TCP粘包和拆包

         TCP（transport control protocol，传输控制协议）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往接收端的包，更有效的发到对方，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，接收端，就难于分辨出来了，必须提供科学的拆包机制。即面向流的通信是无消息保护边界的。
        图解TCP的粘包和拆包

    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。

      特别要注意的是，如果TCP的接受滑窗非常小，而数据包D1和D2比较大，很有可能会发生第五种情况，即服务端分多次才能将D1和D2包完全接受，期间发生多次拆包。

     

     粘包、拆包问题的解决方案：定义通信协议

     目前业界主流的协议(protocol)方案可以归纳如下：

     1 定长协议：假设我们规定每3个字节，表示一个有效报文。

     2.特殊字符分隔符协议：在包尾部增加回车或者空格符等特殊字符进行分割 。

     3.长度编码：将消息分为消息头和消息体，消息头中用一个int型数据(4字节)，表示消息体长度的字段。在解析时，先读取内容长度Length，其值为实际消息体内容(Content)占用的字节数，之后必须读取到这么多字节的内容，才认为是一个完整的数据报文。

     

     下面我用长度编码方式解决TCP的粘包、拆包问题

     首先看一下不使用长度编码协议的时候会发生什么问题？

     Server端测试代码   

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());

            ChannelFuture channelFuture = serverBootstrap.bind(8899).sync();
            channelFuture.channel().closeFuture().sync();
        } finally {
            bossGroup.shutdownGracefully();
            workerGroup.shutdownGracefully();
        }
    }
}

public class MyServerHandler extends SimpleChannelInboundHandler<ByteBuf> {
    private int count;
    @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("服务端接收到的消息内容： " + message);
        System.out.println("服务端接收到的消息数量： " + (++this.count));
        ByteBuf responseByteBuf = Unpooled.copiedBuffer(UUID.randomUUID().toString(), Charset.forName("utf-8"));
        ctx.writeAndFlush(responseByteBuf);
    }
    @Override
    public void exceptionCaught(ChannelHandlerContext ctx, Throwable cause) throws Exception {
        cause.printStackTrace();
        ctx.close();
    }
}

public class MyServerInitializer extends ChannelInitializer<SocketChannel> {

    @Override
    protected void initChannel(SocketChannel ch) throws Exception {
        System.out.println(this);

        ChannelPipeline pipeline = ch.pipeline();

        pipeline.addLast(new MyServerHandler());
    }
}

     Client端测试代码

public class MyClient {

    public static void main(String[] args) throws Exception{
        EventLoopGroup eventLoopGroup = new NioEventLoopGroup();

        try {
            Bootstrap bootstrap = new Bootstrap();
            bootstrap.group(eventLoopGroup).channel(NioSocketChannel.class).
                    handler(new MyClientInitializer());

            ChannelFuture channelFuture = bootstrap.connect("localhost", 8899).sync();
            channelFuture.channel().closeFuture().sync();
        } finally {
            eventLoopGroup.shutdownGracefully();
        }
    }
}

public class MyClientHandler extends SimpleChannelInboundHandler<ByteBuf> {
    private int count;
    @Override
    public void channelActive(ChannelHandlerContext ctx) throws Exception {
        for (int i = 0; i < 10; ++i) {
            ByteBuf buffer = Unpooled.copiedBuffer("sent from client ", 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("客户端接收到的消息内容： " + message);
        System.out.println("客户端接收到的消息数量： " + (++this.count));
    }
    @Override
    public void exceptionCaught(ChannelHandlerContext ctx, Throwable cause) throws Exception {
        cause.printStackTrace();
        ctx.close();
    }
}

public class MyClientInitializer extends ChannelInitializer<SocketChannel> {

    @Override
    protected void initChannel(SocketChannel ch) throws Exception {
        ChannelPipeline pipeline = ch.pipeline();

        pipeline.addLast(new MyClientHandler());
    }
}

          启动服务端，然后

          第一次启动客户端，服务端日志：

          第二次启动客户端，服务端日志：

          第三次启动客户端，服务端日志：

         从结果中可以看出发生了粘包问题。

 

         用长度编码方式解决TCP的粘包和拆包问题

         自定义协议CustomProtocol，解码器MyDecoder，编码器MyEncoder

public class CustomProtocol {

    private int length;

    private byte[] content;

    public int getLength() {
        return length;
    }

    public void setLength(int length) {
        this.length = length;
    }

    public byte[] getContent() {
        return content;
    }

    public void setContent(byte[] content) {
        this.content = content;
    }
}

public class MyDecoder extends ReplayingDecoder<Void> {

    @Override
    protected void decode(ChannelHandlerContext ctx, ByteBuf in, List<Object> out) throws Exception {

        int length = in.readInt();

        byte[] content = new byte[length];
        in.readBytes(content);

        CustomProtocol personProtocol = new CustomProtocol();
        personProtocol.setLength(length);
        personProtocol.setContent(content);

        out.add(personProtocol);
    }
}

public class MyEncoder extends MessageToByteEncoder<CustomProtocol> {

    @Override
    protected void encode(ChannelHandlerContext ctx, CustomProtocol msg, ByteBuf out) throws Exception {

        out.writeInt(msg.getLength());
        out.writeBytes(msg.getContent());
    }
}

          Server端测试代码   

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 ChannelInitializer<SocketChannel>() {
                        @Override
                        protected void initChannel(SocketChannel ch) throws Exception {
                            ChannelPipeline pipeline = ch.pipeline();

                            pipeline.addLast(new MyDecoder());
                            pipeline.addLast(new MyEncoder());

                            pipeline.addLast(new MyClientHandler());
                        }
                    });

            ChannelFuture channelFuture = serverBootstrap.bind(8899).sync();
            channelFuture.channel().closeFuture().sync();
        } finally {
            bossGroup.shutdownGracefully();
            workerGroup.shutdownGracefully();
        }
    }
}

public class MyServerHandler extends SimpleChannelInboundHandler<CustomProtocol> {

    private int count;

    @Override
    protected void channelRead0(ChannelHandlerContext ctx, CustomProtocol msg) throws Exception {
        int length = msg.getLength();
        byte[] content = msg.getContent();

        System.out.println("服务端接收到的数据：");
        System.out.println("长度: " + length);
        System.out.println("内容：" + new String(content, Charset.forName("utf-8")));

        System.out.println("服务端接收到的消息数量：" + (++this.count));

        String responseMessage = UUID.randomUUID().toString();
        int responseLength = responseMessage.getBytes("utf-8").length;
        byte[] responseContent = responseMessage.getBytes("utf-8");

        CustomProtocol personProtocol = new CustomProtocol();
        personProtocol.setLength(responseLength);
        personProtocol.setContent(responseContent);

        ctx.writeAndFlush(personProtocol);
    }

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

          Client端测试代码   

public class MyClient {

    public static void main(String[] args) 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 ch) throws Exception {
                            ChannelPipeline pipeline = ch.pipeline();

                            pipeline.addLast(new MyDecoder());
                            pipeline.addLast(new MyEncoder());

                            pipeline.addLast(new MyClientHandler());
                        }
                    });
            ChannelFuture channelFuture = bootstrap.connect("localhost", 8899).sync();
            channelFuture.channel().closeFuture().sync();
        } finally {
            eventLoopGroup.shutdownGracefully();
        }
    }
}

public class MyClientHandler extends SimpleChannelInboundHandler<CustomProtocol> {

    private int count;

    @Override
    public void channelActive(ChannelHandlerContext ctx) throws Exception {
        for (int i = 0; i < 10; ++i) {
            String messageToBeSent = "sent from client ";
            byte[] content = messageToBeSent.getBytes(Charset.forName("utf-8"));
            int length = messageToBeSent.getBytes(Charset.forName("utf-8")).length;

            CustomProtocol personProtocol = new CustomProtocol();
            personProtocol.setLength(length);
            personProtocol.setContent(content);

            ctx.writeAndFlush(personProtocol);
        }
    }

    @Override
    protected void channelRead0(ChannelHandlerContext ctx, CustomProtocol msg) throws Exception {
        int length = msg.getLength();
        byte[] content = msg.getContent();

        System.out.println("客户端接收到的消息： ");

        System.out.println("长度: " + length);
        System.out.println("内容：" + new String(content, Charset.forName("utf-8")));

        System.out.println("客户端接受到的消息数量：" + (++this.count));
    }

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

 测试结果：  

 结果分析：没有发生Tcp的粘包和拆包。