service zygote /system/bin/app_process -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

int main(int argc, char* const argv[])
{
    ....
   // 1、創(chuàng)建了AppRuntime 對象
    AppRuntime runtime(argv[0], computeArgBlockSize(argc, argv));
    // Process command line arguments
    // ignore argv[0]
    argc--;
    argv++;
    // 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) {
            zygote = true;
            niceName = ZYGOTE_NICE_NAME;
        } 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.setTo(arg + 12);
        } else if (strncmp(arg, "--", 2) != 0) {
            className.setTo(arg);
            break;
        } else {
            --i;
            break;
        }
    }
 ......
 ......
    if (zygote) {
      //2、調(diào)用runtime的start函數(shù)
        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.");
    }
}

void AndroidRuntime::start(const char* className, const Vector<String8>& options, bool zygote)
{
    ....
   ....
   ....
    /* start the virtual machine 3、啟動Java虛擬機(jī)*/
    JniInvocation jni_invocation;
    jni_invocation.Init(NULL);
    JNIEnv* env;
    if (startVm(&mJavaVM, &env, zygote) != 0) {
        return;
    }
    onVmCreated(env);
    .....
   /*
     * Register android functions.
      *Register android native functions with the VM.
     * 4、向虛擬機(jī)注冊android本地函數(shù)。
     */
    if (startReg(env) < 0) {
        ALOGE("Unable to register all android natives\n");
        return;
    }
  .....
   .....
.....
    /*
     * Start VM.  This thread becomes the main thread of the VM, and will
     * not return until the VM exits.
     */
    char* slashClassName = toSlashClassName(className != NULL ? 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 {
        //5、使用env對象，通過JNI調(diào)用startClass這個(gè)類的main函數(shù)，
        //而startClass這個(gè)類就是之前傳起來的 
        //com.android.internal.os.ZygoteInit
            env->CallStaticVoidMethod(startClass, startMeth, strArray);
    }

public static void main(String argv[]) {
      //1、創(chuàng)建一個(gè)ZygoteServer對象
        ZygoteServer zygoteServer = new ZygoteServer();
        final Runnable caller;
        try {
            ....
            ....
            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]);
                }
            }
           //2、注冊一個(gè)本地ServerSocket,名為 "zygote"
            zygoteServer.registerServerSocket(socketName);
                if (!enableLazyPreload) {
                bootTimingsTraceLog.traceBegin("ZygotePreload");
                EventLog.writeEvent(LOG_BOOT_PROGRESS_PRELOAD_START,
                    SystemClock.uptimeMillis());
              //preload執(zhí)行預(yù)加載，在preload函數(shù)中主要調(diào)用了 
              //preloadClasses和 preloadResources();加載一些系統(tǒng)資源和系統(tǒng)類，
                preload(bootTimingsTraceLog);
                EventLog.writeEvent(LOG_BOOT_PROGRESS_PRELOAD_END,
                    SystemClock.uptimeMillis());
                bootTimingsTraceLog.traceEnd(); // ZygotePreload
            } else {
                Zygote.resetNicePriority();
            }
            if (startSystemServer) {
              //調(diào)用fork函數(shù)，啟創(chuàng)建啟動SystemServer進(jìn)程
                Runnable r = forkSystemServer(abiList, socketName, zygoteServer);
                // {@code r == null} in the parent (zygote) process, and {@code r != null} in the
                // child (system_server) process.
              //這句很關(guān)鍵如果在父進(jìn)程zygote，則返回null,如果在system_server進(jìn)程則不會null，
              //所以當(dāng)在zygote進(jìn)程的之后，該條件不滿足
                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.
          //在zygote進(jìn)程中，調(diào)用runSelectLoop進(jìn)入無線循環(huán),如果是子進(jìn)程則返回caller 對象
            caller = zygoteServer.runSelectLoop(abiList);
        } catch (Throwable ex) {
            Log.e(TAG, "System zygote died with exception", ex);
            throw ex;
        } finally {
          //如果不是在zygote進(jìn)程則退出循環(huán)進(jìn)入到這里，關(guān)閉socket server
            zygoteServer.closeServerSocket();
        }
        // We're in the child process and have exited the select loop. Proceed to execute the
        // command.
      //如果是在子進(jìn)程中，通過返回的caller 對象，調(diào)用caller 的run函數(shù)。
        if (caller != null) {
            caller.run();
        }
    }