Android启动过程中的第一个进程init，在启动过程中会启动两个关键的系统服务进程ServiceManager和Zygote。本文要介绍的就是Zygote进程的启动，Zygote俗称孵化器，专门用于生产(启动)新的进程。Zygote是在Init.rc(aosp/system/core/rootdir)里描述并由init进程启动的。相关代码如下:       

service zygote /system/bin/app_process -Xzygote /system/bin --zygote --start-system-server
    class main
    socket zygote stream 660 root system
    onrestart write /sys/android_power/request_state wake
    onrestart write /sys/power/state on
    onrestart restart media
    onrestart restart netd

       zygote的path路径为/system/bin/app_process，-Xzygote /system/bin --zygote --start-system-server是它的参数，所属class是main，同属同一个class的进程会同时启动或者停止，会启动一个socket名为zygote，重启时将做四个操作。

       下面是Zygote进程启动的大致时序图:

     Android启动过程中，init进程使用aosp/system/core/init/Init_parser.c解析init.rc脚本文件。

     step1，循环启动init.rc文件描述的所有状态不为SVC_DISABLED的service，fork()新进程。

     step2，启动Zygote，执行app_main.cpp的main()方法，

    // Parse runtime arguments.  Stop at first unrecognized option.
    bool zygote = false;
    bool startSystemServer = false;
    bool application = false;
    const char* parentDir = NULL;
    const char* niceName = NULL;
    const char* className = NULL;
    while (i < argc) {//main的参数匹配
        const char* arg = argv[i++];
        if (!parentDir) {
            parentDir = arg;
        } else if (strcmp(arg, "--zygote") == 0) {
            zygote = true;
            niceName = "zygote";
        } else if (strcmp(arg, "--start-system-server") == 0) {
            startSystemServer = true;
        } else if (strcmp(arg, "--application") == 0) {
            application = true;
        } else if (strncmp(arg, "--nice-name=", 12) == 0) {
            niceName = arg + 12;
        } else {
            className = arg;
            break;
        }
    }

    if (niceName && *niceName) {
        setArgv0(argv0, niceName);
        set_process_name(niceName);
    }

    runtime.mParentDir = parentDir;

    if (zygote) {
        runtime.start("com.android.internal.os.ZygoteInit",
                startSystemServer ? "start-system-server" : "");
    } else if (className) {
        ......
    } else {
        ......
    }

    /* start the virtual machine */
    JniInvocation jni_invocation;
    jni_invocation.Init(NULL);
    JNIEnv* env;
    if (startVm(&mJavaVM, &env) != 0) {//step4，创建JVM
        return;
    }
    onVmCreated(env);

    /*
     * Register android functions.
     */
    if (startReg(env) < 0) {//step6，在JVM中注册android本地方法
        ALOGE("Unable to register all android natives\n");
        return;
    }

step9，registerZygoteSocket()注册服务端socket，这个socket就是前面提到的zygote，在init进程解析init.rc时创建。

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

            registerZygoteSocket();
            .......
            preload();
            .......

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

            if (argv[1].equals("start-system-server")) {
                startSystemServer();//step11
            } else if (!argv[1].equals("")) {
                throw new RuntimeException(argv[0] + USAGE_STRING);
            }            runSelectLoop();

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

  private static boolean startSystemServer()
            throws MethodAndArgsCaller, RuntimeException {
         ......
        /* Hardcoded command line to start the system server */
        String args[] = {
            "--setuid=1000",
            "--setgid=1000",
            "--setgroups=1001,1002,1003,1004,1005,1006,1007,1008,1009,1010,1018,1032,3001,3002,3003,3006,3007",
            "--capabilities=" + capabilities + "," + capabilities,
            "--runtime-init",
            "--nice-name=system_server",
            "com.android.server.SystemServer",
        };
        ZygoteConnection.Arguments parsedArgs = null;

        int pid;

        try {
            .......

            /* 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) {
            handleSystemServerProcess(parsedArgs);//step12
        }

        return true;
    }

参考：

http://blog.csdn.net/luoshengyang/article/details/6768304

www.cnblogs.com/bastard/archive/2012/08/28/2660389.html

右键复制图片地址，在浏览器中打开即可查看大图。

未完待续，有不对的地方，请指正。

Android4.4 framework分析——Zygote进程的启动过程