java使用epoll方式实现一个C/S架构的应用；要求：服务器同时支持多个客户端连接，一个客户端发送消息，服务器转发到其他客户端并回显消息；

首先，Java中使用NIO来实现epoll方式，NIO使用Selector作为IO多路复用器，实现了异步非阻塞IO操作。下面是一个简单的C/S架构的应用的示例代码，实现了一个简单的聊天室。

服务端代码：

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.nio.charset.Charset;
import java.util.Iterator;

public class Server {

    public static void main(String[] args) throws IOException {
        // 创建ServerSocketChannel，用于监听客户端连接请求
        ServerSocketChannel serverSocketChannel = ServerSocketChannel.open();
        serverSocketChannel.socket().bind(new InetSocketAddress("localhost", 8000));
        serverSocketChannel.configureBlocking(false);

        // 创建Selector，用于监听所有Channel上的事件
        Selector selector = Selector.open();

        // 将ServerSocketChannel注册到Selector上，监听OP_ACCEPT事件
        serverSocketChannel.register(selector, SelectionKey.OP_ACCEPT);

        while (true) {
            int readyChannels = selector.select();
            if (readyChannels == 0) continue;

            Iterator<SelectionKey> iterator = selector.selectedKeys().iterator();
            while (iterator.hasNext()) {
                SelectionKey key = iterator.next();
                iterator.remove();
                handle(key);
            }
        }
    }

    private static void handle(SelectionKey key) throws IOException {
        if (key.isAcceptable()) {
            // 如果是OP_ACCEPT事件，表示有新的客户端连接请求
            ServerSocketChannel serverSocketChannel = (ServerSocketChannel) key.channel();
            SocketChannel socketChannel = serverSocketChannel.accept();
            socketChannel.configureBlocking(false);

            // 将SocketChannel注册到Selector上
            socketChannel.register(key.selector(), SelectionKey.OP_READ);

            // 给客户端发送欢迎消息
            socketChannel.write(Charset.defaultCharset().encode("欢迎来到聊天室！\n"));
        } else if (key.isReadable()) {
            // 如果是OP_READ事件，表示客户端发送了消息
            SocketChannel socketChannel = (SocketChannel) key.channel();
            ByteBuffer byteBuffer = ByteBuffer.allocate(1024);
            StringBuilder content = new StringBuilder();
            while (socketChannel.read(byteBuffer) > 0) {
                byteBuffer.flip();
                content.append(Charset.defaultCharset().decode(byteBuffer));
            }

            // 如果读到的消息不为空，则广播给其他客户端
            if (content.length() > 0) {
                System.out.println("收到来自" + socketChannel.getRemoteAddress() + "的消息：" + content);
                for (SelectionKey selectionKey : key.selector().keys()) {
                    if (selectionKey.channel() instanceof SocketChannel && selectionKey.channel() != socketChannel) {
                        SocketChannel targetChannel = (SocketChannel) selectionKey.channel();
                        targetChannel.write(Charset.defaultCharset().encode(socketChannel.getRemoteAddress() + "：" + content));
                    }
                }
            }
        }
    }
}

客户端代码：

import java.io.IOException;
import java.net.InetSocketAddress;
import java.nio.ByteBuffer;
import java.nio.channels.SocketChannel;
import java.nio.charset.Charset;
import java.util.Scanner;

public class Client {

    public static void main(String[] args) throws IOException {
        SocketChannel socketChannel = SocketChannel.open();
        socketChannel.configureBlocking(false);
        socketChannel.connect(new InetSocketAddress("localhost", 8000));
        while (!socketChannel.finishConnect()) {}

        // 启动一个新的线程，用于接收服务器的消息
        new Thread(() -> {
            ByteBuffer byteBuffer = ByteBuffer.allocate(1024);
            try {
                while (socketChannel.read(byteBuffer) > 0) {
                    byteBuffer.flip();
                    System.out.print(Charset.defaultCharset().decode(byteBuffer));
                    byteBuffer.clear();
                }
            } catch (IOException e) {
                e.printStackTrace();
            }
        }).start();

        // 从标准输入读入消息并发送到服务器
        Scanner scanner = new Scanner(System.in);
        while (scanner.hasNextLine()) {
            String line = scanner.nextLine();
            if (line.equals("quit")) {
                break;
            }
            socketChannel.write(Charset.defaultCharset().encode(line + "\n"));
        }
        socketChannel.close();
    }
}

这样，我们就实现了一个基于epoll方式的C/S架构的应用，可以同时支持多个客户端连接，一个客户端发送消息，服务器会将消息转发到其他客户端并回显消息。