项目结构

服务端

package com.nio.fixed.server;

import io.netty.bootstrap.ServerBootstrap;
import io.netty.buffer.ByteBuf;
import io.netty.buffer.Unpooled;
import io.netty.channel.ChannelFuture;
import io.netty.channel.ChannelInitializer;
import io.netty.channel.ChannelOption;
import io.netty.channel.nio.NioEventLoopGroup;
import io.netty.channel.socket.SocketChannel;
import io.netty.channel.socket.nio.NioServerSocketChannel;
import io.netty.handler.codec.DelimiterBasedFrameDecoder;
import io.netty.handler.codec.FixedLengthFrameDecoder;
import io.netty.handler.codec.string.StringDecoder;
import io.netty.handler.logging.LogLevel;
import io.netty.handler.logging.LoggingHandler;

public class EchoServer {
    public void bind(int port) throws Exception{
        // 配置服务端的ＮＩＯ线程
        NioEventLoopGroup groupBig = new NioEventLoopGroup();
        NioEventLoopGroup groupSmall  = new NioEventLoopGroup();

        try {
            ServerBootstrap bootstrap = new ServerBootstrap();
            bootstrap.group(groupBig,groupSmall)
                    .channel(NioServerSocketChannel.class)
                    .option(ChannelOption.SO_BACKLOG,100)
                    .handler(new LoggingHandler(LogLevel.INFO))
                    .childHandler(new ChannelInitializer<SocketChannel>() {
                        @Override
                        protected void initChannel(SocketChannel socketChannel) throws Exception {
                            socketChannel.pipeline().addLast(new FixedLengthFrameDecoder(20));
                            socketChannel.pipeline().addLast(new StringDecoder());
                            socketChannel.pipeline().addLast(new EchoServerHandler());
                        }
                    });
            // 绑定端口，同步等待成功
            ChannelFuture future = bootstrap.bind(port).sync();
            // 等待服务端监听端口关闭
            future.channel().closeFuture().sync();
        } catch (Exception e) {
            e.printStackTrace();
        }finally {
            // 退出，释放资源
           groupBig.shutdownGracefully();
           groupSmall.shutdownGracefully();
        }
    }

    public static void main(String[] args)throws Exception {
        int port = 8056;
        if (args != null && args.length > 0){
            try {
                port = Integer.valueOf(args[0]);
            }catch (NumberFormatException e){
                e.printStackTrace();
            }
        }
        new EchoServer().bind(port);
    }
}

package com.nio.fixed.server;

import io.netty.buffer.ByteBuf;
import io.netty.buffer.Unpooled;
import io.netty.channel.ChannelHandlerContext;
import io.netty.channel.ChannelInboundHandlerAdapter;

public class EchoServerHandler extends ChannelInboundHandlerAdapter {
    int counter = 0;

    @Override
    public void channelRead(ChannelHandlerContext ctx, Object msg) throws Exception {
        // 打印接收到的消息
        System.out.println("服务端的计数器：" + ++counter + "  ,客户端发来的消息是：【 " + msg + "】");

    }

    @Override
    public void exceptionCaught(ChannelHandlerContext ctx, Throwable cause) throws Exception {
        cause.printStackTrace();
        ctx.close();
    }

}

客户端使用Telnet：

【1】解决Telnet命令不能使用问题

输入命令：

回车输入要给服务端发送的消息：

运行结果：

固定了长度，当输入的码流大于指定的长度就会发送给服务端进行输出

Netty分隔符解码器的使用

https://blog.csdn.net/wildwolf_001/article/details/81187066