().vmLibrary());

        // Enable the sampling profiler.
        if (SamplingProfilerIntegration.isEnabled()) {
            SamplingProfilerIntegration.start();
            mProfilerSnapshotTimer = new Timer();
            mProfilerSnapshotTimer.schedule(new TimerTask() {
                @Override
                public void run() {
                    SamplingProfilerIntegration.writeSnapshot("system_server", null);
                }
            }, SNAPSHOT_INTERVAL, SNAPSHOT_INTERVAL);
        }

        // Mmmmmm... more memory!
        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);

        // Some devices rely on runtime fingerprint generation, so make sure
        // we've defined it before booting further.
        Build.ensureFingerprintProperty();

        // Within the system server, it is an error to access Environment paths without
        // explicitly specifying a user.
        Environment.setUserRequired(true);

        // Ensure binder calls into the system always run at foreground priority.
        BinderInternal.disableBackgroundScheduling(true);

        // Prepare the main looper thread (this thread).
        android.os.Process.setThreadPriority(
                android.os.Process.THREAD_PRIORITY_FOREGROUND);
        android.os.Process.setCanSelfBackground(false);
        Looper.prepareMainLooper();

        // Initialize native services.
        System.loadLibrary("android_servers");

        // Check whether we failed to shut down last time we tried.
        // This call may not return.
        performPendingShutdown();

        // Initialize the system context.
        createSystemContext();

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

        // Start services.
        try {
            startBootstrapServices();
            startCoreServices();
            startOtherServices();
        } catch (Throwable ex) {
            Slog.e("System", "******************************************");
            Slog.e("System", "************ Failure starting system services", ex);
            throw ex;
        }

        // For debug builds, log event loop stalls to dropbox for analysis.
        if (StrictMode.conditionallyEnableDebugLogging()) {
            Slog.i(TAG, "Enabled StrictMode for system server main thread.");
        }

        // Loop forever.
        Looper.loop();
        throw new RuntimeException("Main thread loop unexpectedly exited");
    }

　　这里执行完成后，层层返回，最后回到上面的Step 3中的ZygoteInit.main函数中，接下来它就要调用runSelectLoopMode函数进入一个无限循环在前面Step 4中创建的socket接口上等待ActivityManagerService请求创建新的应用程序进程了。

Step 10. ZygoteInit.runSelectLoopMode

这个函数定义在frameworks/base/core/java/com/android/internal/os/ZygoteInit.java文件中：

    /**
     * Runs the zygote process's select loop. Accepts new connections as
     * they happen, and reads commands from connections one spawn-request's
     * worth at a time.
     *
     * @throws MethodAndArgsCaller in a child process when a main() should
     * be executed.
     */
    private static void runSelectLoop(String abiList) throws MethodAndArgsCaller {
        ArrayList<FileDescriptor> fds = new ArrayList<FileDescriptor>();
        ArrayList<ZygoteConnection> peers = new ArrayList<ZygoteConnection>();

        fds.add(sServerSocket.getFileDescriptor());
        peers.add(null);

        while (true) {
            StructPollfd[] pollFds = new StructPollfd[fds.size()];
            for (int i = 0; i < pollFds.length; ++i) {
                pollFds[i] = new StructPollfd();
                pollFds[i].fd = fds.get(i);
                pollFds[i].events = (short) POLLIN;
            }
            try {
                Os.poll(pollFds, -1);
            } catch (ErrnoException ex) {
                throw new RuntimeException("poll failed", ex);
            }
            for (int i = pollFds.length - 1; i >= 0; --i) {
                if ((pollFds[i].revents & POLLIN) == 0) {
                    continue;
                }
                if (i == 0) {
                    ZygoteConnection newPeer = acceptCommandPeer(abiList);
                    peers.add(newPeer);
                    fds.add(newPeer.getFileDesciptor());
                } else {
                    boolean done = peers.get(i).runOnce();
                    if (done) {
                        peers.remove(i);
                        fds.remove(i);
                    }
                }
            }
        }
    }

　　这个函数就是在等待ActivityManagerService来连接这个Socket，然后调用ZygoteConnection.runOnce函数来创建新的应用程序。

这样，Zygote进程就启动完成了，学习到这里，我们终于都对Android系统中的进程有了一个深刻的认识了，这里总结一下：

1. 系统启动时init进程会创建Zygote进程，Zygote进程负责后续Android应用程序框架层的其它进程的创建和启动工作。

2. Zygote进程会首先创建一个SystemServer进程，SystemServer进程负责启动系统的关键服务，如包管理服务PackageManagerService和应用程序组件管理服务ActivityManagerService。

3. 当我们需要启动一个Android应用程序时，ActivityManagerService会通过Socket进程间通信机制，通知Zygote进程为这个应用程序创建一个新的进程。