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[Netty學習筆記]十一、TCP粘包、拆包及解決方案

概述

TCP是面向連接配接、面向流的，可以提供高可靠性服務。收發兩端(用戶端和伺服器端)都要有一一成對的socket。是以，發送端為了将多個發給接收端的包更有效的發給對方，使用了優化方法，将多次間隔較小且資料量小的資料合成為一個大的資料塊，然後進行封包。這樣做雖然提高了效率，但是接收端就很難分辨出接收到的包是否為一個完整的資料包了。因為面向流的通信時無消息保護邊界的。

由于TCP無消息保護邊界，需要在接收端處理消息邊界問題。是以會有拆包、粘包問題，如下圖

[Netty學習筆記]十一、TCP粘包、拆包及解決方案

說明：

假設用戶端分别發送了兩個資料包D1和D2給服務端，由于服務端一次讀取到的位元組數是不确定的，是以可能存在以下四種情況：

  1. 伺服器端分兩次讀取到了兩個獨立的資料包，分别是D1和D2,沒有粘包、拆包問題
  2. 伺服器端一次接受到了兩個資料包，D1和D2粘合在一起，稱為TCP粘包
  3. 伺服器端分兩次讀取到了資料包，第一次讀取到了完整的D1包和D2包的部分内容，第二次讀取到了D2包的剩餘内容，這稱之為TCP拆包。
  4. 伺服器端分兩次讀取到了資料包，第一次讀取到了D1包的部分内容，第二次讀取到了D1包的剩餘部分内容和完整的D2包
代碼示範TCP粘包、拆包現象

伺服器端:

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("伺服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();
    }
}

用戶端代碼

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 {
        //使用用戶端發送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);
    }
}
用戶端循環5次 向伺服器端發送消息，通過檢視伺服器端接收到的消息完整程度即可判斷粘包、拆包現象

啟動了服務端，并且啟動了三個用戶端，輸出結果：

//用戶端1啟動後伺服器端輸出

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

伺服器端接收到的消息量=1

//用戶端2啟動後伺服器端輸出

from client:from client1

伺服器端接收到的消息量=1

from client:from client2

伺服器端接收到的消息量=2

from client:from client3 from client4 from client5

伺服器端接收到的消息量=3

// 用戶端3啟動後伺服器端輸出

from client:from client1

伺服器端接收到的消息量=1

from client:from client2

伺服器端接收到的消息量=2

from client:from client3

伺服器端接收到的消息量=3

from client:from client4

伺服器端接收到的消息量=4

from client:from client5

伺服器端接收到的消息量=5

可以發現，輸出結果中粘包、拆包等情況都發生了。

解決方案

隻要确定了伺服器端每次傳輸的資料的長度，那麼伺服器端一次讀取到的就是完整的資料，就可以避免TCP拆包、粘包問題。

通常使用自定義協定+編解碼器來解決

解決方案示例：

伺服器端：

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();
    }
}

用戶端代碼

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();
    }
}

自定義對象及編解碼器

package com.wojiushiwo.tcp.protocol;

import lombok.Data;

/**
 * Created by myk
 * 2020/1/29 下午9:47
 */
@Data
public class MessageProtocol {
	//儲存每次資料發送的長度
    private int length;
	//儲存每次發送資料記憶體
    private byte[] content;

}

//編解碼器
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 {

        //這裡的in代表的類型是ReplayingDecoderByteBuf
        int length = in.readInt();
        byte[] content = new byte[length];
        in.readBytes(content);

        MessageProtocol messageProtocol = new MessageProtocol();
        messageProtocol.setContent(content);
        messageProtocol.setLength(length);
        //封裝成 MessageProtocol 對象，放入 out， 傳遞下一個 handler 業務處理
        out.add(messageProtocol);

    }
}

伺服器端啟動後，啟動了三個用戶端，輸出結果無誤

//用戶端1啟動後，伺服器端輸出

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]

//用戶端2啟動後，伺服器端輸出

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]

//用戶端3啟動後，伺服器端輸出

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]

通過輸出結果，可以看出粘包、拆包問題得到了解決。

netty學習筆記 netty tcp 粘包 拆包
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