用NIO技术代替一客户一线程技术改写Echo项目的客户机/服务器设计，请给出代码

以下是使用NIO技术改写Echo项目的客户机/服务器设计的Java代码示例：

服务器端代码：

import java.io.IOException;
import java.net.InetSocketAddress;
import java.nio.ByteBuffer;
import java.nio.channels.SelectionKey;
import java.nio.channels.Selector;
import java.nio.channels.ServerSocketChannel;
import java.nio.channels.SocketChannel;
import java.util.Iterator;

public class NioEchoServer {
    private static final int BUF_SIZE = 1024;
    private static final int PORT = 8080;
    private static final int TIMEOUT = 3000;

    public static void main(String[] args) {
        selector();
    }

    public static void selector() {
        Selector selector = null;
        ServerSocketChannel ssc = null;
        try {
            // 创建选择器
            selector = Selector.open();
            // 打开监听通道
            ssc = ServerSocketChannel.open();
            // 绑定端口
            ssc.socket().bind(new InetSocketAddress(PORT));
            // 设置为非阻塞模式
            ssc.configureBlocking(false);
            // 注册选择器并监听接受事件
            ssc.register(selector, SelectionKey.OP_ACCEPT);
            System.out.println("Server started and listen on port " + PORT + "...");

            while (true) {
                // 等待事件
                if (selector.select(TIMEOUT) == 0) {
                    System.out.print(".");
                    continue;
                }
                // 获取事件集合
                Iterator<SelectionKey> iter = selector.selectedKeys().iterator();
                while (iter.hasNext()) {
                    SelectionKey key = iter.next();
                    // 处理事件
                    handleKey(key);
                    iter.remove();
                }
            }
        } catch (IOException e) {
            e.printStackTrace();
        } finally {
            try {
                if (selector != null) {
                    selector.close();
                }
                if (ssc != null) {
                    ssc.close();
                }
            } catch (IOException e) {
                e.printStackTrace();
            }
        }
    }

    public static void handleKey(SelectionKey key) throws IOException {
        if (key.isAcceptable()) {
            // 处理接受事件
            ServerSocketChannel ssc = (ServerSocketChannel) key.channel();
            SocketChannel sc = ssc.accept();
            sc.configureBlocking(false);
            sc.register(key.selector(), SelectionKey.OP_READ);
        }
        if (key.isReadable()) {
            // 处理读事件
            SocketChannel sc = (SocketChannel) key.channel();
            ByteBuffer buf = ByteBuffer.allocate(BUF_SIZE);
            int len = sc.read(buf);
            if (len > 0) {
                buf.flip();
                byte[] bytes = new byte[buf.remaining()];
                buf.get(bytes);
                String msg = new String(bytes, "UTF-8");
                System.out.println("Server received: " + msg);
                sc.register(key.selector(), SelectionKey.OP_WRITE);
                // 将读取到的数据写回客户端
                ByteBuffer writeBuf = ByteBuffer.wrap(bytes);
                sc.write(writeBuf);
            } else if (len == -1) {
                System.out.println("Client closed...");
                sc.close();
            }
        }
        if (key.isWritable()) {
            // 处理写事件
            SocketChannel sc = (SocketChannel) key.channel();
            ByteBuffer buf = (ByteBuffer) key.attachment();
            buf.flip();
            sc.write(buf);
            if (!buf.hasRemaining()) {
                key.interestOps(SelectionKey.OP_READ);
            }
            buf.compact();
        }
    }
}

客户端代码：

import java.io.IOException;
import java.net.InetSocketAddress;
import java.nio.ByteBuffer;
import java.nio.channels.SocketChannel;

public class NioEchoClient {
    private static final int BUF_SIZE = 1024;
    private static final int PORT = 8080;

    public static void main(String[] args) {
        try {
            // 打开通道
            SocketChannel sc = SocketChannel.open();
            // 设置为非阻塞模式
            sc.configureBlocking(false);
            // 连接服务器
            sc.connect(new InetSocketAddress(PORT));
            while (!sc.finishConnect()) {
                System.out.print(".");
            }
            System.out.println("Client connected...");

            // 发送数据
            ByteBuffer buf = ByteBuffer.allocate(BUF_SIZE);
            buf.put("Hello, NIO!".getBytes());
            buf.flip();
            sc.write(buf);

            // 接收数据
            buf.clear();
            int len = sc.read(buf);
            if (len > 0) {
                buf.flip();
                byte[] bytes = new byte[buf.remaining()];
                buf.get(bytes);
                String msg = new String(bytes, "UTF-8");
                System.out.println("Client received: " + msg);
            }
            // 关闭通道
            sc.close();
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}

以上代码示例中，服务器端使用Selector监听ACCEPT、READ和WRITE事件，客户端使用SocketChannel与服务器建立连接，并发送和接收数据。通过NIO技术，可以实现多个客户端连接共享少量线程的并发处理。