TCP 粘包和拆包

• 简介

1) TCP是面向连接的，面向流的，提供高可靠性服务。收发两端（客户端和服务器端）都要有一一成对的socket，因此，发送端为了将多个发给接收端的包，更有效的发给对方，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，也就是我们所说的粘包、拆包问题

假设客户端分别发送了两个数据包D1和D2给服务端，由于服务端一次读取到字节数是不确定的，故可能存在以下四种情况：
1) 服务端分两次读取到了两个独立的数据包，分别是D1和D2，没有粘包和拆包
2) 服务端一次接受到了两个数据包，D1和D2粘合在一起，称之为TCP粘包
3) 服务端分两次读取到了数据包，第一次读取到了完整的D1包和D2包的部分内容，第二次读取到了D2包的剩余内容，这称之为TCP拆包
4) 服务端分两次读取到了数据包，第一次读取到了D1包的部分内容D1_1，第二次读取到了D1包的剩余部分内容D1_2和完整的D2包

• 编写服务端

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 com.ychen.netty.netty.tcp.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();
        }
 
    }
}
 
public class MyServerInitializer extends ChannelInitializer<SocketChannel> {
 
    @Override
    protected void initChannel(SocketChannel ch) throws Exception {
        ChannelPipeline pipeline = ch.pipeline();
        pipeline.addLast(new MyServerHandler());
    }
}
 
public class MyServerHandler extends SimpleChannelInboundHandler<ByteBuf>{
 
    private int count;
 
    @Override
    public void exceptionCaught(ChannelHandlerContext ctx, Throwable cause) throws Exception {
        cause.printStackTrace();
        ctx.close();
    }
 
    @Override
    protected void channelRead0(ChannelHandlerContext ctx, ByteBuf msg) throws Exception {
 
        byte[] buffer = new byte[msg.readableBytes()];
        msg.readBytes(buffer);
 
        //将buffer转成字符串
        String message = new String(buffer, Charset.forName("utf-8"));
 
        System.out.println("服务器接收到数据 " + message);
        System.out.println("服务器接收到消息量=" + (++this.count));
 
        //服务器回送数据给客户端, 回送一个随机id ,
        ByteBuf responseByteBuf = Unpooled.copiedBuffer(UUID.randomUUID().toString() + " ", Charset.forName("utf-8"));
        ctx.writeAndFlush(responseByteBuf);
 
    }
}

• 编写客户端

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 com.ychen.netty.netty.tcp.MyClientInitializer()); //自定义一个初始化类
 
            ChannelFuture channelFuture = bootstrap.connect("localhost", 7000).sync();
            channelFuture.channel().closeFuture().sync();
 
        }finally {
            group.shutdownGracefully();
        }
    }
}
 
public class MyClientInitializer extends ChannelInitializer<SocketChannel> {
    @Override
    protected void initChannel(SocketChannel ch) throws Exception {
 
        ChannelPipeline pipeline = ch.pipeline();
        pipeline.addLast(new MyClientHandler());
    }
}
 
public class MyClientHandler extends SimpleChannelInboundHandler<ByteBuf> {
 
    private int count;
 
    @Override
    public void channelActive(ChannelHandlerContext ctx) throws Exception {
        //使用客户端发送10条数据 hello,server 编号
        for(int i= 0; i< 10; ++i) {
            ByteBuf buffer = Unpooled.copiedBuffer("hello,server " + i, Charset.forName("utf-8"));
            ctx.writeAndFlush(buffer);
        }
    }
 
    @Override
    protected void channelRead0(ChannelHandlerContext ctx, ByteBuf msg) throws Exception {
 
        byte[] buffer = new byte[msg.readableBytes()];
        msg.readBytes(buffer);
 
        String message = new String(buffer, Charset.forName("utf-8"));
        System.out.println("客户端接收到消息=" + message);
        System.out.println("客户端接收消息数量=" + (++this.count));
    }
 
    @Override
    public void exceptionCaught(ChannelHandlerContext ctx, Throwable cause) throws Exception {
        cause.printStackTrace();
        ctx.close();
    }
}

• 测试：启动服务端，再启动多个客户端