SelectionKey.OP_CONNECT
SelectionKey.OP_ACCEPT
SelectionKey.OP_READ
SelectionKey.OP_WRITE

int interestSet = SelectionKey.OP_READ | SelectionKey.OP_WRITE;

key.attachment(); //返回SelectionKey的attachment，attachment可以在注册channel的时候指定。
key.channel(); // 返回该SelectionKey对应的channel。
key.selector(); // 返回该SelectionKey对应的Selector。
key.interestOps(); //返回代表需要Selector监控的IO操作的bit mask
key.readyOps(); // 返回一个bit mask，代表在相应channel上可以进行的IO操作。

public class Client {
    public static void main(String[] args) {
        //这是服务端的IP和端口
        final String DEFAULT_SERVER_HOST = "127.0.0.1";
        final int DEFAULT_SERVER_PORT = 8888;
        //创建Socket
        try (Socket socket = new Socket(DEFAULT_SERVER_HOST, DEFAULT_SERVER_PORT)) {
            //接收消息
            BufferedReader reader = new BufferedReader(
                    new InputStreamReader(socket.getInputStream())
            );
            //发送消息
            BufferedWriter writer = new BufferedWriter(
                    new OutputStreamWriter(socket.getOutputStream())
            );
            //获取用户输入的消息
            BufferedReader userReader = new BufferedReader(
                    new InputStreamReader(System.in)
            );
            String msg = null;
            //循环的话客户端就可以一直输入消息，不然执行完try catch会自动释放资源，也就是断开连接
            while (true) {
                String input = userReader.readLine();
                //写入客户端要发送的消息。因为服务端用readLine获取消息，其以\n为终点，所以要在消息最后加上\n
                writer.write(input + "\n");
                writer.flush();
                msg = reader.readLine();
                System.out.println(msg);
                //如果客户端输入quit就可以跳出循环、断开连接了
                if(input.equals("quit")){
                    break;
                }
            }
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}

public class Server {
    public static void main(String[] args) {
        final int DEFAULT_PORT = 8888;
        //创建ServerSocket监听8888端口
        try (ServerSocket serverSocket = new ServerSocket(DEFAULT_PORT)) {
            System.out.println("ServerSocket Start,The Port is:" + DEFAULT_PORT);
            while (true) {//不停地监听该端口
                //阻塞式的监听，如果没有客户端请求就一直停留在这里
                Socket socket = serverSocket.accept();
                System.out.println("Client[" + socket.getPort() + "]Online");
                //接收消息
                BufferedReader reader = new BufferedReader(
                        new InputStreamReader(socket.getInputStream())
                );
                //发送消息
                BufferedWriter writer = new BufferedWriter(
                        new OutputStreamWriter(socket.getOutputStream())
                );

                String msg = null;
                while ((msg = reader.readLine()) != null) {
                    System.out.println("Client[" + socket.getPort() + "]:" + msg);
                    //写入服务端要发送的消息
                    writer.write("Server:" + msg + "\n");
                    writer.flush();
                    //如果客户端的消息是quit代表他退出了，并跳出循环，不用再接收他的消息了。如果客户端再次连接就会重新上线
                    if (msg.equals("quit")) {
                        System.out.println("Client[" + socket.getPort() + "]:Offline");
                        break;
                    }
                }
            }
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}

public class ChatServer {
    private int DEFAULT_PORT = 8888;
    /**
     * 创建一个Map存储在线用户的信息。这个map可以统计在线用户、针对这些用户可以转发其他用户发送的消息
     * 因为会有多个线程操作这个map，所以为了安全起见用ConcurrentHashMap
     * 在这里key就是客户端的端口号，但在实际中肯定不会用端口号区分用户，如果是web的话一般用session。
     * value是IO的Writer，用以存储客户端发送的消息
     */
    private Map<Integer, Writer> map=new ConcurrentHashMap<>();
    /**
     * 创建线程池，线程上限为10个，如果第11个客户端请求进来，服务器会接收但是不会去分配线程处理它。
     * 前10个客户端的聊天记录，它看不见。当有一个客户端下线时，这第11个客户端就会被分配线程，服务器显示在线
     * 大家可以把10再设置小一点，测试看看
     * */
    private ExecutorService executorService= Executors.newFixedThreadPool(10);
    //客户端连接时往map添加客户端
    public void addClient(Socket socket) throws IOException {
        if (socket != null) {
            BufferedWriter writer = new BufferedWriter(
                    new OutputStreamWriter(socket.getOutputStream())
            );
            map.put(socket.getPort(), writer);
            System.out.println("Client["+socket.getPort()+"]:Online");
        }
    }

    //断开连接时map里移除客户端
    public void removeClient(Socket socket) throws Exception {
        if (socket != null) {
            if (map.containsKey(socket.getPort())) {
                map.get(socket.getPort()).close();
                map.remove(socket.getPort());
            }
            System.out.println("Client[" + socket.getPort() + "]Offline");
        }
    }

    //转发客户端消息，这个方法就是把消息发送给在线的其他的所有客户端
    public void sendMessage(Socket socket, String msg) throws IOException {
        //遍历在线客户端
        for (Integer port : map.keySet()) {
            //发送给在线的其他客户端
            if (port != socket.getPort()) {
                Writer writer = map.get(port);
                writer.write(msg);
                writer.flush();
            }
        }
    }

    //接收客户端请求，并分配Handler去处理请求
    public void start() {
        try (ServerSocket serverSocket = new ServerSocket(DEFAULT_PORT)) {
            System.out.println("Server Start,The Port is:"+DEFAULT_PORT);
            while (true){
                //等待客户端连接
                Socket socket=serverSocket.accept();
                //为客户端分配一个ChatHandler线程
                executorService.execute(new ChatHandler(this,socket));
            }
        } catch (IOException e) {
            e.printStackTrace();
        }
    }

    public static void main(String[] args) {
        ChatServer server=new ChatServer();
        server.start();
    }
}

public class ChatHandler implements Runnable {
    private ChatServer server;
    private Socket socket;

    //构造函数，ChatServer通过这个分配Handler线程
    public ChatHandler(ChatServer server, Socket socket) {
        this.server = server;
        this.socket = socket;
    }

    @Override
    public void run() {
        try {
            //往map里添加这个客户端
            server.addClient(socket);
            //读取这个客户端发送的消息
            BufferedReader reader = new BufferedReader(
                    new InputStreamReader(socket.getInputStream())
            );
            String msg = null;
            while ((msg = reader.readLine()) != null) {
                //这样拼接是为了让其他客户端也能看清是谁发送的消息
                String sendmsg = "Client[" + socket.getPort() + "]:" + msg;
                //服务器打印这个消息
                System.out.println(sendmsg);
                //将收到的消息转发给其他在线客户端
                server.sendMessage(socket, sendmsg + "\n");
                if (msg.equals("quit")) {
                    break;
                }
            }
        } catch (IOException e) {
            e.printStackTrace();
        } finally {
            //如果用户退出或者发生异常，就在map中移除该客户端
            try {
                server.removeClient(socket);
            } catch (Exception e) {
                e.printStackTrace();
            }
        }
    }
}

public class ChatClient {
    private BufferedReader reader;
    private BufferedWriter writer;
    private Socket socket;
    //发送消息给服务器
    public void sendToServer(String msg) throws IOException {
        //发送之前，判断socket的输出流是否关闭
        if (!socket.isOutputShutdown()) {
            //如果没有关闭就把用户键入的消息放到writer里面
            writer.write(msg + "\n");
            writer.flush();
        }
    }
    //从服务器接收消息
    public String receive() throws IOException {
        String msg = null;
        //判断socket的输入流是否关闭
        if (!socket.isInputShutdown()) {
            //没有关闭的话就可以通过reader读取服务器发送来的消息。注意：如果没有读取到消息线程会阻塞在这里
            msg = reader.readLine();
        }
        return msg;
    }

    public void start() {
        //和服务创建连接
        try {
            socket = new Socket("127.0.0.1", 8888);
            reader=new BufferedReader(
                    new InputStreamReader(socket.getInputStream())
            );
            writer=new BufferedWriter(
                    new OutputStreamWriter(socket.getOutputStream())
            );
            //新建一个线程去监听用户输入的消息
            new Thread(new UserInputHandler(this)).start();
            /**
             * 不停的读取服务器转发的其他客户端的信息
             * 记录一下之前踩过的小坑：
             * 这里一定要创建一个msg接收信息，如果直接用receive()方法判断和输出receive()的话会造成有的消息不会显示
             * 因为receive()获取时，在返回之前是阻塞的，一旦接收到消息才会返回，也就是while这里是阻塞的，一旦有消息就会进入到while里面
             * 这时候如果输出的是receive(),那么上次获取的信息就会丢失，然后阻塞在System.out.println
             * */
            String msg=null;
            while ((msg=receive())!=null){
                System.out.println(msg);
            }
        } catch (IOException e) {
            e.printStackTrace();
        }finally {
            try {
               if(writer!=null){
                   writer.close();
               }
            } catch (IOException e) {
                e.printStackTrace();
            }
        }
    }

    public static void main(String[] args) {
        new ChatClient().start();
    }
}

public class UserInputHandler implements Runnable {
    private ChatClient client;

    public UserInputHandler(ChatClient client) {
        this.client = client;
    }

    @Override
    public void run() {
        try {
            //接收用户输入的消息
            BufferedReader reader = new BufferedReader(
                    new InputStreamReader(System.in)
            );
            //不停的获取reader中的System.in，实现了等待用户输入的效果
            while (true) {
                String input = reader.readLine();
                //向服务器发送消息
                client.sendToServer(input);
                if (input.equals("quit"))
                    break;
            }
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}

public class ChatServer {
    //设置缓冲区的大小，这里设置为1024个字节
    private static final int BUFFER = 1024;

    //Channel都要配合缓冲区进行读写，所以这里创建一个读缓冲区和一个写缓冲区
    //allocate()静态方法就是设置缓存区大小的方法
    private ByteBuffer read_buffer = ByteBuffer.allocate(BUFFER);
    private ByteBuffer write_buffer = ByteBuffer.allocate(BUFFER);

    //为了监听端口更灵活，再不写死了，用一个构造函数设置需要监听的端口号
    private int port;

    public ChatServer(int port) {
        this.port = port;
    }

    private void start() {
        //创建ServerSocketChannel和Selector并打开
        try (ServerSocketChannel server = ServerSocketChannel.open(); Selector selector = Selector.open()) {
            //【重点,实现NIO编程模型的关键】configureBlocking设置ServerSocketChannel为非阻塞式调用,Channel默认的是阻塞的调用方式
            server.configureBlocking(false);
            //绑定监听端口,这里不是给ServerSocketChannel绑定，而是给ServerSocket绑定，socket()就是获取通道原生的ServerSocket或Socket
            server.socket().bind(new InetSocketAddress(port));

            //把server注册到Selector并监听Accept事件
            server.register(selector, SelectionKey.OP_ACCEPT);
            System.out.println("启动服务器，监听端口:" + port);

            while (true) {
                //select()会返回此时触发了多少个Selector监听的事件
                if(selector.select()>0) {
                    //获取这些已经触发的事件,selectedKeys()返回的是触发事件的所有信息
                    Set<SelectionKey> selectionKeys = selector.selectedKeys();
                    //循环处理这些事件
                    for (SelectionKey key : selectionKeys) {
                        handles(key, selector);
                    }
                    //处理完后清空selectedKeys，避免重复处理
                    selectionKeys.clear();
                }
            }
        } catch (IOException e) {
            e.printStackTrace();
        }
    }

    //处理事件的方法
    private void handles(SelectionKey key, Selector selector) throws IOException {
        //当触发了Accept事件，也就是有客户端请求进来
        if (key.isAcceptable()) {
            //获取ServerSocketChannel
            ServerSocketChannel server = (ServerSocketChannel) key.channel();
            //然后通过accept()方法接收客户端的请求，这个方法会返回客户端的SocketChannel，这一步和原生的ServerSocket类似
            SocketChannel client = server.accept();
            client.configureBlocking(false);

            //把客户端的SocketChannel注册到Selector，并监听Read事件
            client.register(selector, SelectionKey.OP_READ);
            System.out.println("客户端[" + client.socket().getPort() + "]上线啦！");
        }
        //当触发了Read事件，也就是客户端发来了消息
        if (key.isReadable()) {
            SocketChannel client = (SocketChannel) key.channel();
            //获取消息
            String msg = receive(client);
            System.out.println("客户端[" + client.socket().getPort() + "]:" + msg);
            //把消息转发给其他客户端
            sendMessage(client, msg, selector);
            //判断用户是否退出
            if (msg.equals("quit")) {
                //解除该事件的监听
                key.cancel();
                //更新Selector
                selector.wakeup();
                System.out.println("客户端[" + client.socket().getPort() + "]下线了！");
            }
        }
    }

    //编码方式设置为utf-8，下面字符和字符串互转时用得到
    private Charset charset = Charset.forName("UTF-8");

    //接收消息的方法
    private String receive(SocketChannel client) throws IOException {
        //用缓冲区之前先清空一下,避免之前的信息残留
        read_buffer.clear();
        //把通道里的信息读取到缓冲区，用while循环一直读取，直到读完所有消息。因为没有明确的类似\n这样的结尾，所以要一直读到没有字节为止
        while (client.read(read_buffer) > 0) ;
        //把消息读取到缓冲区后，需要转换buffer的读写状态，不明白的看看前面的Buffer的讲解
        read_buffer.flip();
        return String.valueOf(charset.decode(read_buffer));
    }

    //转发消息的方法
    private void sendMessage(SocketChannel client, String msg, Selector selector) throws IOException {
        msg = "客户端[" + client.socket().getPort() + "]:" + msg;
        //获取所有客户端,keys()与前面的selectedKeys不同，这个是获取所有已经注册的信息，而selectedKeys获取的是触发了的事件的信息
        for (SelectionKey key : selector.keys()) {
            //排除服务器和本客户端并且保证key是有效的，isValid()会判断Selector监听是否正常、对应的通道是保持连接的状态等
            if (!(key.channel() instanceof ServerSocketChannel) && !client.equals(key.channel()) && key.isValid()) {
                SocketChannel otherClient = (SocketChannel) key.channel();
                write_buffer.clear();
                write_buffer.put(charset.encode(msg));
                write_buffer.flip();
                //把消息写入到缓冲区后，再把缓冲区的内容写到客户端对应的通道中
                while (write_buffer.hasRemaining()) {
                    otherClient.write(write_buffer);
                }
            }
        }
    }

    public static void main(String[] args) {
        new ChatServer(8888).start();
    }
}

public class ChatClient {
    private static final int BUFFER = 1024;
    private ByteBuffer read_buffer = ByteBuffer.allocate(BUFFER);
    private ByteBuffer write_buffer = ByteBuffer.allocate(BUFFER);
    //声明成全局变量是为了方便下面一些工具方法的调用，就不用try with resource了
    private SocketChannel client;
    private Selector selector;

    private Charset charset = Charset.forName("UTF-8");

    private void start() {
        try  {
            client=SocketChannel.open();
            selector=Selector.open();
            client.configureBlocking(false);
            //注册channel，并监听SocketChannel的Connect事件
            client.register(selector, SelectionKey.OP_CONNECT);
            //请求服务器建立连接
            client.connect(new InetSocketAddress("127.0.0.1", 8888));
            //和服务器一样，不停的获取触发事件，并做相应的处理
            while (true) {
                selector.select();
                Set<SelectionKey> selectionKeys = selector.selectedKeys();
                for (SelectionKey key : selectionKeys) {
                    handle(key);
                }
                selectionKeys.clear();
            }
        } catch (IOException e) {
            e.printStackTrace();
        }catch (ClosedSelectorException e){
            //当用户输入quit时，在send()方法中，selector会被关闭，而在上面的无限while循环中，可能会使用到已经关闭了的selector。
            //所以这里捕捉一下异常，做正常退出处理就行了。不会对服务器造成影响
        }
    }

    private void handle(SelectionKey key) throws IOException {
        //当触发connect事件，也就是服务器和客户端建立连接
        if (key.isConnectable()) {
            SocketChannel client = (SocketChannel) key.channel();
            //finishConnect()返回true，说明和服务器已经建立连接。如果是false，说明还在连接中，还没完全连接完成
            if(client.finishConnect()){
                //新建一个新线程去等待用户输入
                new Thread(new UserInputHandler(this)).start();
            }
            //连接建立完成后，注册read事件，开始监听服务器转发的消息
            client.register(selector,SelectionKey.OP_READ);
        }
        //当触发read事件，也就是获取到服务器的转发消息
        if(key.isReadable()){
            SocketChannel client = (SocketChannel) key.channel();
            //获取消息
            String msg = receive(client);
            System.out.println(msg);
            //判断用户是否退出
            if (msg.equals("quit")) {
                //解除该事件的监听
                key.cancel();
                //更新Selector
                selector.wakeup();
            }
        }
    }
    //获取消息
    private String receive(SocketChannel client) throws IOException{
        read_buffer.clear();
        while (client.read(read_buffer)>0);
        read_buffer.flip();
        return String.valueOf(charset.decode(read_buffer));
    }

    //发送消息
    public void send(String msg) throws IOException{
        if(!msg.isEmpty()){
            write_buffer.clear();
            write_buffer.put(charset.encode(msg));
            write_buffer.flip();
            while (write_buffer.hasRemaining()){
                client.write(write_buffer);
            }
            if(msg.equals("quit")){
                selector.close();
            }
        }
    }

    public static void main(String[] args) {
        new ChatClient().start();
    }
}

public class UserInputHandler implements Runnable {
    ChatClient client;
    public UserInputHandler(ChatClient chatClient) {
        this.client=chatClient;
    }
    @Override
    public void run() {
        BufferedReader read=new BufferedReader(
                new InputStreamReader(System.in)
        );
        while (true){
            try {
                String input=read.readLine();
                client.send(input);
                if(input.equals("quit"))
                    break;
            } catch (IOException e) {
                e.printStackTrace();
            }
        }
    }
}

Future<Integer> readResult=clientChannel.read(buffer)

public class ChatServer {
    //设置缓冲区字节大小
    private static final int BUFFER = 1024;

    //声明AsynchronousServerSocketChannel和AsynchronousChannelGroup
    private AsynchronousServerSocketChannel serverSocketChannel;
    private AsynchronousChannelGroup channelGroup;

    //在线用户列表。为了并发下的线程安全，所以使用CopyOnWriteArrayList
    //CopyOnWriteArrayList在写时加锁，读时不加锁，而本项目正好在转发消息时需要频繁读取.
    //ClientHandler包含每个客户端的通道，类型选择为ClientHandler是为了在write的时候调用每个客户端的handler
    private CopyOnWriteArrayList<ClientHandler> clientHandlerList;
    //字符和字符串互转需要用到，规定编码方式，避免中文乱码
    private Charset charset = Charset.forName("UTF-8");

    //通过构造函数设置监听端口
    private int port;
    public ChatServer(int port) {
        this.port = port;
        clientHandlerList=new CopyOnWriteArrayList<>();
    }

    public void start() {
        try {
            /**
             *创建一个线程池并把线程池和AsynchronousChannelGroup绑定，前面提到了AsynchronousChannelGroup包括一些系统资源，而线程就是其中一种。
             *为了方便理解我们就暂且把它当作线程池，实际上并不止包含线程池。如果你需要自己选定线程池类型和数量，就可以如下操作
             *如果不需要自定义线程池类型和数量，可以不用写下面两行代码。
             * */
            ExecutorService executorService = Executors.newFixedThreadPool(10);
            channelGroup = AsynchronousChannelGroup.withThreadPool(executorService);
            serverSocketChannel=AsynchronousServerSocketChannel.open(channelGroup);
            serverSocketChannel.bind(new InetSocketAddress("127.0.0.1",port));
            System.out.println("服务器启动：端口【"+port+"】");
            /**
             * AIO中accept可以异步调用，就用上面说到的CompletionHandler方式
             * 第一个参数是辅助参数，回调函数中可能会用上的，如果没有就填null;第二个参数为CompletionHandler接口的实现
             * 这里使用while和System.in.read()的原因：
             * while是为了让服务器保持运行状态，前面的NIO，BIO都有用到while无限循环来保持服务器运行，但是它们用的地方可能更好理解
             * System.in.read()是阻塞式的调用，只是单纯的避免无限循环而让accept频繁被调用，无实际业务功能。
             */
            while (true) {
                serverSocketChannel.accept(null, new AcceptHandler());
                System.in.read();
            }
        } catch (IOException e) {
            e.printStackTrace();
        }finally {
            if(serverSocketChannel!=null){
                try {
                    serverSocketChannel.close();
                } catch (IOException e) {
                    e.printStackTrace();
                }
            }
        }
    }

    //AsynchronousSocketChannel为accept返回的类型，Object为辅助参数类型，没有就填Object
    private class AcceptHandler implements CompletionHandler<AsynchronousSocketChannel,Object>{
        //如果成功，执行的回调方法
        @Override
        public void completed(AsynchronousSocketChannel clientChannel, Object attachment) {
            //如果服务器没关闭，在接收完当前客户端的请求后，再次调用,以接着接收其他客户端的请求
            if(serverSocketChannel.isOpen()){
                serverSocketChannel.accept(null,this);
            }
            //如果客户端的channel没有关闭
            if(clientChannel!=null&&clientChannel.isOpen()){
                //这个就是异步read和write要用到的handler,并传入当前客户端的channel
                ClientHandler handler=new ClientHandler(clientChannel);
                //把新用户添加到在线用户列表里
                clientHandlerList.add(handler);
                System.out.println(getPort(clientChannel)+"上线啦！");
                ByteBuffer buffer=ByteBuffer.allocate(BUFFER);
                //异步调用read,第一个buffer是存放读到数据的容器，第二个是辅助参数。
                //因为真正的处理是在handler里的回调函数进行的，辅助参数会直接传进回调函数，所以为了方便使用，buffer就当作辅助参数
                clientChannel.read(buffer,buffer,handler);
            }
        }
        //如果失败，执行的回调方法
        @Override
        public void failed(Throwable exc, Object attachment) {
            System.out.println("连接失败"+exc);
        }
    }

    private class ClientHandler implements CompletionHandler<Integer, ByteBuffer>{
        private AsynchronousSocketChannel clientChannel;
        public ClientHandler(AsynchronousSocketChannel clientChannel) {
            this.clientChannel = clientChannel;
        }
        @Override
        public void completed(Integer result, ByteBuffer buffer) {
            if(buffer!=null){
                //如果read返回的结果小于等于0，而buffer不为空，说明客户端通道出现异常，做下线操作
                if(result<=0){
                    removeClient(this);
                }else {
                    //转换buffer读写模式并获取消息
                    buffer.flip();
                    String msg=String.valueOf(charset.decode(buffer));
                    //在服务器上打印客户端发来的消息
                    System.out.println(getPort(clientChannel)+msg);
                    //把消息转发给其他客户端
                    sendMessage(clientChannel,getPort(clientChannel)+msg);
                    buffer=ByteBuffer.allocate(BUFFER);

                    //如果用户输入的是退出，就从在线列表里移除。否则接着监听这个用户发送消息
                    if(msg.equals("quit"))
                        removeClient(this);
                    else
                        clientChannel.read(buffer, buffer, this);
                }
            }
        }

        @Override
        public void failed(Throwable exc, ByteBuffer attachment) {
            System.out.println("客户端读写异常："+exc);
        }
    }

    //转发消息的方法
    private void sendMessage(AsynchronousSocketChannel clientChannel,String msg){
        for(ClientHandler handler:clientHandlerList){
            if(!handler.clientChannel.equals(clientChannel)){
                ByteBuffer buffer=charset.encode(msg);
                //write不需要buffer当辅助参数，因为写到客户端的通道就完事了，而读还需要回调函数转发给其他客户端。
                handler.clientChannel.write(buffer,null,handler);
            }
        }
    }
    //根据客户端channel获取对应端口号的方法
    private String getPort(AsynchronousSocketChannel clientChannel){
        try {
            InetSocketAddress address=(InetSocketAddress)clientChannel.getRemoteAddress();
            return "客户端["+address.getPort()+"]:";
        } catch (IOException e) {
            e.printStackTrace();
            return "客户端[Undefined]:";
        }
    }
    //移除客户端
    private void removeClient(ClientHandler handler){
        clientHandlerList.remove(handler);
        System.out.println(getPort(handler.clientChannel)+"断开连接...");
        if(handler.clientChannel!=null){
            try {
                handler.clientChannel.close();
            } catch (IOException e) {
                e.printStackTrace();
            }
        }
    }

    public static void main(String[] args) {
        new ChatServer(8888).start();
    }
}

public class ChatClient {
    private static final int BUFFER = 1024;
    private AsynchronousSocketChannel clientChannel;
    private Charset charset = Charset.forName("UTF-8");

    private String host;
    private int port;
    //设置服务器IP和端口
    public ChatClient(String host, int port) {
        this.host = host;
        this.port = port;
    }

    public void start() {
        try {
            clientChannel = AsynchronousSocketChannel.open();
            //连接服务器
            Future<Void> future = clientChannel.connect(new InetSocketAddress(host, port));
            future.get();
            //新建一个线程去等待用户输入
            new Thread(new UserInputHandler(this)).start();
            ByteBuffer buffer=ByteBuffer.allocate(BUFFER);
            //无限循环让客户端保持运行状态
            while (true){
                //获取服务器发来的消息并存入到buffer
                Future<Integer> read=clientChannel.read(buffer);
                if(read.get()>0){
                    buffer.flip();
                    String msg=String.valueOf(charset.decode(buffer));
                    System.out.println(msg);
                    buffer.clear();
                }else {
                    //如果read的结果小于等于0说明和服务器连接出现异常
                    System.out.println("服务器断开连接");
                    if(clientChannel!=null){
                        clientChannel.close();
                    }
                    System.exit(-1);
                }
            }
        } catch (IOException | InterruptedException | ExecutionException e) {
            e.printStackTrace();
        }
    }

    public void send(String msg) {
        if (msg.isEmpty())
            return;
        ByteBuffer buffer = charset.encode(msg);
        Future<Integer> write=clientChannel.write(buffer);
        try {
            //获取发送结果，如果get方法发生异常说明发送失败
            write.get();
        } catch (ExecutionException|InterruptedException e) {
            System.out.println("消息发送失败");
            e.printStackTrace();
        }
    }

    public static void main(String[] args) {
        new ChatClient("127.0.0.1",8888).start();
    }
}

public class UserInputHandler implements Runnable {
    ChatClient client;
    public UserInputHandler(ChatClient chatClient) {
        this.client=chatClient;
    }
    @Override
    public void run() {
        BufferedReader read=new BufferedReader(
                new InputStreamReader(System.in)
        );
        while (true){
            try {
                String input=read.readLine();
                client.send(input);
                if(input.equals("quit"))
                    break;
            } catch (IOException e) {
                e.printStackTrace();
            }
        }
    }
}