ZygoteInit to java world

先熟悉下：http://blog.csdn.net/jianguo_liao19840726/article/details/15810633

 public static void main(String argv[]) {
            registerZygoteSocket();  
            preloadClasses();      
            preloadResources();    
            if (argv[1].equals("true")) {
                startSystemServer();
            }
            if (ZYGOTE_FORK_MODE) {
                runForkMode();
            } else {
                runSelectLoopMode();
            }
            closeServerSocket();
    }

上面被我删除了一些代码，主要有关方法说明：

1、registerZygoteSocket();注册zygote的socket、即提供socket的服务端

2、preloadClasses();  preloadResources(); 预加载类和资源

3、startSystemServer(); 启动system_server进程

4、运行的孵化器进程的socket的I/O多路复用select循环，等待接受新连接

5、closeServerSocket();关闭socket

先看注册socket，此方法在其注释中已经很好的进行了解释

    /**
     * Registers a server socket for zygote command connections
     *
     * @throws RuntimeException when open fails
     */
    private static void registerZygoteSocket() {
        if (sServerSocket == null) {
            int fileDesc;
            try {
                String env = System.getenv(ANDROID_SOCKET_ENV);
                fileDesc = Integer.parseInt(env);
            } catch (RuntimeException ex) {
                throw new RuntimeException(
                        ANDROID_SOCKET_ENV + " unset or invalid", ex);
            }

            try {
                sServerSocket = new LocalServerSocket(
                        createFileDescriptor(fileDesc));
            } catch (IOException ex) {
                throw new RuntimeException(
                        "Error binding to local socket '" + fileDesc + "'", ex);
            }
        }
    }

有关预加类

   private static void preloadClasses() {
        final VMRuntime runtime = VMRuntime.getRuntime();

        InputStream is = ZygoteInit.class.getClassLoader().getResourceAsStream(
                PRELOADED_CLASSES);
               //PRELOADED_CLASSES常量值为：preloaded-classes在framework/base下面
    
                BufferedReader br
                    = new BufferedReader(new InputStreamReader(is), 256);

                int count = 0;
                String line;
                while ((line = br.readLine()) != null) {
                    // Skip comments and blank lines.
                    line = line.trim();
                    if (line.startsWith("#") || line.equals("")) {
                        continue;
                    }
           
                    try {
                        //通过反射来加载类
                        Class.forName(line);
                     }
               }
    }

加载资源是加载framework-res.apk中的资源，即我们常用的com.android.R.XXX资源，系统默认资源

startSystemServer();先不讲，放到下一个帖子来讲

    private static void runSelectLoopMode() throws MethodAndArgsCaller {
        ArrayList<FileDescriptor> fds = new ArrayList();
        ArrayList<ZygoteConnection> peers = new ArrayList();
        FileDescriptor[] fdArray = new FileDescriptor[4];

        fds.add(sServerSocket.getFileDescriptor());
        peers.add(null);

        int loopCount = GC_LOOP_COUNT;
        while (true) {
            int index;

            /*
             * Call gc() before we block in select().
             * It's work that has to be done anyway, and it's better
             * to avoid making every child do it.  It will also
             * madvise() any free memory as a side-effect.
             *
             * Don't call it every time, because walking the entire
             * heap is a lot of overhead to free a few hundred bytes.
             */
            if (loopCount <= 0) {
                gc();
                loopCount = GC_LOOP_COUNT;
            } else {
                loopCount--;
            }


            try {
                fdArray = fds.toArray(fdArray);
                index = selectReadable(fdArray);
            } catch (IOException ex) {
                throw new RuntimeException("Error in select()", ex);
            }

            if (index < 0) {
                throw new RuntimeException("Error in select()");
            } else if (index == 0) {
                ZygoteConnection newPeer = acceptCommandPeer();
                peers.add(newPeer);
                fds.add(newPeer.getFileDesciptor());
            } else {
                boolean done;
                done = peers.get(index).runOnce();

                if (done) {
                    peers.remove(index);
                    fds.remove(index);
                }
            }
        }
    }

其中最重要的一句selectReadable见上面，我们点进去这个方法

    static native int selectReadable(FileDescriptor[] fds) throws IOException;

从上面来看是个native方法，我们就不深究了，但是从注释来看，我们也能大致了解到这个方法的意思了，下面是两个方法的注释

   /**
     * Runs the zygote process's select loop. Accepts new connections as
     * they happen, and reads commands from connections one spawn-request's
     * worth at a time.

 /**
     * Invokes select() on the provider array of file descriptors (selecting
     * for readability only). Array elements of null are ignored.
     *

从上面的注释来看就是一个socket的读写I/O的多路复用，即接收来自socket的客户短的并发的请求，因为是在while中，所以我们来看看，select返回值的相应的处理

if (index < 0) {
                throw new RuntimeException("Error in select()");
            } else if (index == 0) {
                ZygoteConnection newPeer = acceptCommandPeer();
                peers.add(newPeer);
                fds.add(newPeer.getFileDesciptor());
            } else {
                boolean done;
                done = peers.get(index).runOnce();

                if (done) {
                    peers.remove(index);
                    fds.remove(index);
                }
            }

从上面来看，先是链接上，然后ZygoteConnection.runOnce()来处理客户端的请求