在简单聊天室的代码中修改ChatServerHandler类，就可以模拟多人聊天的功能

package com.cppdy.server;

import io.netty.channel.Channel;
import io.netty.channel.ChannelHandlerContext;
import io.netty.channel.SimpleChannelInboundHandler;
import io.netty.channel.group.ChannelGroup;
import io.netty.channel.group.DefaultChannelGroup;
import io.netty.util.concurrent.GlobalEventExecutor;

public class ChatServerHandler extends SimpleChannelInboundHandler<String> {

    // 存放channel的集合
    public static ChannelGroup channels = new DefaultChannelGroup(GlobalEventExecutor.INSTANCE);

    @Override
    protected void channelRead0(ChannelHandlerContext ctx, String msg) throws Exception {

        System.out.println(msg);

        for (Channel channel : channels) {
            if (!channel.equals(ctx.channel())) {
                channel.writeAndFlush("[Server]-" + "[" + channel.remoteAddress() + "]" + msg);
            }
        }

    }

    @Override
    public void handlerAdded(ChannelHandlerContext ctx) throws Exception {
        System.out.println(ctx.channel().remoteAddress()+"--连接上了");
        channels.add(ctx.channel());
    }

    @Override
    public void handlerRemoved(ChannelHandlerContext ctx) throws Exception {
        System.out.println(ctx.channel().remoteAddress()+"--退出了");
        channels.remove(ctx.channel());
    }
    
    @Override
    public void channelActive(ChannelHandlerContext ctx) throws Exception {
        System.out.println(ctx.channel().remoteAddress()+"--上线了");
    }
    
    @Override
    public void channelInactive(ChannelHandlerContext ctx) throws Exception {
        System.out.println(ctx.channel().remoteAddress()+"--掉线了");
    }
    
    @Override
    public void exceptionCaught(ChannelHandlerContext ctx, Throwable cause) throws Exception {
        System.out.println(ctx.channel().remoteAddress()+"--error");
    }

}

先启动ChatServer类，再启动2次ChatClient类，就可以模拟两个客户端互发消息的功能。