java网络通信：异步非阻塞I/O (NIO)

首先是channel，是一个双向的全双工的通道，可同时读写，而输入输出流都是单工的，要么读要么写。Channel分为两大类，分别是用于网络数据的SelectableChannel和用于文件操作的FileChannel。

注意：在java NIO库中，所有的数据都是用缓冲区处理，常用的是ByteBuffer。

多路复用器Selector：

Selector会不断轮询注册在其上的Channel，如果某个Channel上又新的连接接入、读和写事件，这个Channel就处于就绪状态，通过SelectorKey可以获取就绪Channel的集合。底层使用了epoll()实现，没有最大连接句柄的限制。

服务端代码：

public class TimeServer {
    public static void main(String[] args) throws IOException {
        int port = 8080;
        MultiplexerTimeServer timeServer = new MultiplexerTimeServer(port);
        new Thread(timeServer, "NIO-MultiplexerTimeServer-001").start();
    }
}
public class MultiplexerTimeServer implements Runnable {

    private Selector selector;
    private ServerSocketChannel servChannel;
    private volatile boolean stop;

    public MultiplexerTimeServer(int port) {
        try {
            selector = Selector.open();
            servChannel = ServerSocketChannel.open();
            servChannel.configureBlocking(false);//设置非阻塞模式
            servChannel.socket().bind(new InetSocketAddress(port), 1024);
            servChannel.register(selector, SelectionKey.OP_ACCEPT);//将Channel注册到selector，监听accept事件
            System.out.println("The time server is start in port : " + port);
        } catch (IOException e) {
            e.printStackTrace();
            System.exit(1);
        }
    }

    public void stop() {
    this.stop = true;
    }

    @Override
    public void run() {
    while (!stop) {
        try {
            selector.select(1000);//每隔一秒轮询一次
            Set<SelectionKey> selectedKeys = selector.selectedKeys();
            Iterator<SelectionKey> it = selectedKeys.iterator();
            SelectionKey key = null;
            while (it.hasNext()) {//如果有新的客户端接入
                key = it.next();
                it.remove();
                try {
                    handleInput(key);//处理请求
                } catch (Exception e) {
                    if (key != null) {
                        key.cancel();
                        if (key.channel() != null)
                            key.channel().close();
                    }
                }
            }
        } catch (Throwable t) {
            t.printStackTrace();
        }
    }

    // 多路复用器关闭后，所有注册在上面的Channel和Pipe等资源都会被自动去注册并关闭，所以不需要重复释放资源
    if (selector != null)
        try {
        selector.close();
        } catch (IOException e) {
        e.printStackTrace();
        }
    }

    private void handleInput(SelectionKey key) throws IOException {
        if (key.isValid()) {
            // 处理新接入的请求消息
            if (key.isAcceptable()) {
                // 获取客户端的连接通道
                ServerSocketChannel ssc = (ServerSocketChannel) key.channel();
                SocketChannel sc = ssc.accept();
                sc.configureBlocking(false);
                //将新连接注册到selector，并监听读事件
                sc.register(selector, SelectionKey.OP_READ);
            }
            if (key.isReadable()) {
                //读取通道数据写入字节缓存
                SocketChannel sc = (SocketChannel) key.channel();
                ByteBuffer readBuffer = ByteBuffer.allocate(1024);
                int readBytes = sc.read(readBuffer);
                if (readBytes > 0) {
                    readBuffer.flip();
                    byte[] bytes = new byte[readBuffer.remaining()];
                    readBuffer.get(bytes);//字节缓存写入字节数组
                    String body = new String(bytes, "UTF-8");
                    System.out.println("The time server receive order : " + body);
                    String currentTime = "QUERY TIME ORDER".equalsIgnoreCase(body)
                            ? new java.util.Date(System.currentTimeMillis()).toString()
                            : "BAD ORDER";
                    doWrite(sc, currentTime);//向socketchannel写入数据
                } else if (readBytes < 0) {
                    // 对端链路关闭
                    key.cancel();
                    sc.close();
                } 
            }
        }
    }

    private void doWrite(SocketChannel channel, String response)
        throws IOException {
        if (response != null && response.trim().length() > 0) {
            byte[] bytes = response.getBytes();
            ByteBuffer writeBuffer = ByteBuffer.allocate(bytes.length);
            writeBuffer.put(bytes);
            writeBuffer.flip();
            channel.write(writeBuffer);
        }
    }
}

客户端：

public class TimeClient {
    
    public static void main(String[] args) {
        int port = 8080;
        new Thread(new TimeClientHandle("127.0.0.1", port), "TimeClient-001").start();
    }
}
public class TimeClientHandle implements Runnable {

    private String host;
    private int port;

    private Selector selector;
    private SocketChannel socketChannel;

    private volatile boolean stop;

    public TimeClientHandle(String host, int port) {
        this.host = host == null ? "127.0.0.1" : host;
        this.port = port;
        try {
            selector = Selector.open();
            socketChannel = SocketChannel.open();
            socketChannel.configureBlocking(false);
        } catch (IOException e) {
            e.printStackTrace();
            System.exit(1);
        }
    }

    @Override
    public void run() {
        try {
            doConnect();
        } catch (IOException e) {
            e.printStackTrace();
            System.exit(1);
        }
        while (!stop) {
            try {
                selector.select(1000);
                Set<SelectionKey> selectedKeys = selector.selectedKeys();
                Iterator<SelectionKey> it = selectedKeys.iterator();
                SelectionKey key = null;
                while (it.hasNext()) {
                    key = it.next();
                    it.remove();
                    try {
                        handleInput(key);
                    } catch (Exception e) {
                        if (key != null) {
                            key.cancel();
                            if (key.channel() != null)
                                key.channel().close();
                        }
                    }
                }
            } catch (Exception e) {
                e.printStackTrace();
                System.exit(1);
            }
        }
        // 多路复用器关闭后，所有注册在上面的Channel和Pipe等资源都会被自动去注册并关闭，所以不需要重复释放资源
        if (selector != null)
            try {
                selector.close();
            } catch (IOException e) {
                e.printStackTrace();
            }

    }
    private void doConnect() throws IOException {
        // 如果直接连接成功，则注册到多路复用器上，发送请求消息，读应答
        if (!socketChannel.connect(new InetSocketAddress(host, port))) {
            socketChannel.register(selector, SelectionKey.OP_READ);
            doWrite(socketChannel);
        } else//没有直接连接成功，不代表失败，而是说明服务器还没有返回TCP握手的应答消息，所以注册OP_CONNECT事件，监听消息。
            socketChannel.register(selector, SelectionKey.OP_CONNECT);    }

    private void handleInput(SelectionKey key) throws IOException {
        if (key.isValid()) {
            // 判断是否连接成功
            SocketChannel sc = (SocketChannel) key.channel();
            if (key.isConnectable()) {//判断是否有连接事件，是的话，说明未连接
                if (sc.finishConnect()) {//再连接
                    sc.register(selector, SelectionKey.OP_READ);
                    doWrite(sc);
                } else
                    System.exit(1);// 连接失败，进程退出
            }
            if (key.isReadable()) {
                ByteBuffer readBuffer = ByteBuffer.allocate(1024);
                int readBytes = sc.read(readBuffer);
                if (readBytes > 0) {
                    readBuffer.flip();
                    byte[] bytes = new byte[readBuffer.remaining()];
                    readBuffer.get(bytes);
                    String body = new String(bytes, "UTF-8");
                    System.out.println("Now is : " + body);
                    this.stop = true;
                } else if (readBytes < 0) {
                    // 对端链路关闭
                    key.cancel();
                    sc.close();
                }
            }
        }

    }
    private void doWrite(SocketChannel sc) throws IOException {
        byte[] req = "QUERY TIME ORDER".getBytes();
        ByteBuffer writeBuffer = ByteBuffer.allocate(req.length);
        writeBuffer.put(req);
        writeBuffer.flip();
        sc.write(writeBuffer);
        if (!writeBuffer.hasRemaining())
            System.out.println("Send order 2 server succeed.");
    }
}