使用Netty實現一個小型服務器（作爲數據中轉站）

項目需要，要寫一個服務器作爲數據中轉站，服務器接收客戶端發送的數據，再轉發給其他客戶端。
客戶端可以自己寫一個測試是否可以連接和接收數據，也可以是4G模塊或者單片機啥的。

客戶端代碼：

我的項目目錄結構：

代碼如下：
NettyServer.java

import io.netty.bootstrap.ServerBootstrap;
import io.netty.channel.ChannelFuture;
import io.netty.channel.ChannelInitializer;
import io.netty.channel.ChannelPipeline;
import io.netty.channel.EventLoopGroup;
import io.netty.channel.nio.NioEventLoopGroup;
import io.netty.channel.socket.SocketChannel;
import io.netty.channel.socket.nio.NioServerSocketChannel;
import io.netty.handler.codec.string.StringDecoder;
import io.netty.handler.codec.string.StringEncoder;
import io.netty.util.CharsetUtil;

public class NettyServer {

    public static void startServer(String hostName, int port){
        startServer0(hostName,port);
    }

    private static void startServer0(String hostname, int port)  {
        EventLoopGroup bossGroup = new NioEventLoopGroup();
        EventLoopGroup workerGroup = new NioEventLoopGroup();

        try {
            ServerBootstrap serverBootstrap = new ServerBootstrap();
            serverBootstrap.group(bossGroup,workerGroup)
                    .channel(NioServerSocketChannel.class)
                    .childHandler(new ChannelInitializer<SocketChannel>() {
                        @Override
                        protected void initChannel(SocketChannel socketChannel) throws Exception {
                            ChannelPipeline pipeline = socketChannel.pipeline();
                            pipeline.addLast(new StringEncoder(CharsetUtil.UTF_8));
                            pipeline.addLast(new StringDecoder(CharsetUtil.UTF_8));
                            pipeline.addLast(new NettyServerHandler());
                        }
                    });
            ChannelFuture channelFuture = serverBootstrap.bind(hostname, port).sync();
            System.out.println("服務提供者已經啓動");
            channelFuture.channel().closeFuture().sync();
        } catch (InterruptedException e) {
            e.printStackTrace();
        }finally {
            bossGroup.shutdownGracefully();
            workerGroup.shutdownGracefully();
        }

    }
}

NettyServerHandler.java

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 NettyServerHandler extends SimpleChannelInboundHandler<String> {

    private static ChannelGroup channelGroup = new DefaultChannelGroup(GlobalEventExecutor.INSTANCE);

    /**
     * 服務器端收到任何一個客戶端的消息都會觸發這個方法
     * 連接的客戶端向服務器端發送消息，那麼其他客戶端都收到此消息，自己收到【自己】+消息
     */
    @Override
    protected void channelRead0(ChannelHandlerContext channelHandlerContext, String msg) throws Exception {
        Channel channel = channelHandlerContext.channel();

        channelGroup.forEach(ch -> {
            if(channel != ch){
                ch.writeAndFlush(channel.remoteAddress() +" 發送的消息:" +msg+" \n");
            }else {
                ch.writeAndFlush(" 【自己】"+msg +" \n");
            }
        });
    }

    //表示服務端與客戶端連接建立
    @Override
    public void handlerAdded(ChannelHandlerContext ctx) throws Exception {
        Channel channel = ctx.channel();

        /**
         * 調用channelGroup的writeAndFlush其實就相當於channelGroup中的每個channel都writeAndFlush
         *
         * 先去廣播，再將自己加入到channelGroup中
         */
        channelGroup.writeAndFlush(" 【客戶端】 -" +channel.remoteAddress() +" 加入\n");
        channelGroup.add(channel);
    }

    @Override
    public void handlerRemoved(ChannelHandlerContext ctx) throws Exception {
        Channel channel = ctx.channel();
        channelGroup.writeAndFlush(" 【客戶端】 -" +channel.remoteAddress() +" 離開\n");

        //驗證一下每次客戶端斷開連接，連接自動地從channelGroup中刪除調。
        System.out.println(channelGroup.size());
        //當客戶端和服務端斷開連接的時候，下面的那段代碼netty會自動調用，所以不需要人爲的去調用它
        //channelGroup.remove(channel);
    }

    //連接處於活動狀態
    @Override
    public void channelActive(ChannelHandlerContext ctx) throws Exception {
        Channel channel = ctx.channel();
        System.out.println(channel.remoteAddress() +" 上線了");
    }

    @Override
    public void channelInactive(ChannelHandlerContext ctx) throws Exception {
        Channel channel = ctx.channel();
        System.out.println(channel.remoteAddress() +" 下線了");
    }

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

啓動類：ServerMain.java

public class ServerMain {

    public static void main(String[] args) {
        NettyServer.startServer("ip",port);
    }
}

客戶端代碼：

NettyClient.java

import io.netty.bootstrap.Bootstrap;
import io.netty.channel.Channel;
import io.netty.channel.ChannelInitializer;
import io.netty.channel.ChannelPipeline;
import io.netty.channel.EventLoopGroup;
import io.netty.channel.nio.NioEventLoopGroup;
import io.netty.channel.socket.SocketChannel;
import io.netty.channel.socket.nio.NioSocketChannel;
import io.netty.handler.codec.string.StringDecoder;
import io.netty.handler.codec.string.StringEncoder;
import io.netty.util.CharsetUtil;

import java.io.BufferedReader;
import java.io.InputStreamReader;

public class NettyClient {

    public static void startClient(String hostName, int port) throws Exception {
        initClient(hostName, port);
    }

    private static void initClient(String hostName, int port) throws Exception{
        EventLoopGroup eventLoopGroup = new NioEventLoopGroup();

        try {
            Bootstrap bootstrap = new Bootstrap();
            bootstrap.group(eventLoopGroup)
                    .channel(NioSocketChannel.class)
                    .handler(new ChannelInitializer<SocketChannel>() {
                        @Override
                        protected void initChannel(SocketChannel socketChannel) throws Exception {
                            ChannelPipeline pipeline = socketChannel.pipeline();
                            pipeline.addLast(new StringDecoder(CharsetUtil.UTF_8));
                            pipeline.addLast(new StringEncoder(CharsetUtil.UTF_8));
                            pipeline.addLast(new NettyClientHandler());
                        }
                    });

            Channel channel = bootstrap.connect(hostName, port).sync().channel();

            //標準輸入
            BufferedReader bufferedReader = new BufferedReader(new InputStreamReader(System.in));

            //利用死循環，不斷讀取客戶端在控制檯上的輸入內容
            for (;;){
                channel.writeAndFlush(bufferedReader.readLine() +"\r\n");
            }
        }catch (Exception e){
            e.printStackTrace();
        }finally {
            eventLoopGroup.shutdownGracefully();
        }

    }
}

NettyClientHandler.java

import io.netty.channel.ChannelHandlerContext;
import io.netty.channel.SimpleChannelInboundHandler;

public class NettyClientHandler extends SimpleChannelInboundHandler<String> {
    @Override
    protected void channelRead0(ChannelHandlerContext channelHandlerContext, String s) throws Exception {
        System.out.println("...."+s);
    }

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

啓動類：ClientMain.java

public class ClientMain {

    public static void main(String[] args) throws Exception {
        NettyClient.startClient("ip",port);
    }
}

附：打包流程
https://blog.csdn.net/weixin_42089175/article/details/89113271
我是按照這篇教程操作的

附：上傳jar包到雲服務器
scp 本地jar包路徑 root@雲服務器ip:想要上傳到的雲服務器的路徑

附：在雲服務器持續在後臺運行jar包
nohup java -jar jar包名稱 >/dev/null 2>&1 &

附：查看該端口接收和轉發的數據
telnet ip port