//ZygoteInit.java
public static void main(String argv[]) {
    try {
        ......
        
        /* ①注册zygote使用的socket */
        registerZygoteSocket(socketName);
        EventLog.writeEvent(LOG_BOOT_PROGRESS_PRELOAD_START,
            SystemClock.uptimeMillis());

        /* ②预加载类和资源 */
        preload();
        
        ......
        
        /* ③启动system_server进程 */
        if (startSystemServer) {
            startSystemServer(abiList, socketName);
        }

        ......

        /* ④运行zygote进程的select循环，当有新的连接请求时就接受 */
        runSelectLoop(abiList);

        closeServerSocket();
    } catch (MethodAndArgsCaller caller) {
        /* ⑤在system_serer进程中，以反射方式调用SystemServer类的main方法 */
        caller.run();
    } catch (RuntimeException ex) {
        Log.e(TAG, "Zygote died with exception", ex);
        closeServerSocket();
        throw ex;
    }
}

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);
        }
    }
}

static void preload() {
    Log.d(TAG, "begin preload");
    preloadClasses();
    preloadResources();
    preloadOpenGL();
    preloadSharedLibraries();
    preloadTextResources();
    // Ask the WebViewFactory to do any initialization that must run in the zygote process,
    // for memory sharing purposes.
    WebViewFactory.prepareWebViewInZygote();
    Log.d(TAG, "end preload");
}

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
    );
    /* 硬编码相关启动参数 */
    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",//设置进程名为system_server
        "--runtime-args",
        "com.android.server.SystemServer",//实际启动的类
    };
    ZygoteConnection.Arguments parsedArgs = null;

    int pid;

    try {
        parsedArgs = new ZygoteConnection.Arguments(args);
        ZygoteConnection.applyDebuggerSystemProperty(parsedArgs);
        ZygoteConnection.applyInvokeWithSystemProperty(parsedArgs);

        /* fork一个子进程，也就是system_server进程 */
        pid = Zygote.forkSystemServer(
                parsedArgs.uid, parsedArgs.gid,
                parsedArgs.gids,
                parsedArgs.debugFlags,
                null,
                parsedArgs.permittedCapabilities,
                parsedArgs.effectiveCapabilities);
    } catch (IllegalArgumentException ex) {
        throw new RuntimeException(ex);
    }

    /* 对于子进程，也就是system_server，进入下面的代码 */
    if (pid == 0) {
        /* 参考本文最开头的init.zygote32_64.rc文件，会启动2个zygote，第二个的名字是"zygote_secondary" */
        if (hasSecondZygote(abiList)) { 
            waitForSecondaryZygote(socketName);
        }

        /* 把剩下的工作交给system_server进程处理 */
        handleSystemServerProcess(parsedArgs);
    }

    return true;
}

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) {
                /* 等待并接受客户端的连接请求 */
                ZygoteConnection newPeer = acceptCommandPeer(abiList);
                peers.add(newPeer);
                fds.add(newPeer.getFileDesciptor());
            } else {
                /* 将客户端的后续请求交给ZygoteConnection的runOnce()方法处理 */
                boolean done = peers.get(i).runOnce();
                if (done) {
                    peers.remove(i);
                    fds.remove(i);
                }
            }
        }
    }
}