public class EchoServer {

    public static void main(String[] args) throws Exception {
        // 创建两个 EventLoopGroup，一个用于接收客户端连接，一个用于处理客户端数据
        EventLoopGroup bossGroup = new NioEventLoopGroup(1); // 主 Reactor
        EventLoopGroup workerGroup = new NioEventLoopGroup(); // 从 Reactor

        try {
            ServerBootstrap b = new ServerBootstrap();
            b.group(bossGroup, workerGroup)
             .channel(NioServerSocketChannel.class) // 使用 NIO 传输 Channel
             .childHandler(new ChannelInitializer<SocketChannel>() {
                 @Override
                 public void initChannel(SocketChannel ch) throws Exception {
                     // 添加自定义的 ChannelHandler
                     ch.pipeline().addLast(new EchoServerHandler());
                 }
             });

            // 绑定端口，同步等待成功
            ChannelFuture f = b.bind(8080).sync();

            // 等待服务端监听端口关闭
            f.channel().closeFuture().sync();
        } finally {
            // 优雅关闭 EventLoopGroup
            bossGroup.shutdownGracefully();
            workerGroup.shutdownGracefully();
        }
    }
}

class EchoServerHandler extends ChannelInboundHandlerAdapter {

    @Override
    public void channelRead(ChannelHandlerContext ctx, Object msg) {
        // 处理读事件
        ctx.write(msg); // 写回数据
    }

    @Override
    public void channelReadComplete(ChannelHandlerContext ctx) {
        // 读完毕事件
        ctx.flush(); // 刷新后才将数据发出到 SocketChannel
    }

    @Override
    public void exceptionCaught(ChannelHandlerContext ctx, Throwable cause) {
        // 异常事件
        cause.printStackTrace();
        ctx.close(); // 关闭这个 Channel
    }
}