Reactor模型：网络线程模型演进

一，阻塞IO线程池模型（BIO）

这是传统的网络编程方案所采用的线程模型。

即有一个主循环，socket.accept阻塞等待，当建立连接后，创建新的线程/从线程池中取一个，把该socket连接交由新线程全权处理。

这种方案优点即实现简单，缺点则是方案的伸缩性受到线程数的限制。

// 循环监听
while (true) {
    // 阻塞监听客户端请求
    client = server.accept();
    System.out.println(client.getRemoteSocketAddress() + "客户端连接成功!");
    // 将该客户端请求通过线程池放入HandlMsg线程中进行处理
    executorService.execute(new HandleMsg(client));
}

二，Reactor单线程模型

有了NIO后，可以采用IO多路复用机制了。

这是一个单Reactor单线程模型，时序图见下文，该方案只有一个线程，所有Channel的连接均注册在了该Reactor上，由一个线程全权负责所有的任务。

这种方案实现简单，且不受线程数的限制，但受限于使用场景，仅适合于IO密集的应用，不太适合CPU密集的应用，且适合于CPU资源紧张的应用上。

三，Reactor线程池模型

Reactor负责全部IO任务（包括每个Channel的连接和读写），线程池负责业务逻辑的处理。

虽然该方案可以充分利用CPU资源，但是这个方案比单线程版本多了进出Thread Pool的两次上下文切换。

四，主从Reactor模型（Netty的线程模型）

MainReactor负责连接任务，SubReactor负责IO读写、业务计算。
MainReactor和每个SubReactor都是单独的线程，可以调整SubReactor的数量适应CPU资源紧张的应用。
该方案有一个不太明显的缺点，即Session没有分优先级，所有Session平等对待均分到所有的线程中，这样可能会导致优先级低耗资源的Session堵塞高优先级的Session。（ TODO 看下Netty的优化）

五，主从Reactor线程池模型

和主从Reactor模型相比，只是把业务计算放到线程池里了，IO读写还是在SubReactor线程里。

该模型可以更为灵活的适应大多应用场景，通过：调整SubReactor数量、调整Thread Pool参数等。

注意：

如果将IO读写放到线程池里，可能会出现问题：SubReactor选中读就绪事件立马交给线程池，但线程还没来得及read，Channel由于仍然读就绪被select出来重复执行。
上图这样把Channel的读写放在SubReactor，那么此SubReactor上不同Channel的读写会阻塞，但可能效率很高也问题不大。

主从Reactor线程池模型代码示例（调试过了，注意细节见注释）

客户端

public class ReactorClient {

    public static void main(String[] args) throws IOException, InterruptedException {
        for (int i = 0; i < 4; i++) {
            new Thread(() -> {
                try {
                    send();
                } catch (IOException e) {
                    e.printStackTrace();
                }
            }).start();
        }
    }

    static void send() throws IOException {
        // 阻塞模式读写
        SocketChannel socketChannel = SocketChannel.open();
        socketChannel.connect(new InetSocketAddress("127.0.0.1", 9090));
        ByteBuffer writeBuff = ByteBuffer.allocate(20);

        /**
         * 分配太小时，客户端表现：接收数据不完整，但正常退出；服务端表现：业务读写正常，但业务结束后会收到2次读就绪事件，一次读返-1，关闭channel，一次读就会报java.io.IOException: Connection reset by peer
         * TODO 研究下这个原理和如何分配大小
         */
        ByteBuffer readBuff = ByteBuffer.allocate(2000);

        writeBuff.put(("i am client " + Thread.currentThread().getName()).getBytes());
        writeBuff.flip();

        new Thread(new Runnable() {
            @Override
            @SneakyThrows
            public void run() {
                socketChannel.write(writeBuff);
                System.out.println(Thread.currentThread().getName() + " 已发送数据，等待返回");
                readBuff.clear();

                // 阻塞等服务端消息
                socketChannel.read(readBuff);

                readBuff.flip();
                System.out.println(Thread.currentThread().getName() + " 接受服务端消息：" + new String(readBuff.array()));

                // 正常来讲应放入finally
                socketChannel.close();
            }
        }).start();
    }
}

服务端

/**
 * 主从Reactor多线程模型
 */
public class MainSubReactorMultiThread {

    private static final int SUB_COUNT = 4;

    public static void main(String[] args) {
        MainSubReactorMultiThread.MainReactor mainReactor = new MainSubReactorMultiThread.MainReactor(9090);
        mainReactor.run();
    }

    /**
     * 选择就绪的连接事件
     */
    public static class MainReactor implements Runnable {
        ServerSocketChannel serverSocketChannel;
        Selector selector;

        public MainReactor(int port) {
            try {
                serverSocketChannel = ServerSocketChannel.open();
                selector = Selector.open();
                serverSocketChannel.socket().bind(new InetSocketAddress(port));
                serverSocketChannel.configureBlocking(false);
                // 注册了连接事件
                SelectionKey selectionKey = serverSocketChannel.register(selector, SelectionKey.OP_ACCEPT);
                // 并且在selectionKey对象附加了一个Acceptor对象，这是用来处理连接请求的类
                selectionKey.attach(new MainSubReactorMultiThread.Acceptor(serverSocketChannel));
            } catch (IOException e) {
                e.printStackTrace();
            }
        }

        @Override
        public void run() {
            while (true) {
                try {
                    System.out.println("【" + DateUtil.getCurrentDateAndTime() + "】" + "thread:" + Thread.currentThread().getName() + ", " + "mainSelector, 开始监听");
                    selector.select();
                    System.out.println("【" + DateUtil.getCurrentDateAndTime() + "】" + "thread:" + Thread.currentThread().getName() + ", " + "mainSelector, 监听到连接件");
                    Set<SelectionKey> selectionKeys = selector.selectedKeys();
                    Iterator<SelectionKey> iterator = selectionKeys.iterator();
                    while (iterator.hasNext()) {
                        SelectionKey selectionKey = iterator.next();
                        // 这里因为是通过attach附加了事件响应的Runnable，所以不用区分事件类型
                        dispatcher(selectionKey);
                        iterator.remove();
                    }
                } catch (IOException e) {
                    e.printStackTrace();
                }
            }
        }

        private void dispatcher(SelectionKey selectionKey) {
            Runnable runnable = (Runnable) selectionKey.attachment();
            // 同线程执行
            runnable.run();
        }
    }

    /**
     * 选择就绪的读写事件
     */
    public static class SubReactor implements Runnable {
        Selector subSelector;
        int index;

        public SubReactor(Selector subSelector, int index) {
            this.subSelector = subSelector;
            this.index = index;
        }

        @Override
        public void run() {
            while (true) {
                try {
                    System.out.println("【" + DateUtil.getCurrentDateAndTime() + "】" + "thread:" + Thread.currentThread().getName() + ", " + "subSelector:" + index + ", 开始监听");
                    int selectNum = subSelector.select();
                    if (selectNum != 0) {
                        System.out.println("【" + DateUtil.getCurrentDateAndTime() + "】" + "thread:" + Thread.currentThread().getName() + ", " + "subSelector:" + index + ", 监听到就绪事件:" + JSON.toJSONString(subSelector.selectedKeys()));
                    } else {
                        System.out.println("【" + DateUtil.getCurrentDateAndTime() + "】" + "thread:" + Thread.currentThread().getName() + ", " + "subSelector:" + index + ", 未监听到事件，继续轮训");
                        continue;
                    }
                    Set<SelectionKey> selectionKeys = subSelector.selectedKeys();
                    Iterator<SelectionKey> iterator = selectionKeys.iterator();
                    while (iterator.hasNext()) {
                        SelectionKey selectionKey = iterator.next();
                        // 这里因为是通过attach附加了事件响应的Runnable，所以不用区分事件类型
                        dispatcher(selectionKey);
                        iterator.remove();
                    }
                } catch (IOException e) {
                    e.printStackTrace();
                }
            }
        }

        @SneakyThrows
        private void dispatcher(SelectionKey selectionKey) {
            while (true) {
                Runnable runnable = (Runnable) selectionKey.attachment();
                if (runnable != null) {
                    // 同线程执行
                    runnable.run();
                    return;
                }
                System.out.println("【" + DateUtil.getCurrentDateAndTime() + "】" + "thread:" + Thread.currentThread().getName() + ", " + "subSelector:" + index + ", runnable对象未添加完成，等待10ms");
                Thread.sleep(10);
            }

            /**
             * 可能在Acceptor里刚注册channel到selector就被reactor选中执行了，这时注册channel的地方还没执行attach方法，runnable会报NPE，所以要判空
             */
//            Runnable runnable = (Runnable) selectionKey.attachment();
//            runnable.run();
        }
    }

    /**
     * 处理连接
     */
    public static class Acceptor implements Runnable {

        private static Selector[] subSelector = new Selector[SUB_COUNT];

        private ServerSocketChannel serverSocketChannel;

        /**
         * 单线程不会冲突
         */
        private int index = -1;

        @SneakyThrows
        public Acceptor(ServerSocketChannel serverSocketChannel) {
            for (int i = 0; i < SUB_COUNT; i++) {
                subSelector[i] = Selector.open();
                SubReactor subReactor = new SubReactor(subSelector[i], i);
                new Thread(subReactor).start();
            }
            this.serverSocketChannel = serverSocketChannel;
        }

        @Override
        public void run() {
            try {
                SocketChannel socketChannel = serverSocketChannel.accept();
                socketChannel.configureBlocking(false);
                int ind = getNextIndex();
                /**
                 * 本来以为没必要的，但如果不wakeup，会在下一步register阻塞！底层在等待synchronized同步锁
                 * TODO 研究下原理
                 */
                subSelector[ind].wakeup();
                SelectionKey selectionKey = socketChannel.register(subSelector[ind], SelectionKey.OP_READ);
                selectionKey.attach(new MainSubReactorMultiThread.ThreadPollWorkHandler(socketChannel));
                System.out.println("【" + DateUtil.getCurrentDateAndTime() + "】" + "thread:" + Thread.currentThread().getName() + ", " + "客户端已连接：" + socketChannel.getRemoteAddress());
            } catch (IOException e) {
                e.printStackTrace();
            }
        }

        private int getNextIndex() {
            if (index++ == SUB_COUNT - 1) {
                index = 0;
            }
            return index;
        }
    }

    /**
     * 处理读写
     */
    public static class ThreadPollWorkHandler implements Runnable {

        private static ExecutorService executorService = Executors.newCachedThreadPool();

        private SocketChannel socketChannel;

        public ThreadPollWorkHandler(SocketChannel socketChannel) {
            this.socketChannel = socketChannel;
        }

       
        @Override
        public void run() {
            try {
                System.out.println("【" + DateUtil.getCurrentDateAndTime() + "】" + "socketChannel:" + socketChannel.hashCode() + ", " + "开始处理socket读");

                /**
                 * 读数据
                 */
                ByteBuffer byteBuffer = ByteBuffer.allocate(1024);
                int readLength = socketChannel.read(byteBuffer);
                if (readLength == -1) {
                    System.out.println("【" + DateUtil.getCurrentDateAndTime() + "】" + "socketChannel:" + socketChannel.hashCode() + ", " + "客户端已关闭，关闭此通道");
                    socketChannel.close();
                    return;
                }
                String message = new String(byteBuffer.array(), StandardCharsets.UTF_8);
                System.out.println("【" + DateUtil.getCurrentDateAndTime() + "】" + "socketChannel:" + socketChannel.hashCode() + ", 客户端:" + socketChannel.getRemoteAddress() + ", socket读完成: " + message);

                /**
                 * 线程池处理业务计算
                 */
                TaskHandler taskHandler = new TaskHandler(socketChannel, message);
                Future<String> taskResult = executorService.submit(taskHandler);

                /**
                 * 写数据
                 */
                ByteBuffer writeBuffer = ByteBuffer.wrap((socketChannel.getRemoteAddress() + ":" + taskResult.get()).getBytes(StandardCharsets.UTF_8));
                socketChannel.write(writeBuffer);
                System.out.println("【" + DateUtil.getCurrentDateAndTime() + "】" + "socketChannel:" + socketChannel.hashCode() + ", " + "已返回客户端数据，请求处理最终完成");
            } catch (Exception e) {
                e.printStackTrace();
            }
        }
    }

    static class TaskHandler implements Callable<String> {

        private SocketChannel socketChannel;

        private String parameter;

        public TaskHandler(SocketChannel socketChannel, String parameter) {
            this.socketChannel = socketChannel;
            this.parameter = parameter;
        }

        @Override
        public String call() throws Exception {
            System.out.println("【" + DateUtil.getCurrentDateAndTime() + "】线程池Thread:" + Thread.currentThread().getName() + "socketChannel:" + socketChannel.hashCode() + ", 客户端:" + socketChannel.getRemoteAddress() + ", 开始处理业务计算 参数: " + parameter);
            Thread.sleep(1000);
            String result = String.format("response(%s) for (%s)", RandomStringUtils.randomAlphanumeric(30), parameter).trim();
            System.out.println("【" + DateUtil.getCurrentDateAndTime() + "】线程池Thread:" + Thread.currentThread().getName() + "socketChannel:" + socketChannel.hashCode() + ", 客户端:" + socketChannel.getRemoteAddress() + ", 业务计算完成 返回: " + result);
            return result;
        }
    }
}