Netty 框架实现简单的 C/S 结构网络程序：TCP 半包问题解决方案

以下是一个使用 Netty 框架实现的简单 C/S 结构的网络程序，处理了 TCP 半包问题（粘包、拆包）：

服务端代码：

public class Server {
    public static void main(String[] args) throws InterruptedException {
        EventLoopGroup bossGroup = new NioEventLoopGroup(1);
        EventLoopGroup workerGroup = new NioEventLoopGroup();
        try {
            ServerBootstrap b = new ServerBootstrap();
            b.group(bossGroup, workerGroup)
                    .channel(NioServerSocketChannel.class)
                    .childHandler(new ChannelInitializer<SocketChannel>() {
                        @Override
                        public void initChannel(SocketChannel ch) throws Exception {
                            ChannelPipeline pipeline = ch.pipeline();
                            pipeline.addLast(new LengthFieldBasedFrameDecoder(1024, 0, 4, 0, 4));
                            pipeline.addLast(new StringDecoder());
                            pipeline.addLast(new LengthFieldPrepender(4));
                            pipeline.addLast(new StringEncoder());
                            pipeline.addLast(new ServerHandler());
                        }
                    })
                    .option(ChannelOption.SO_BACKLOG, 128)
                    .childOption(ChannelOption.SO_KEEPALIVE, true);

            ChannelFuture f = b.bind(8888).sync();
            f.channel().closeFuture().sync();
        } finally {
            workerGroup.shutdownGracefully();
            bossGroup.shutdownGracefully();
        }
    }
}

class ServerHandler extends ChannelInboundHandlerAdapter {
    @Override
    public void channelRead(ChannelHandlerContext ctx, Object msg) throws Exception {
        String request = (String) msg;
        System.out.println('Server received: ' + request);
        String response = 'Hello, ' + request + '!';
        ctx.writeAndFlush(response);
    }

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

客户端代码：

public class Client {
    public static void main(String[] args) throws InterruptedException {
        EventLoopGroup group = new NioEventLoopGroup();
        try {
            Bootstrap b = new Bootstrap();
            b.group(group)
                    .channel(NioSocketChannel.class)
                    .handler(new ChannelInitializer<SocketChannel>() {
                        @Override
                        public void initChannel(SocketChannel ch) throws Exception {
                            ChannelPipeline pipeline = ch.pipeline();
                            pipeline.addLast(new LengthFieldBasedFrameDecoder(1024, 0, 4, 0, 4));
                            pipeline.addLast(new StringDecoder());
                            pipeline.addLast(new LengthFieldPrepender(4));
                            pipeline.addLast(new StringEncoder());
                            pipeline.addLast(new ClientHandler());
                        }
                    });

            ChannelFuture f = b.connect('localhost', 8888).sync();
            f.channel().writeAndFlush('world');
            f.channel().closeFuture().sync();
        } finally {
            group.shutdownGracefully();
        }
    }
}

class ClientHandler extends ChannelInboundHandlerAdapter {
    @Override
    public void channelRead(ChannelHandlerContext ctx, Object msg) throws Exception {
        String response = (String) msg;
        System.out.println('Client received: ' + response);
    }

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

在服务端和客户端的 ChannelPipeline 中添加了 LengthFieldBasedFrameDecoder 和 LengthFieldPrepender，用于处理 TCP 半包问题。LengthFieldBasedFrameDecoder 可以根据长度字段自动处理粘包和拆包，而 LengthFieldPrepender 则在消息前添加长度字段，以便服务端正确解析消息。

在服务端的 ServerHandler 中，将客户端发送的消息打印出来，并回复一个 Hello 消息。在客户端的 ClientHandler 中，将服务端回复的消息打印出来。

运行客户端和服务端程序，可以看到客户端发送的消息被服务端正确接收并回复了一个 Hello 消息。