2. Android系统进程启动流程

public static void main(String argv[]) {
        ...
        try {
            //设置DDMS可用
            RuntimeInit.enableDdms(); //1
            //初始化启动参数
            boolean startSystemServer = false;
            String socketName = "zygote";
            String abiList = null;
            //2
            for (int i = 1; i < argv.length; i++) {
                if ("start-system-server".equals(argv[i])) {
                    startSystemServer = 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.");
            }
            //注册Zygote用的Socket
            registerZygoteSocket(socketName); //3
            //预加载类和资源
            preload(); //4
            ...
            //启动SystemServer进程
            if (startSystemServer) {
                startSystemServer(abiList, socketName); //5
            }
            //监听socket,启动新的应用进程  
            runSelectLoop(abiList); //6
            closeServerSocket();
        } catch (MethodAndArgsCaller caller) {
            //通过反射调用SystemServer#main()
            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);
                //不是使用IP和端口、而是使用fd创建socket
                sServerSocket = new LocalServerSocket(fd); //1
            } catch (IOException ex) {
                throw new RuntimeException(
                        "Error binding to local socket '" + fileDesc + "'", ex);
            }
        }
    }

static void preload() {
        beginIcuCachePinning();
        preloadClasses(); //加载所需的各种class文件
        preloadResources(); //加载资源文件
        preloadOpenGL(); //初始化OpenGL
        preloadSharedLibraries(); //加载系统Libraries
        preloadTextResources(); //加载文字资源
        // Ask the WebViewFactory to do any initialization that must run in the zygote process,
        // for memory sharing purposes.
        WebViewFactory.prepareWebViewInZygote(); //初始化WebView
        endIcuCachePinning();
        warmUpJcaProviders();
        Log.d(TAG, "end preload");
    }

private static boolean startSystemServer(String abiList, String socketName)
            throws MethodAndArgsCaller, RuntimeException {
        long capabilities = posixCapabilitiesAsBits(
            OsConstants.CAP_IPC_LOCK,
            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
        );
        /* Containers run without this capability, so avoid setting it in that case */
        if (!SystemProperties.getBoolean(PROPERTY_RUNNING_IN_CONTAINER, false)) {
            capabilities |= posixCapabilitiesAsBits(OsConstants.CAP_BLOCK_SUSPEND);
        }
        /* Hardcoded command line to start the system server */
        /* 1 */
        String args[] = {
            "--setuid=1000",
            "--setgid=1000",
            "--setgroups=1001,1002,1003,1004,1005,1006,1007,1008,1009,1010,1018,1021,1032,3001,3002,3003,3006,3007,3009,3010",
            "--capabilities=" + capabilities + "," + capabilities,
            "--nice-name=system_server",
            "--runtime-args",
            "com.android.server.SystemServer",
        };
        ZygoteConnection.Arguments parsedArgs = null;
        int pid;
        try {
            /* 2 */
            parsedArgs = new ZygoteConnection.Arguments(args); 
            /** 打开系统调试属性*/
            ZygoteConnection.applyDebuggerSystemProperty(parsedArgs);
            ZygoteConnection.applyInvokeWithSystemProperty(parsedArgs);
            /* 3 */
            // 请求fork SystemServer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);
        }
        // pid为0表示子进程，即SystemServer进程，从此SystemServer进程与Zygote进程分道扬镳
        if (pid == 0) {
            if (hasSecondZygote(abiList)) {
                waitForSecondaryZygote(socketName);
            }
            handleSystemServerProcess(parsedArgs); //4
        }
        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()); //1
        peers.add(null);
        while (true) {
            StructPollfd[] pollFds = new StructPollfd[fds.size()];
            for (int i = 0; i < pollFds.length; ++i) {  //2
                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) {  //3
                if ((pollFds[i].revents & POLLIN) == 0) {
                    continue;
                }
                if (i == 0) {
                    ZygoteConnection newPeer = acceptCommandPeer(abiList);  //4
                    peers.add(newPeer);
                    fds.add(newPeer.getFileDesciptor());
                } else {
                    boolean done = peers.get(i).runOnce();  //5
                    if (done) {
                        peers.remove(i);
                        fds.remove(i);
                    }
                }
            }
        }
    }