介绍

1.TCP是面向连接、面向流的，提供高可靠性服务，收发两端(客户端和服务端)都要有一 一成对的socket，因此，发送端为了将多个发送给接收端的包，更有效的发送给对方，使用了优化方法(Nagle算法)，将多次间隔较小且数据量小的数据，合并成一个大的数据块，然后进行封包。这样做虽然提高了效率，但是接收端就难于分辨出完整的数据包了，因为面向流的通信是无消息保护边界的
2.由于TCP无消息保护边界，需要在接收端处理消息边界问题，也就是我们说的粘包、拆包问题


问题演示

接收方可能将数据分为多次接收，不能保证消息的准确

解决

1.使用自定义协议 + 编解码器来解决
2.关键就是要解决 服务器端每次读取数据长度的问题，这个问题解决，就不会出现服务器多读或少读数据的问题，从而避免TCP 粘包、拆包

自定义协议包

//协议包
public class MessageProtocol {
    private int len;
    private byte[] content;
    public int getLen() {
        return len;
    }
    public void setLen(int len) {
        this.len = len;
    }
    public byte[] getContent() {
        return content;
    }
    public void setContent(byte[] content) {
        this.content = content;
    }
}

ClientInitializer

public class MyClientInitializer extends ChannelInitializer<SocketChannel> {
    @Override
    protected void initChannel(SocketChannel ch) throws Exception {
        ChannelPipeline pipeline = ch.pipeline();
        pipeline.addLast(new MyMessageEncoder());
        pipeline.addLast(new MyClientHandler());
    }
}

Encoder

public class MyMessageEncoder extends MessageToByteEncoder<MessageProtocol> {
    @Override
    protected void encode(ChannelHandlerContext ctx, MessageProtocol msg, ByteBuf out) throws Exception {
        System.out.println("MyMessageEncoder--encode");
        out.writeInt(msg.getLen());
        out.writeBytes(msg.getContent());
    }
}

ClientHandler

public class MyClientHandler extends SimpleChannelInboundHandler<MessageProtocol> {
    private int count;
    @Override
    protected void channelRead0(ChannelHandlerContext ctx, MessageProtocol msg) throws Exception {
    }
    @Override
    public void channelActive(ChannelHandlerContext ctx) throws Exception {
        //发送10条数据
        for (int i = 0; i < 5; i++) {
            String msg = "zzzzzzz";
            byte[] bytes = msg.getBytes(StandardCharsets.UTF_8);
            int length = msg.getBytes(StandardCharsets.UTF_8).length;
            //创建协议包
            MessageProtocol messageProtocol = new MessageProtocol();
            messageProtocol.setLen(length);
            messageProtocol.setContent(bytes);
            ctx.writeAndFlush(messageProtocol);
        }
    }
}

Client

public class MyClient {
    public static void main(String[] args) throws Exception {
        EventLoopGroup group = new NioEventLoopGroup();
        try {
            Bootstrap bootstrap = new Bootstrap();
            bootstrap.group(group).channel(NioSocketChannel.class)
                    .handler(new MyClientInitializer()); //自定义一个初始化类
            ChannelFuture channelFuture = bootstrap.connect("localhost", 7000).sync();
            channelFuture.channel().closeFuture().sync();
        } finally {
            group.shutdownGracefully();
        }
    }
}

ServerInitializer

public class MyServerInitializer extends ChannelInitializer<SocketChannel> {
    @Override
    protected void initChannel(SocketChannel ch) throws Exception {
        ch.pipeline().addLast(new MyMessageDecoder()).addLast(new MyServerHandler());
    }
}

Decoder

public class MyMessageDecoder extends ReplayingDecoder<Void> {
    @Override
    protected void decode(ChannelHandlerContext ctx, ByteBuf in, List<Object> out) throws Exception {
        System.out.println("decode");
        //将得到的二进制字节码转化为messageProtocol
        int len = in.readInt();
        byte[] content = new byte[len];
        in.readBytes(content);
        //封装成messageProtocol对象,传递给下一个handler进行处理
        MessageProtocol messageProtocol = new MessageProtocol();
        messageProtocol.setLen(len);
        messageProtocol.setContent(content);
        out.add(messageProtocol);
    }
}

ServerHandler

public class MyServerHandler extends SimpleChannelInboundHandler<MessageProtocol> {
    private int count;
    @Override
    protected void channelRead0(ChannelHandlerContext ctx, MessageProtocol msg) throws Exception {
        int len = msg.getLen();
        byte[] content = msg.getContent();
        String s = new String(content, StandardCharsets.UTF_8);
        System.out.println(s);
        System.out.println("++++++++++++++++++");
        System.out.println(++count);
    }
}

Server

public class MyServer {
    public static void main(String[] args) throws Exception{
        EventLoopGroup bossGroup = new NioEventLoopGroup(1);
        EventLoopGroup workerGroup = new NioEventLoopGroup();
        try {
            ServerBootstrap serverBootstrap = new ServerBootstrap();
            serverBootstrap.group(bossGroup,workerGroup).channel(NioServerSocketChannel.class).childHandler(new MyServerInitializer()); //自定义一个初始化类
            ChannelFuture channelFuture = serverBootstrap.bind(7000).sync();
            channelFuture.channel().closeFuture().sync();
        }finally {
            bossGroup.shutdownGracefully();
            workerGroup.shutdownGracefully();
        }
    }
}

发送5次就分5次接收，不会存在粘包，拆包问题