public static void main(String argv[]) {
    ZygoteServer zygoteServer = null;

    // 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);
    }

    Runnable caller;
    try {
        // Store now for StatsLogging later.
        final long startTime = SystemClock.elapsedRealtime();
        final boolean isRuntimeRestarted = "1".equals(
                SystemProperties.get("sys.boot_completed"));

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

        boolean startSystemServer = false;
        String zygoteSocketName = "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)) {
                zygoteSocketName = argv[i].substring(SOCKET_NAME_ARG.length());
            } else {
                throw new RuntimeException("Unknown command line argument: " + argv[i]);
            }
        }

        final boolean isPrimaryZygote = zygoteSocketName.equals(Zygote.PRIMARY_SOCKET_NAME);
        if (!isRuntimeRestarted) {
            if (isPrimaryZygote) {
                FrameworkStatsLog.write(FrameworkStatsLog.BOOT_TIME_EVENT_ELAPSED_TIME_REPORTED,
                        BOOT_TIME_EVENT_ELAPSED_TIME__EVENT__ZYGOTE_INIT_START,
                        startTime);
            } else if (zygoteSocketName.equals(Zygote.SECONDARY_SOCKET_NAME)) {
                FrameworkStatsLog.write(FrameworkStatsLog.BOOT_TIME_EVENT_ELAPSED_TIME_REPORTED,
                        BOOT_TIME_EVENT_ELAPSED_TIME__EVENT__SECONDARY_ZYGOTE_INIT_START,
                        startTime);
            }
        }

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

        // 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());

            // 预加载系统启动服务
            preload(bootTimingsTraceLog);

            EventLog.writeEvent(LOG_BOOT_PROGRESS_PRELOAD_END,
                    SystemClock.uptimeMillis());
            bootTimingsTraceLog.traceEnd(); // ZygotePreload
        }

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

        bootTimingsTraceLog.traceEnd(); // ZygoteInit

        Zygote.initNativeState(isPrimaryZygote);

        ZygoteHooks.stopZygoteNoThreadCreation();

        zygoteServer = new ZygoteServer(isPrimaryZygote);

        // 如果是启动SystemServer，则fork出SystemServer进程（Android Framework的核心进程）
        // isPrimaryZygote判断是否是 parent (zygote) process
        // abiList: 应用程序二进制接口字符串列表
        // zygoteSocketName: zygoteSocket名称（默认：zygote）
        // zygoteServer: Zygote or system_server 进程的ServerSocket对象
        if (startSystemServer) {
            Runnable r = forkSystemServer(abiList, zygoteSocketName, zygoteServer);

            // {@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.
        // 早在fork子进程之前select轮询就一直在 zygote 进程中
        // 这个阻塞状态会一直存在，用于等待连接，处理来自AMS申请进程创建的请求
        caller = zygoteServer.runSelectLoop(abiList);
    } catch (Throwable ex) {
        Log.e(TAG, "System zygote died with exception", ex);
        throw ex;
    } finally {
        if (zygoteServer != null) {
            zygoteServer.closeServerSocket();
        }
    }

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

static void preload(TimingsTraceLog bootTimingsTraceLog) {
    // 预加载frameworks/base/preloaded-classes和framework_res.apk资源
    Log.d(TAG, "begin preload");
    bootTimingsTraceLog.traceBegin("BeginPreload");
    beginPreload();
    bootTimingsTraceLog.traceEnd(); // BeginPreload
    bootTimingsTraceLog.traceBegin("PreloadClasses");
    preloadClasses();
    bootTimingsTraceLog.traceEnd(); // PreloadClasses
    bootTimingsTraceLog.traceBegin("CacheNonBootClasspathClassLoaders");
    cacheNonBootClasspathClassLoaders();
    bootTimingsTraceLog.traceEnd(); // CacheNonBootClasspathClassLoaders
    bootTimingsTraceLog.traceBegin("PreloadResources");
    preloadResources();
    bootTimingsTraceLog.traceEnd(); // PreloadResources
    Trace.traceBegin(Trace.TRACE_TAG_DALVIK, "PreloadAppProcessHALs");
    nativePreloadAppProcessHALs();
    Trace.traceEnd(Trace.TRACE_TAG_DALVIK);
    Trace.traceBegin(Trace.TRACE_TAG_DALVIK, "PreloadGraphicsDriver");
    maybePreloadGraphicsDriver();
    Trace.traceEnd(Trace.TRACE_TAG_DALVIK);
    preloadSharedLibraries();
    preloadTextResources();
    // Ask the WebViewFactory to do any initialization that must run in the zygote process,
    // for memory sharing purposes.
    WebViewFactory.prepareWebViewInZygote();
    endPreload();
    warmUpJcaProviders();
    Log.d(TAG, "end preload");

    sPreloadComplete = true;
}