用 Java Nio 实现一个和Netty一样的功能Server端和client端

要实现一个和Netty一样的功能Server端和client端，可以使用Java NIO来进行实现。下面是一个简单的示例代码：

### Server端

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;
import java.util.Set;

public class NioServer {
    private Selector selector;
    private ByteBuffer readBuffer = ByteBuffer.allocate(1024);
    private ByteBuffer writeBuffer = ByteBuffer.allocate(1024);

    public NioServer(int port) {
        try {
            // 创建ServerSocketChannel对象并绑定端口
            ServerSocketChannel serverSocketChannel = ServerSocketChannel.open();
            serverSocketChannel.socket().bind(new InetSocketAddress(port));
            serverSocketChannel.configureBlocking(false);

            // 创建Selector对象
            selector = Selector.open();

            // 将ServerSocketChannel注册到Selector上，并设置为监听OP_ACCEPT事件
            serverSocketChannel.register(selector, SelectionKey.OP_ACCEPT);
            System.out.println("Server started, listening on port " + port);
        } catch (IOException e) {
            e.printStackTrace();
            System.exit(1);
        }
    }

    public void start() {
        try {
            while (true) {
                // 阻塞等待事件的发生
                selector.select();

                // 获取发生事件的SelectionKey集合
                Set<SelectionKey> selectionKeys = selector.selectedKeys();
                Iterator<SelectionKey> iterator = selectionKeys.iterator();

                while (iterator.hasNext()) {
                    SelectionKey selectionKey = iterator.next();
                    iterator.remove();

                    if (selectionKey.isAcceptable()) {
                        // ServerSocketChannel可以接收客户端连接
                        ServerSocketChannel serverSocketChannel = (ServerSocketChannel) selectionKey.channel();
                        SocketChannel socketChannel = serverSocketChannel.accept();
                        socketChannel.configureBlocking(false);
                        socketChannel.register(selector, SelectionKey.OP_READ);
                        System.out.println("Client " + socketChannel.getRemoteAddress() + " connected.");
                    } else if (selectionKey.isReadable()) {
                        // SocketChannel可以读取数据
                        SocketChannel socketChannel = (SocketChannel) selectionKey.channel();
                        readBuffer.clear();
                        int numRead = socketChannel.read(readBuffer);
                        if (numRead == -1) {
                            // 客户端关闭连接
                            selectionKey.cancel();
                            socketChannel.close();
                            System.out.println("Client " + socketChannel.getRemoteAddress() + " disconnected.");
                        } else {
                            // 处理读取到的数据
                            String request = new String(readBuffer.array(), 0, numRead);
                            System.out.println("Received request from client " + socketChannel.getRemoteAddress() + ": " + request);
                            socketChannel.register(selector, SelectionKey.OP_WRITE);
                        }
                    } else if (selectionKey.isWritable()) {
                        // SocketChannel可以写入数据
                        SocketChannel socketChannel = (SocketChannel) selectionKey.channel();
                        writeBuffer.clear();
                        String response = "Hello from server!";
                        writeBuffer.put(response.getBytes());
                        writeBuffer.flip();
                        socketChannel.write(writeBuffer);
                        socketChannel.register(selector, SelectionKey.OP_READ);
                    }
                }
            }
        } catch (IOException e) {
            e.printStackTrace();
            System.exit(1);
        }
    }

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

### Client端

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.SocketChannel;
import java.util.Iterator;
import java.util.Set;

public class NioClient {
    private Selector selector;
    private ByteBuffer readBuffer = ByteBuffer.allocate(1024);
    private ByteBuffer writeBuffer = ByteBuffer.allocate(1024);

    public NioClient(String host, int port) {
        try {
            // 创建SocketChannel对象并连接服务器
            SocketChannel socketChannel = SocketChannel.open();
            socketChannel.configureBlocking(false);
            socketChannel.connect(new InetSocketAddress(host, port));

            // 创建Selector对象
            selector = Selector.open();

            // 将SocketChannel注册到Selector上，并设置为监听OP_CONNECT事件
            socketChannel.register(selector, SelectionKey.OP_CONNECT);
            System.out.println("Connecting to server " + host + ":" + port);
        } catch (IOException e) {
            e.printStackTrace();
            System.exit(1);
        }
    }

    public void start() {
        try {
            while (true) {
                // 阻塞等待事件的发生
                selector.select();

                // 获取发生事件的SelectionKey集合
                Set<SelectionKey> selectionKeys = selector.selectedKeys();
                Iterator<SelectionKey> iterator = selectionKeys.iterator();

                while (iterator.hasNext()) {
                    SelectionKey selectionKey = iterator.next();
                    iterator.remove();

                    if (selectionKey.isConnectable()) {
                        // SocketChannel已连接到服务器
                        SocketChannel socketChannel = (SocketChannel) selectionKey.channel();
                        if (socketChannel.isConnectionPending()) {
                            socketChannel.finishConnect();
                        }
                        socketChannel.configureBlocking(false);
                        socketChannel.register(selector, SelectionKey.OP_WRITE);
                        System.out.println("Connected to server " + socketChannel.getRemoteAddress());
                    } else if (selectionKey.isReadable()) {
                        // SocketChannel可以读取数据
                        SocketChannel socketChannel = (SocketChannel) selectionKey.channel();
                        readBuffer.clear();
                        int numRead = socketChannel.read(readBuffer);
                        if (numRead == -1) {
                            // 服务器关闭连接
                            selectionKey.cancel();
                            socketChannel.close();
                            System.out.println("Server " + socketChannel.getRemoteAddress() + " disconnected.");
                        } else {
                            // 处理读取到的数据
                            String response = new String(readBuffer.array(), 0, numRead);
                            System.out.println("Received response from server " + socketChannel.getRemoteAddress() + ": " + response);
                            socketChannel.register(selector, SelectionKey.OP_WRITE);
                        }
                    } else if (selectionKey.isWritable()) {
                        // SocketChannel可以写入数据
                        SocketChannel socketChannel = (SocketChannel) selectionKey.channel();
                        writeBuffer.clear();
                        String request = "Hello from client!";
                        writeBuffer.put(request.getBytes());
                        writeBuffer.flip();
                        socketChannel.write(writeBuffer);
                        socketChannel.register(selector, SelectionKey.OP_READ);
                    }
                }
            }
        } catch (IOException e) {
            e.printStackTrace();
            System.exit(1);
        }
    }

    public static void main(String[] args) {
        NioClient client = new NioClient("localhost", 8888);
        client.start();
    }
}

这个示例代码实现了一个简单的NIO Server和Client，可以接收客户端连接，读取客户端发送的数据，并回复一条消息。虽然它没有Netty那么强大，但是可以作为一个参考来了解Java NIO的基本原理和使用方法。