1.Server

public class NioServer {

    private static final Map<String, SocketChannel> clientMap = new HashMap<>();

    public static void main(String[] args) throws Exception {

        ServerSocketChannel serverSocketChannel = ServerSocketChannel.open();
        serverSocketChannel.configureBlocking(false);
        ServerSocket serverSocket = serverSocketChannel.socket();
        serverSocket.bind(new InetSocketAddress(8888));

        Selector selector = Selector.open();
        serverSocketChannel.register(selector, SelectionKey.OP_ACCEPT);

        while (true) {
            try {


                selector.select();

                Set<SelectionKey> selectionKeys = selector.selectedKeys();

                selectionKeys.forEach(selectionKey -> {
                    final SocketChannel client;
                    try {
                        if (selectionKey.isAcceptable()) {
                            ServerSocketChannel server = (ServerSocketChannel) selectionKey.channel();
                            client = server.accept();
                            client.configureBlocking(false);
                            client.register(selector, SelectionKey.OP_READ);

                            clientMap.put("【" + UUID.randomUUID().toString() + "】", client);
                        } else if (selectionKey.isReadable()) {
                            client = (SocketChannel) selectionKey.channel();
                            ByteBuffer buffer = ByteBuffer.allocate(1024);

                            int count = client.read(buffer);
                            if (count > 0) {
                                buffer.flip();

                                Charset charset = Charset.forName("utf-8");
                                String receivedMsg = String.valueOf(charset.decode(buffer).array());
                                System.out.println("client " + client + " ~ " + receivedMsg);

                                String senderKey = null;
                                for (Map.Entry<String, SocketChannel> entry : clientMap.entrySet()) {
                                    if (client == entry.getValue()) {
                                        senderKey = entry.getKey();
                                        break;
                                    }
                                }

                                for (Map.Entry<String, SocketChannel> entry : clientMap.entrySet()) {
                                    SocketChannel value = entry.getValue();
                                    ByteBuffer writeBuffer = ByteBuffer.allocate(1024);

                                    writeBuffer.put((senderKey + ":" + receivedMsg).getBytes());
                                    writeBuffer.flip();

                                    value.write(writeBuffer);
                                }
                            }
                        }
                        // very important
                        selectionKeys.clear();
                    } catch (Exception e) {
                        e.printStackTrace();
                    }
                });
            } catch (Exception e) {
                e.printStackTrace();
            }
        }
    }
}

2.Client

public class NioClient {
    public static void main(String[] args) throws Exception {
        try {
            SocketChannel socketChannel = SocketChannel.open();
            socketChannel.configureBlocking(false);

            Selector selector = Selector.open();
            socketChannel.register(selector, SelectionKey.OP_CONNECT);
            socketChannel.connect(new InetSocketAddress("localhost", 8888));

            while (true) {
                selector.select();
                Set<SelectionKey> selectionKeys = selector.selectedKeys();
                for (SelectionKey selectionKey : selectionKeys) {
                    if (selectionKey.isConnectable()) {
                        SocketChannel client = (SocketChannel) selectionKey.channel();

                        if (client.isConnectionPending()) {
                            client.finishConnect();

                            ByteBuffer writeBuffer = ByteBuffer.allocate(1024);
                            writeBuffer.put((LocalDateTime.now() + " 连接成功").getBytes());
                            writeBuffer.flip();
                            client.write(writeBuffer);

                            ExecutorService executorService = Executors.newSingleThreadExecutor(Executors.defaultThreadFactory());
                            executorService.submit(() -> {
                                while (true) {
                                    try {
                                        writeBuffer.clear();
                                        InputStreamReader input = new InputStreamReader(System.in);
                                        BufferedReader br = new BufferedReader(input);

                                        String sendMsg = br.readLine();
                                        writeBuffer.put(sendMsg.getBytes());
                                        writeBuffer.flip();
                                        client.write(writeBuffer);
                                    } catch (Exception e) {
                                        e.printStackTrace();
                                    }
                                }
                            });
                        }

                        client.register(selector, SelectionKey.OP_READ);
                    } else if (selectionKey.isReadable()) {
                        SocketChannel client = (SocketChannel) selectionKey.channel();

                        ByteBuffer readBuffer = ByteBuffer.allocate(1024);
                        int count = client.read(readBuffer);
                        if (count > 0) {
                            String receivedMsg = new String(readBuffer.array(), 0, count);
                            System.out.println(receivedMsg);
                        }
                    }
                }
                selectionKeys.clear();
            }

        } catch (Exception e) {
            e.printStackTrace();
        }
    }
}