[Netty学习笔记]十一、TCP粘包、拆包及解决方案

概述

TCP是面向连接、面向流的，可以提供高可靠性服务。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，将多次间隔较小且数据量小的数据合成为一个大的数据块，然后进行封包。这样做虽然提高了效率，但是接收端就很难分辨出接收到的包是否为一个完整的数据包了。因为面向流的通信时无消息保护边界的。

由于TCP无消息保护边界，需要在接收端处理消息边界问题。因此会有拆包、粘包问题，如下图

在这里插入图片描述

说明：

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的字节数是不确定的，因此可能存在以下四种情况：

服务器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、拆包问题
服务器端一次接受到了两个数据包，D1和D2粘合在一起，称为TCP粘包
服务器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。
服务器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，第二次读取到了D1包的剩余部分内容和完整的D2包

代码演示TCP粘包、拆包现象

服务器端:

package com.wojiushiwo.tcp.packingunpacking;

import com.wojiushiwo.codec.CustomCodecServerHandler;
import com.wojiushiwo.codec.LongDecoder;
import com.wojiushiwo.codec.LongEncoder;
import io.netty.bootstrap.ServerBootstrap;
import io.netty.channel.*;
import io.netty.channel.nio.NioEventLoopGroup;
import io.netty.channel.socket.SocketChannel;
import io.netty.channel.socket.nio.NioServerSocketChannel;

/**
 * Created by myk
 * 2020/1/29 下午8:26
 */
public class PackingUnpackingServer {
    public static void main(String[] args) {
        EventLoopGroup bossGroup = new NioEventLoopGroup(1);
        EventLoopGroup workerGroup = new NioEventLoopGroup();
        try {

            ServerBootstrap serverBootstrap = new ServerBootstrap();
            serverBootstrap.group(bossGroup, workerGroup)
                    .channel(NioServerSocketChannel.class)
                    .option(ChannelOption.SO_BACKLOG, 128)
                    .childHandler(new ChannelInitializer<SocketChannel>() {
                        @Override
                        protected void initChannel(SocketChannel ch) throws Exception {

                            ChannelPipeline pipeline = ch.pipeline();
                            pipeline.addLast(new PackingUnpackingServerHandler());
                        }
                    });

            ChannelFuture channelFuture = serverBootstrap.bind(8091).sync();
            channelFuture.channel().closeFuture().sync();

        } catch (Exception e) {
            e.printStackTrace();
        } finally {
            bossGroup.shutdownGracefully();
            workerGroup.shutdownGracefully();
        }
    }
}
//handler
package com.wojiushiwo.tcp.packingunpacking;

import io.netty.buffer.ByteBuf;
import io.netty.channel.ChannelHandlerContext;
import io.netty.channel.ChannelInboundHandlerAdapter;
import io.netty.util.CharsetUtil;

/**
 * Created by myk
 * 2020/1/29 下午8:27
 */
public class PackingUnpackingServerHandler extends ChannelInboundHandlerAdapter {

    private int count = 0;

    @Override
    public void channelRead(ChannelHandlerContext ctx, Object msg) throws Exception {
        ByteBuf buf = (ByteBuf) msg;
        System.out.println("from client:" + buf.toString(CharsetUtil.UTF_8));
        System.out.println("服务器端接收到的消息量=" + (++this.count));
    }


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

客户端代码

package com.wojiushiwo.tcp.packingunpacking;

import io.netty.bootstrap.Bootstrap;
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.NioSocketChannel;

/**
 * Created by myk
 * 2020/1/29 下午8:26
 */
public class PackingUnpackingClient {
    public static void main(String[] args) {
        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 PackingUnpackingClientHandler());
                        }
                    });


            ChannelFuture channelFuture = bootstrap.connect("127.0.0.1", 8091).sync();
            channelFuture.channel().closeFuture().sync();

        } catch (Exception e) {
            e.printStackTrace();
        } finally {
            eventLoopGroup.shutdownGracefully();
        }
    }
}

//handler
package com.wojiushiwo.tcp.packingunpacking;

import io.netty.buffer.Unpooled;
import io.netty.channel.ChannelHandlerContext;
import io.netty.channel.ChannelInboundHandlerAdapter;
import io.netty.util.CharsetUtil;

/**
 * Created by myk
 * 2020/1/29 下午8:27
 */
public class PackingUnpackingClientHandler extends ChannelInboundHandlerAdapter {


    @Override
    public void channelActive(ChannelHandlerContext ctx) throws Exception {
        //使用客户端发送5次消息
        for (int i = 0; i < 5; i++) {
            ctx.writeAndFlush(Unpooled.copiedBuffer("from client" + (i + 1) + " ", CharsetUtil.UTF_8));
        }
    }

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

客户端循环5次向服务器端发送消息，通过查看服务器端接收到的消息完整程度即可判断粘包、拆包现象

启动了服务端，并且启动了三个客户端，输出结果：

//客户端1启动后服务器端输出

from client:from client1 from client2 from client3 from client4 from client5

服务器端接收到的消息量=1

//客户端2启动后服务器端输出

from client:from client1

服务器端接收到的消息量=1

from client:from client2

服务器端接收到的消息量=2

from client:from client3 from client4 from client5

服务器端接收到的消息量=3

// 客户端3启动后服务器端输出

from client:from client1

服务器端接收到的消息量=1

from client:from client2

服务器端接收到的消息量=2

from client:from client3

服务器端接收到的消息量=3

from client:from client4

服务器端接收到的消息量=4

from client:from client5
服务器端接收到的消息量=5

可以发现，输出结果中粘包、拆包等情况都发生了。

解决方案

只要确定了服务器端每次传输的数据的长度，那么服务器端一次读取到的就是完整的数据，就可以避免TCP拆包、粘包问题。

通常使用自定义协议+编解码器来解决

解决方案示例：

服务器端：

package com.wojiushiwo.tcp.protocol;

import io.netty.bootstrap.ServerBootstrap;
import io.netty.channel.*;
import io.netty.channel.nio.NioEventLoopGroup;
import io.netty.channel.socket.SocketChannel;
import io.netty.channel.socket.nio.NioServerSocketChannel;

/**
 * Created by myk
 * 2020/1/29 下午9:53
 */
public class Server {
    public static void main(String[] args) {
        EventLoopGroup bossGroup = new NioEventLoopGroup(1);
        EventLoopGroup workerGroup = new NioEventLoopGroup();
        try {

            ServerBootstrap serverBootstrap = new ServerBootstrap();
            serverBootstrap.group(bossGroup, workerGroup)
                    .channel(NioServerSocketChannel.class)
                    .option(ChannelOption.SO_BACKLOG, 128)
                    .childHandler(new ChannelInitializer<SocketChannel>() {
                        @Override
                        protected void initChannel(SocketChannel ch) throws Exception {

                            ChannelPipeline pipeline = ch.pipeline();
                            pipeline.addLast("encoder",new PackingUnPackingEncoder());
                            pipeline.addLast("decoder",new PackingUnpackingDecoder());
                            pipeline.addLast(new ServerHandler());
                        }
                    });

            ChannelFuture channelFuture = serverBootstrap.bind(8091).sync();
            channelFuture.channel().closeFuture().sync();

        } catch (Exception e) {
            e.printStackTrace();
        } finally {
            bossGroup.shutdownGracefully();
            workerGroup.shutdownGracefully();
        }
    }
}
//handler
package com.wojiushiwo.tcp.protocol;

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

/**
 * Created by myk
 * 2020/1/29 下午9:53
 */
public class ServerHandler extends SimpleChannelInboundHandler<MessageProtocol> {


    @Override
    protected void channelRead0(ChannelHandlerContext ctx, MessageProtocol msg) throws Exception {
        System.out.println("from client[length:" + msg.getLength() + "内容:" + new String(msg.getContent(), CharsetUtil.UTF_8) + "]");
    }


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

客户端代码

package com.wojiushiwo.tcp.protocol;

import io.netty.bootstrap.Bootstrap;
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.NioSocketChannel;

/**
 * Created by myk
 * 2020/1/29 下午9:53
 */
public class Client {
    public static void main(String[] args) {
        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("encoder", new PackingUnPackingEncoder());
                            pipeline.addLast("decoder", new PackingUnpackingDecoder());
                            pipeline.addLast(new ClientHandler());
                        }
                    });


            ChannelFuture channelFuture = bootstrap.connect("127.0.0.1", 8091).sync();
            channelFuture.channel().closeFuture().sync();

        } catch (Exception e) {
            e.printStackTrace();
        } finally {
            eventLoopGroup.shutdownGracefully();
        }
    }
}

//handler
package com.wojiushiwo.tcp.protocol;

import io.netty.channel.ChannelHandlerContext;
import io.netty.channel.ChannelInboundHandlerAdapter;

/**
 * Created by myk
 * 2020/1/29 下午9:53
 */
public class ClientHandler extends ChannelInboundHandlerAdapter {

    @Override
    public void channelActive(ChannelHandlerContext ctx) throws Exception {
        for (int i = 0; i < 10; i++) {

            MessageProtocol messageProtocol = new MessageProtocol();
            String content = "hello,packing&unpacking," + (i + 1);
            messageProtocol.setContent(content.getBytes());
            messageProtocol.setLength(content.getBytes().length);
            ctx.writeAndFlush(messageProtocol);

        }
    }

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

自定义对象及编解码器

package com.wojiushiwo.tcp.protocol;

import lombok.Data;

/**
 * Created by myk
 * 2020/1/29 下午9:47
 */
@Data
public class MessageProtocol {
	//保存每次数据发送的长度
    private int length;
	//保存每次发送数据内存
    private byte[] content;

}

//编解码器
package com.wojiushiwo.tcp.protocol;

import io.netty.buffer.ByteBuf;
import io.netty.channel.ChannelHandlerContext;
import io.netty.handler.codec.MessageToByteEncoder;

/**
 * Created by myk
 * 2020/1/29 下午9:48
 */
public class PackingUnPackingEncoder extends MessageToByteEncoder<MessageProtocol> {
    @Override
    protected void encode(ChannelHandlerContext ctx, MessageProtocol msg, ByteBuf out) throws Exception {
        out.writeInt(msg.getLength());
        out.writeBytes(msg.getContent());
    }
}

package com.wojiushiwo.tcp.protocol;

import io.netty.buffer.ByteBuf;
import io.netty.channel.ChannelHandlerContext;
import io.netty.handler.codec.ReplayingDecoder;

import java.util.List;

/**
 * Created by myk
 * 2020/1/29 下午9:49
 */
public class PackingUnpackingDecoder extends ReplayingDecoder<Void> {
    @Override
    protected void decode(ChannelHandlerContext ctx, ByteBuf in, List<Object> out) throws Exception {

        //这里的in代表的类型是ReplayingDecoderByteBuf
        int length = in.readInt();
        byte[] content = new byte[length];
        in.readBytes(content);

        MessageProtocol messageProtocol = new MessageProtocol();
        messageProtocol.setContent(content);
        messageProtocol.setLength(length);
        //封装成 MessageProtocol 对象，放入 out， 传递下一个 handler 业务处理
        out.add(messageProtocol);

    }
}

服务器端启动后，启动了三个客户端，输出结果无误

//客户端1启动后，服务器端输出

from client[length:25内容:hello,packing&unpacking,1]
from client[length:25内容:hello,packing&unpacking,2]
from client[length:25内容:hello,packing&unpacking,3]
from client[length:25内容:hello,packing&unpacking,4]
from client[length:25内容:hello,packing&unpacking,5]
from client[length:25内容:hello,packing&unpacking,6]
from client[length:25内容:hello,packing&unpacking,7]
from client[length:25内容:hello,packing&unpacking,8]
from client[length:25内容:hello,packing&unpacking,9]
from client[length:26内容:hello,packing&unpacking,10]

//客户端2启动后，服务器端输出

from client[length:25内容:hello,packing&unpacking,1]
from client[length:25内容:hello,packing&unpacking,2]
from client[length:25内容:hello,packing&unpacking,3]
from client[length:25内容:hello,packing&unpacking,4]
from client[length:25内容:hello,packing&unpacking,5]
from client[length:25内容:hello,packing&unpacking,6]
from client[length:25内容:hello,packing&unpacking,7]
from client[length:25内容:hello,packing&unpacking,8]
from client[length:25内容:hello,packing&unpacking,9]
from client[length:26内容:hello,packing&unpacking,10]

//客户端3启动后，服务器端输出

from client[length:25内容:hello,packing&unpacking,1]
from client[length:25内容:hello,packing&unpacking,2]
from client[length:25内容:hello,packing&unpacking,3]
from client[length:25内容:hello,packing&unpacking,4]
from client[length:25内容:hello,packing&unpacking,5]
from client[length:25内容:hello,packing&unpacking,6]
from client[length:25内容:hello,packing&unpacking,7]
from client[length:25内容:hello,packing&unpacking,8]
from client[length:25内容:hello,packing&unpacking,9]
from client[length:26内容:hello,packing&unpacking,10]

通过输出结果，可以看出粘包、拆包问题得到了解决。

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