用ServiceSocketChannel 写个服务端，发送消息给客户端

在Java中，你可以使用`java.nio.channels.SocketChannel`和`java.net.ServerSocketChannel`创建一个基于TCP的服务端，用于接收客户端的消息并发送回复。以下是简单的示例代码：

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;

public class SimpleServer {
    private static final int PORT = 9000;
    private Selector selector;
    private ServerSocketChannel serverSocket;

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

    public void start() {
        try {
            // 创建服务器套接字通道
            serverSocket = ServerSocketChannel.open();
            serverSocket.configureBlocking(false);
            serverSocket.socket().bind(new InetSocketAddress(PORT));

            // 初始化Selector
            selector = Selector.open();

            // 注册监听连接请求事件
            serverSocket.register(selector, SelectionKey.OP_ACCEPT);

            System.out.println("Server started on port " + PORT);

            while (true) {
                // 选择可用的键
                selector.select();

                for (SelectionKey key : selector.selectedKeys()) {
                    if (key.isAcceptable()) { // 接受新连接
                        acceptConnection(key);
                    } else if (key.isReadable()) { // 客户端发送数据
                        handleRead(key);
                    }
                }
            }
        } catch (IOException e) {
            e.printStackTrace();
        }
    }

    private void acceptConnection(SelectionKey key) throws IOException {
        ServerSocketChannel server = (ServerSocketChannel) key.channel();
        SocketChannel client = server.accept(); // 接收连接
        client.configureBlocking(false); // 非阻塞模式

        // 绑定新的键到客户端通道，处理读取和写入
        client.register(selector, SelectionKey.OP_READ | SelectionKey.OP_WRITE);
    }

    private void handleRead(SelectionKey key) throws IOException {
        SocketChannel client = (SocketChannel) key.channel();
        ByteBuffer buffer = ByteBuffer.allocate(4096); // 缓冲区大小

        int bytesReceived = client.read(buffer); // 读取数据
        if (bytesReceived > 0) {
            buffer.flip(); // 将缓冲区翻转以便读取数据
            String message = new String(buffer.array(), 0, bytesReceived); // 转换为字符串

            // 在这里处理收到的消息，例如打印或转发给其他服务
            System.out.println("Received from client: " + message);

            // 回应客户端，可以构造一个类似"ACK: " + message 的响应
            sendResponse(client, "ACK: " + message);
        } else if (bytesReceived == -1) { // 关闭连接
            client.close();
            key.cancel();
        } else {
            // 如果读取失败，取消选键
            key.cancel();
        }
    }

    private void sendResponse(SocketChannel client, String response) throws IOException {
        client.write(ByteBuffer.wrap(response.getBytes()));
    }
}

这个例子展示了如何创建一个简单的服务端，它会等待客户端连接，接收客户端发来的消息，并回送确认信息。记得在实际应用中添加错误处理和适当的数据包装。