[系统运维]Android之Zygote启动过程

总体时序

先概述一下总体运行流程，当按电源键，首先是加载系统引导程序BootLoader，然后启动linux内核，再启动init进程，最后Zygote进程启动完成。理论上Android系统中的所有应用程序理论上都是由Zygote启动的。Zygote前期启动启动服务，后期主要fork程序。

init启动流程

• 用户空间的第一个进程，进程号为1（在《深入理解安卓内核思想》的257页里面写的是0，在这记录一下）.
• 职责
  • 创建Zygote
  • 初始化属性服务
  • init文件位于源码目录system/core/init中

init进程的启动三个阶段

1. 启动电源以及系统的启动，加载引导程序BootLoader。
2. 启动Linux内核
3. 启动init进程。
   1. 启动Zygote进程
   2. 初始化启动属性服务。

入口函数

Linux内核启动，启动init进程，进入init的入口函数中，加载init.rc文件：

int main(int argc, char** argv) {
    if (!strcmp(basename(argv[0]), "ueventd")) {
        return ueventd_main(argc, argv);
    }

    if (!strcmp(basename(argv[0]), "watchdogd")) {
        return watchdogd_main(argc, argv);
    }

    if (REBOOT_BOOTLOADER_ON_PANIC) {
        install_reboot_signal_handlers();
    }

    add_environment("PATH", _PATH_DEFPATH);

    bool is_first_stage = (getenv("INIT_SECOND_STAGE") == nullptr);

    if (is_first_stage) {
        boot_clock::time_point start_time = boot_clock::now();

        // Clear the umask. 清理umask
        umask(0);

        // Get the basic filesystem setup we need put together in the initramdisk
        // on / and then we'll let the rc file figure out the rest.
        //创建挂在启动所需要的文件目录
        mount("tmpfs", "/dev", "tmpfs", MS_NOSUID, "mode=0755");
        mkdir("/dev/pts", 0755);
        mkdir("/dev/socket", 0755);
        mount("devpts", "/dev/pts", "devpts", 0, NULL);
        #define MAKE_STR(x) __STRING(x)
        mount("proc", "/proc", "proc", 0, "hidepid=2,gid=" MAKE_STR(AID_READPROC));
        // Don't expose the raw commandline to unprivileged processes.
        chmod("/proc/cmdline", 0440);
        gid_t groups[] = { AID_READPROC };
        setgroups(arraysize(groups), groups);
        mount("sysfs", "/sys", "sysfs", 0, NULL);
        mount("selinuxfs", "/sys/fs/selinux", "selinuxfs", 0, NULL);
        mknod("/dev/kmsg", S_IFCHR | 0600, makedev(1, 11));
        mknod("/dev/random", S_IFCHR | 0666, makedev(1, 8));
        mknod("/dev/urandom", S_IFCHR | 0666, makedev(1, 9));

        // Now that tmpfs is mounted on /dev and we have /dev/kmsg, we can actually
        // talk to the outside world...
        InitKernelLogging(argv);

        LOG(INFO) << "init first stage started!";

        if (!DoFirstStageMount()) {
            LOG(ERROR) << "Failed to mount required partitions early ...";
            panic();
        }

        SetInitAvbVersionInRecovery();

        // Set up SELinux, loading the SELinux policy.
        selinux_initialize(true);

        // We're in the kernel domain, so re-exec init to transition to the init domain now
        // that the SELinux policy has been loaded.
        if (restorecon("/init") == -1) {
            PLOG(ERROR) << "restorecon failed";
            security_failure();
        }

        setenv("INIT_SECOND_STAGE", "true", 1);

        static constexpr uint32_t kNanosecondsPerMillisecond = 1e6;
        uint64_t start_ms = start_time.time_since_epoch().count() / kNanosecondsPerMillisecond;
        setenv("INIT_STARTED_AT", StringPrintf("%" PRIu64, start_ms).c_str(), 1);

        char* path = argv[0];
        char* args[] = { path, nullptr };
        execv(path, args);

        // execv() only returns if an error happened, in which case we
        // panic and never fall through this conditional.
        PLOG(ERROR) << "execv(\"" << path << "\") failed";
        security_failure();
    }

    // At this point we're in the second stage of init.
    InitKernelLogging(argv);
    LOG(INFO) << "init second stage started!";

    // Set up a session keyring that all processes will have access to. It
    // will hold things like FBE encryption keys. No process should override
    // its session keyring.
    keyctl(KEYCTL_GET_KEYRING_ID, KEY_SPEC_SESSION_KEYRING, 1);

    // Indicate that booting is in progress to background fw loaders, etc.
    close(open("/dev/.booting", O_WRONLY | O_CREAT | O_CLOEXEC, 0000));
	//初始化和启动属性服务
    property_init();

    // If arguments are passed both on the command line and in DT,
    // properties set in DT always have priority over the command-line ones.
    process_kernel_dt();
    process_kernel_cmdline();

    // Propagate the kernel variables to internal variables
    // used by init as well as the current required properties.
    export_kernel_boot_props();

    // Make the time that init started available for bootstat to log.
    property_set("ro.boottime.init", getenv("INIT_STARTED_AT"));
    property_set("ro.boottime.init.selinux", getenv("INIT_SELINUX_TOOK"));

    // Set libavb version for Framework-only OTA match in Treble build.
    const char* avb_version = getenv("INIT_AVB_VERSION");
    if (avb_version) property_set("ro.boot.avb_version", avb_version);

    // Clean up our environment.
    unsetenv("INIT_SECOND_STAGE");
    unsetenv("INIT_STARTED_AT");
    unsetenv("INIT_SELINUX_TOOK");
    unsetenv("INIT_AVB_VERSION");

    // Now set up SELinux for second stage.
    selinux_initialize(false);
    selinux_restore_context();

    //创建epoll句柄
    epoll_fd = epoll_create1(EPOLL_CLOEXEC);
    if (epoll_fd == -1) {
        PLOG(ERROR) << "epoll_create1 failed";
        exit(1);
    }
	//设置进程信号处理，用于设置子进程信号处理函数，如果子进程异常退出，init进程会调用
    //中设定的信号处理函数处理。
    signal_handler_init();

    //导入默认的环境变量
    property_load_boot_defaults();
    export_oem_lock_status();
    start_property_service();
    set_usb_controller();

    const BuiltinFunctionMap function_map;
    Action::set_function_map(&function_map);

    Parser& parser = Parser::GetInstance();
    parser.AddSectionParser("service",std::make_unique<ServiceParser>());
    parser.AddSectionParser("on", std::make_unique<ActionParser>());
    parser.AddSectionParser("import", std::make_unique<ImportParser>());
    std::string bootscript = GetProperty("ro.boot.init_rc", "");
    if (bootscript.empty()) {
        //解析init.rc配置文件
        parser.ParseConfig("/init.rc");
        parser.set_is_system_etc_init_loaded(
                parser.ParseConfig("/system/etc/init"));
        parser.set_is_vendor_etc_init_loaded(
                parser.ParseConfig("/vendor/etc/init"));
        parser.set_is_odm_etc_init_loaded(parser.ParseConfig("/odm/etc/init"));
    } else {
        parser.ParseConfig(bootscript);
        parser.set_is_system_etc_init_loaded(true);
        parser.set_is_vendor_etc_init_loaded(true);
        parser.set_is_odm_etc_init_loaded(true);
    }

    // Turning this on and letting the INFO logging be discarded adds 0.2s to
    // Nexus 9 boot time, so it's disabled by default.
    if (false) parser.DumpState();

    ActionManager& am = ActionManager::GetInstance();

    am.QueueEventTrigger("early-init");

    // Queue an action that waits for coldboot done so we know ueventd has set up all of /dev...
    am.QueueBuiltinAction(wait_for_coldboot_done_action, "wait_for_coldboot_done");
    // ... so that we can start queuing up actions that require stuff from /dev.
    am.QueueBuiltinAction(mix_hwrng_into_linux_rng_action, "mix_hwrng_into_linux_rng");
    am.QueueBuiltinAction(set_mmap_rnd_bits_action, "set_mmap_rnd_bits");
    am.QueueBuiltinAction(set_kptr_restrict_action, "set_kptr_restrict");
    am.QueueBuiltinAction(keychord_init_action, "keychord_init");
    am.QueueBuiltinAction(console_init_action, "console_init");

    // Trigger all the boot actions to get us started.
    am.QueueEventTrigger("init");

    // Repeat mix_hwrng_into_linux_rng in case /dev/hw_random or /dev/random
    // wasn't ready immediately after wait_for_coldboot_done
    am.QueueBuiltinAction(mix_hwrng_into_linux_rng_action, "mix_hwrng_into_linux_rng");

    // Don't mount filesystems or start core system services in charger mode.
    std::string bootmode = GetProperty("ro.bootmode", "");
    if (bootmode == "charger") {
        am.QueueEventTrigger("charger");
    } else {
        am.QueueEventTrigger("late-init");
    }

    // Run all property triggers based on current state of the properties.
    am.QueueBuiltinAction(queue_property_triggers_action, "queue_property_triggers");

    while (true) {
        // By default, sleep until something happens.
        int epoll_timeout_ms = -1;

        if (!(waiting_for_prop || ServiceManager::GetInstance().IsWaitingForExec())) {
            //内部遍历执行每个action中携带的command对应的执行函数
            am.ExecuteOneCommand();
        }
        if (!(waiting_for_prop || ServiceManager::GetInstance().IsWaitingForExec())) {
            //重启死去的进程
            restart_processes();

            // If there's a process that needs restarting, wake up in time for that.
            if (process_needs_restart_at != 0) {
                epoll_timeout_ms = (process_needs_restart_at - time(nullptr)) * 1000;
                if (epoll_timeout_ms < 0) epoll_timeout_ms = 0;
            }

            // If there's more work to do, wake up again immediately.
            if (am.HasMoreCommands()) epoll_timeout_ms = 0;
        }

        epoll_event ev;
        int nr = TEMP_FAILURE_RETRY(epoll_wait(epoll_fd, &ev, 1, epoll_timeout_ms));
        if (nr == -1) {
            PLOG(ERROR) << "epoll_wait failed";
        } else if (nr == 1) {
            ((void (*)()) ev.data.ptr)();
        }
    }

    return 0;
}

main函数中做了很多事情，主要做的事情如下：

1. 初始化和启动属性服务
2. 信号处理，详见system/core/init/signal_handler_init.cpp文件
3. 解析init.rc文件，详见system/core/init/init_parse.cpp文件

property_init

代码详见:\system\core\init\property_service.cpp

void property_init() {
    if (__system_property_area_init()) {
        LOG(ERROR) << "Failed to initialize property area";
        exit(1);
    }
}

void start_property_service() {
    property_set("ro.property_service.version", "2");

    property_set_fd = create_socket(PROP_SERVICE_NAME, SOCK_STREAM | SOCK_CLOEXEC | SOCK_NONBLOCK,
                                    0666, 0, 0, NULL);//创建非阻塞的Socket
    if (property_set_fd == -1) {
        PLOG(ERROR) << "start_property_service socket creation failed";
        exit(1);
    }

    listen(property_set_fd, 8);//调用listen函数对对属性进行监听

    register_epoll_handler(property_set_fd, handle_property_set_fd);//当有数据更新时，init进程会调用handle_property_set_fd函数进行处理
}

• 属性服务的初始化

1. 创建非阻塞的Socket
2. 调用listen函数对对属性进行监听
3. 当有数据更新时，init进程会调用handle_property_set_fd函数进行处理

handle_property_set

代码详见:\system\core\init\property_service.cpp

static void handle_property_set(SocketConnection& socket,
                                const std::string& name,
                                const std::string& value,
                                bool legacy_protocol) {
  const char* cmd_name = legacy_protocol ? "PROP_MSG_SETPROP" : "PROP_MSG_SETPROP2";
  if (!is_legal_property_name(name)) {
    LOG(ERROR) << "sys_prop(" << cmd_name << "): illegal property name \"" << name << "\"";
    socket.SendUint32(PROP_ERROR_INVALID_NAME);
    return;
  }

  struct ucred cr = socket.cred();
  char* source_ctx = nullptr;
  getpeercon(socket.socket(), &source_ctx);

    //ctl开头为控制属性
  if (android::base::StartsWith(name, "ctl.")) {
      //校验客户端权限
    if (check_control_mac_perms(value.c_str(), source_ctx, &cr)) {
        //设置控制属性
      handle_control_message(name.c_str() + 4, value.c_str());
      if (!legacy_protocol) {
        socket.SendUint32(PROP_SUCCESS);
      }
    } else {
      LOG(ERROR) << "sys_prop(" << cmd_name << "): Unable to " << (name.c_str() + 4)
                 << " service ctl [" << value << "]"
                 << " uid:" << cr.uid
                 << " gid:" << cr.gid
                 << " pid:" << cr.pid;
      if (!legacy_protocol) {
        socket.SendUint32(PROP_ERROR_HANDLE_CONTROL_MESSAGE);
      }
    }
  } else {//普通属性
      //校验客户端权限
    if (check_mac_perms(name, source_ctx, &cr)) {
      uint32_t result = property_set(name, value);
      if (!legacy_protocol) {
        socket.SendUint32(result);
      }
    } else {
      LOG(ERROR) << "sys_prop(" << cmd_name << "): permission denied uid:" << cr.uid << " name:" << name;
      if (!legacy_protocol) {
        socket.SendUint32(PROP_ERROR_PERMISSION_DENIED);
      }
    }
  }

  freecon(source_ctx);
}


static void handle_property_set_fd() {
    static constexpr uint32_t kDefaultSocketTimeout = 2000; /* ms */

    int s = accept4(property_set_fd, nullptr, nullptr, SOCK_CLOEXEC);
    if (s == -1) {
        return;
    }

    struct ucred cr;
    socklen_t cr_size = sizeof(cr);
    if (getsockopt(s, SOL_SOCKET, SO_PEERCRED, &cr, &cr_size) < 0) {
        close(s);
        PLOG(ERROR) << "sys_prop: unable to get SO_PEERCRED";
        return;
    }

    SocketConnection socket(s, cr);
    uint32_t timeout_ms = kDefaultSocketTimeout;

    uint32_t cmd = 0;
    if (!socket.RecvUint32(&cmd, &timeout_ms)) {
        PLOG(ERROR) << "sys_prop: error while reading command from the socket";
        socket.SendUint32(PROP_ERROR_READ_CMD);
        return;
    }

    switch (cmd) {
    case PROP_MSG_SETPROP: {
        char prop_name[PROP_NAME_MAX];
        char prop_value[PROP_VALUE_MAX];

        //如果Socket读取不到数据就返回
        if (!socket.RecvChars(prop_name, PROP_NAME_MAX, &timeout_ms) ||
            !socket.RecvChars(prop_value, PROP_VALUE_MAX, &timeout_ms)) {
          PLOG(ERROR) << "sys_prop(PROP_MSG_SETPROP): error while reading name/value from the socket";
          return;
        }

        prop_name[PROP_NAME_MAX-1] = 0;
        prop_value[PROP_VALUE_MAX-1] = 0;

        handle_property_set(socket, prop_value, prop_value, true);
        break;
      }

    case PROP_MSG_SETPROP2: {
        std::string name;
        std::string value;
        if (!socket.RecvString(&name, &timeout_ms) ||
            !socket.RecvString(&value, &timeout_ms)) {
          PLOG(ERROR) << "sys_prop(PROP_MSG_SETPROP2): error while reading name/value from the socket";
          socket.SendUint32(PROP_ERROR_READ_DATA);
          return;
        }

        handle_property_set(socket, name, value, false);//处理客户端的请求
        break;
      }

    default:
        LOG(ERROR) << "sys_prop: invalid command " << cmd;
        socket.SendUint32(PROP_ERROR_INVALID_CMD);
        break;
    }
}

• 处理客户端请求

1. 服务属性接收到客户端请求时调用handle_property_set_fd（）处理数据
2. 根据属性分类处理：普通属性、控制属性

init.rc文件

官方注解文档详见system/core/init/Readme.txt

init.rc文件详见system/core/rootdir,其用到了AIL（Android Init Language）语言,其详细含义请自行百度。

在Android8.0中对init.rc文件进行了拆分，每个服务对应一个rc文件。

解析init.rc

zygote启动脚本路径为：system/core/rootdir/init.zygote64.rc

service zygote /system/bin/app_process64 -Xzygote /system/bin --zygote --start-system-server
    class main
    priority -20
    user root
    group root readproc
    socket zygote stream 660 root system
    onrestart write /sys/android_power/request_state wake
    onrestart write /sys/power/state on
    onrestart restart audioserver
    onrestart restart cameraserver
    onrestart restart media
    onrestart restart netd
    onrestart restart wificond
    writepid /dev/cpuset/foreground/tasks

启动脚本中参数介绍：

1. zygote：创建的进程名称
2. /system/bin/app_process64 ：执行的文件路径
3. class main：表示Zygote的classname为main，后面会根据main查找Zygote服务
4. onrestart：当服务启动时需要重启的服务

Service类型语句采用ServiceParser进行解析，ServiceParser代码在system/core/init/service.cpp中，其会针对启动脚本中的每个服务创建对应的实例，然后将所有的对象实例缓存在Service链表中，在启动服务时就会从此列表中查找对应的服务对象。

class main代表的是Zygote服务，所以会遍历前面保存解析Service的链表，查找classname为main（）的服务，然后执行Service中的start（）方法。

Result<Success> Service::Start() {
pid_t pid = -1;
    if (namespace_flags_) {
        pid = clone(nullptr, nullptr, namespace_flags_ | SIGCHLD, nullptr);
    } else {
        pid = fork(); 
    }
    if (pid == 0) { 
    if (!ExpandArgsAndExecv(args_)) {
            PLOG(ERROR) << "cannot execve('" << args_[0] << "')";
        }
    }
}

static bool ExpandArgsAndExecv(const std::vector<std::string>& args) {
    return execv(c_strings[0], c_strings.data()) == 0; //3、
}

在statr（）方法中，首先判断进程是否已经运行，对未运行的进程通过fork（）创建子进程，创建成功后调用ExpandArgsAndExecv（）方法，在ExpandArgsAndExecv（）中调用执行execv（）后Service进程就被启动并进入Service的main（）方法，Zygote进程对应的程序路径为app_main.cpp,在app_main.cpp的main（）方法中调用runtime.start()启动进程。

路径：frameworks\base\cmds\app_process\app_main.c

int main(int argc, char* const argv[])
{
    if (!LOG_NDEBUG) {
      String8 argv_String;
      for (int i = 0; i < argc; ++i) {
        argv_String.append("\"");
        argv_String.append(argv[i]);
        argv_String.append("\" ");
      }
      ALOGV("app_process main with argv: %s", argv_String.string());
    }

    AppRuntime runtime(argv[0], 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.
    //
    // As an exception to the above rule, anything in "spaced commands"
    // goes to the vm even though it has a space in it.
    const char* spaced_commands[] = { "-cp", "-classpath" };
    // Allow "spaced commands" to be succeeded by exactly 1 argument (regardless of -s).
    bool known_command = false;

    int i;
    for (i = 0; i < argc; i++) {
        if (known_command == true) {
          runtime.addOption(strdup(argv[i]));
          ALOGV("app_process main add known option '%s'", argv[i]);
          known_command = false;
          continue;
        }

        for (int j = 0;
             j < static_cast<int>(sizeof(spaced_commands) / sizeof(spaced_commands[0]));
             ++j) {
          if (strcmp(argv[i], spaced_commands[j]) == 0) {
            known_command = true;
            ALOGV("app_process main found known command '%s'", argv[i]);
          }
        }

        if (argv[i][0] != '-') {
            break;
        }
        if (argv[i][1] == '-' && argv[i][2] == 0) {
            ++i; // Skip --.
            break;
        }

        runtime.addOption(strdup(argv[i]));
        ALOGV("app_process main add option '%s'", 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") == 0) {//当前进程是否在Zygote中
            zygote = true;
            niceName = ZYGOTE_NICE_NAME;
        } else if (strcmp(arg, "--start-system-server") == 0) {//是否是systemserver进程
            startSystemServer = true;
        } else if (strcmp(arg, "--application") == 0) {
            application = true;
        } else if (strncmp(arg, "--nice-name=", 12) == 0) {
            niceName.setTo(arg + 12);
        } else if (strncmp(arg, "--", 2) != 0) {
            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);

        if (!LOG_NDEBUG) {
          String8 restOfArgs;
          char* const* argv_new = argv + i;
          int argc_new = argc - i;
          for (int k = 0; k < argc_new; ++k) {
            restOfArgs.append("\"");
            restOfArgs.append(argv_new[k]);
            restOfArgs.append("\" ");
          }
          ALOGV("Class name = %s, args = %s", className.string(), restOfArgs.string());
        }
    } 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) == 0) {
            LOG_ALWAYS_FATAL("app_process: Unable to determine ABI list from property %s.",
                ABI_LIST_PROPERTY);
            return 11;
        }

        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(), true /* setProcName */);
    }

    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.");
    }
}

由于zygote进程是通过forke自己来创建进程，这样zygote进程和他的子进程都可以进入app_main.cpp的main函数中，因此需要去人一下当前进程是在哪个进程。

在app_main文件的main（）方法中，首先根据进程的名称判断当前是否为Zyote进程，并赋值zygote为true，然后调用runtime.start（）启动进程，注意这里的参数传入的是ZygoteInit类的全路径，最后是根据全路径反射执行ZygoteInit方法。

AndroidRuntime

代码路径：frameworks\base\core\jni\AndroidRuntime.cpp

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 = 0; i < options.size(); ++i) {
        if (options[i] == startSystemServer) {
           /* track our progress through the boot sequence */
           const int LOG_BOOT_PROGRESS_START = 3000;
           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, 1);
    }

    //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;
    //启动java虚拟机
    if (startVm(&mJavaVM, &env, zygote) != 0) {
        return;
    }
    onVmCreated(env);

    /*为Java虚拟机注册jni方法
     * Register android functions.
     */
    if (startReg(env) < 0) {
        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() + 1, stringClass, NULL);
    assert(strArray != NULL);
    //从app_main的main函数中找到className为com.android.internal.os.ZygoteInit
    classNameStr = env->NewStringUTF(className);
    assert(classNameStr != NULL);
    env->SetObjectArrayElement(strArray, 0, classNameStr);

    for (size_t i = 0; i < options.size(); ++i) {
        jstring optionsStr = env->NewStringUTF(options.itemAt(i).string());
        assert(optionsStr != NULL);
        env->SetObjectArrayElement(strArray, i + 1, optionsStr);
    }

    /*讲className的“.”替换为“/”
     * Start VM.  This thread becomes the main thread of the VM, and will
     * not return until the VM exits.
     */
    char* slashClassName = toSlashClassName(className);
    //找到ZygoteInit
    jclass startClass = env->FindClass(slashClassName);
    
    if (startClass == NULL) {
        ALOGE("JavaVM unable to locate class '%s'\n", slashClassName);
        /* keep going */
    } else {
        //ZytoteInit的main方法。
        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 {
            //执行main方法。
            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() != 0)
        ALOGW("Warning: VM did not shut down cleanly\n");
}

在AndroidRuntime的start()方法中，执行了Zygote进程的主要逻辑：

1. 启动Java虚拟机
2. 为Java虚拟机注册JNI方法
3. 通过JNI调用Java层ZygoteInit类中的方法完成进程的启动，此时程序由native进入Java层

代码路径：frameworks/base/core/java/com.android.internal.os/ZygoteInit

 public static void main(String argv[]) {
        ZygoteServer zygoteServer = new ZygoteServer();//首先创建并注册Service端的Socket，此Socket用于相应AMS请求创建进程

        // Mark zygote start. This ensures that thread creation will throw
        // an error.
        ZygoteHooks.startZygoteNoThreadCreation();

        // Zygote goes into its own process group.
        try {
            Os.setpgid(0, 0);
        } catch (ErrnoException ex) {
            throw new RuntimeException("Failed to setpgid(0,0)", ex);
        }

        final Runnable caller;
        try {
            // Report Zygote start time to tron unless it is a runtime restart
            if (!"1".equals(SystemProperties.get("sys.boot_completed"))) {
                MetricsLogger.histogram(null, "boot_zygote_init",
                        (int) SystemClock.elapsedRealtime());
            }

            String bootTimeTag = Process.is64Bit() ? "Zygote64Timing" : "Zygote32Timing";
            TimingsTraceLog bootTimingsTraceLog = new TimingsTraceLog(bootTimeTag,
                    Trace.TRACE_TAG_DALVIK);
            bootTimingsTraceLog.traceBegin("ZygoteInit");
            RuntimeInit.enableDdms();

            boolean startSystemServer = false;
            String socketName = "zygote";
            String abiList = null;
            boolean enableLazyPreload = false;
            for (int i = 1; i < argv.length; i++) {
                if ("start-system-server".equals(argv[i])) {
                    startSystemServer = true;
                } else if ("--enable-lazy-preload".equals(argv[i])) {
                    enableLazyPreload = 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.");
            }

            zygoteServer.registerServerSocketFromEnv(socketName);
            // In some configurations, we avoid preloading resources and classes eagerly.
            // In such cases, we will preload things prior to our first fork.
            if (!enableLazyPreload) {
                bootTimingsTraceLog.traceBegin("ZygotePreload");
                EventLog.writeEvent(LOG_BOOT_PROGRESS_PRELOAD_START,
                    SystemClock.uptimeMillis());
                preload(bootTimingsTraceLog);//预加载类和资源
                EventLog.writeEvent(LOG_BOOT_PROGRESS_PRELOAD_END,
                    SystemClock.uptimeMillis());
                bootTimingsTraceLog.traceEnd(); // ZygotePreload
            } else {
                Zygote.resetNicePriority();
            }

            // Do an initial gc to clean up after startup
            bootTimingsTraceLog.traceBegin("PostZygoteInitGC");
            gcAndFinalize();
            bootTimingsTraceLog.traceEnd(); // PostZygoteInitGC

            bootTimingsTraceLog.traceEnd(); // ZygoteInit
            // Disable tracing so that forked processes do not inherit stale tracing tags from
            // Zygote.
            Trace.setTracingEnabled(false, 0);

            Zygote.nativeSecurityInit();

            // Zygote process unmounts root storage spaces.
            Zygote.nativeUnmountStorageOnInit();

            ZygoteHooks.stopZygoteNoThreadCreation();

            if (startSystemServer) {
                Runnable r = forkSystemServer(abiList, socketName, zygoteServer);//调用forkSystemServer（）启动SystemServer进程

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

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

            // The select loop returns early in the child process after a fork and
            // loops forever in the zygote.
            caller = zygoteServer.runSelectLoop(abiList);//执行zygoteServer.runSelectLoop（）循环等待AMS请求创建新的应用进程
        } catch (Throwable ex) {
            Log.e(TAG, "System zygote died with exception", ex);
            throw ex;
        } finally {
            zygoteServer.closeServerSocket();
        }

        // We're in the child process and have exited the select loop. Proceed to execute the
        // command.
        if (caller != null) {
            caller.run();
        }
    }

程序进入Java层执行ZygoteInit.main（）方法，在main（）中主要执行：

1. 首先创建并注册Service端的Socket，此Socket用于相应AMS请求创建进程
2. 预加载类和资源
3. 调用forkSystemServer（）启动SystemServer进程
4. 执行zygoteServer.runSelectLoop（）循环等待AMS请求创建新的应用进程

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