tcp的粘包和拆包示例以及使用LengthFieldFrameDecoder来解决的方法

粘包和拆包是什么?
TCP协议是一种字节流协议,没有记录边界,我们在接收消息的时候,不能人为接收到的数据包就是一个整包消息

当客户端向服务器端发送多个消息数据的时候,TCP协议可能将多个消息数据合并成一个数据包进行发送,这就是粘包

当客户端向服务器端发送的消息过大的时候,tcp协议可能将一个数据包拆成多个数据包来进行发送,这就是拆包

以下一netty为例,展示一下tcp粘包和拆包的例子:
ServerBusinessHanler:

import io.netty.buffer.ByteBuf;
import io.netty.buffer.Unpooled;
import io.netty.channel.ChannelHandlerContext;
import io.netty.channel.SimpleChannelInboundHandler;

import java.nio.charset.Charset;
import java.util.UUID;

public class ServerBusinessHanler extends SimpleChannelInboundHandler<ByteBuf> {

    int count = 0;

    @Override
    protected void channelRead0(ChannelHandlerContext ctx, ByteBuf msg) throws Exception {
        byte[] bytes = new byte[msg.readableBytes()];
        msg.readBytes(bytes);
        String message = new String(bytes, Charset.forName("UTF-8"));
        System.out.println("从客户端收到的字符串:" + message);
        System.out.println("服务端接收到的请求数:" + (++count));
        ctx.writeAndFlush(Unpooled.copiedBuffer(UUID.randomUUID().toString(),Charset.forName("UTF-8")) );

    }

    @Override
    public void exceptionCaught(ChannelHandlerContext ctx, Throwable cause) throws Exception {
        System.out.println(cause.getStackTrace());
        ctx.channel().close();
    }

}

Server:

import io.netty.bootstrap.ServerBootstrap;
import io.netty.channel.ChannelInitializer;
import io.netty.channel.EventLoopGroup;
import io.netty.channel.nio.NioEventLoopGroup;
import io.netty.channel.socket.SocketChannel;
import io.netty.channel.socket.nio.NioServerSocketChannel;
import java.net.InetSocketAddress;


public class Server {
    public static void main(String[] args) throws InterruptedException {
        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 {
                        
                            ch.pipeline().addLast(new ServerBusinessHanler());
                        }
                    });
            serverBootstrap.bind(new InetSocketAddress(8899)).sync().channel().closeFuture().sync();

        } finally {
            bossGroup.shutdownGracefully();
            workerGroup.shutdownGracefully();
        }

    }
}    

ClientBusinessHandler:

import io.netty.buffer.ByteBuf;
import io.netty.buffer.Unpooled;
import io.netty.channel.ChannelHandlerContext;
import io.netty.channel.SimpleChannelInboundHandler;    
import java.nio.charset.Charset;

public class ClientBusinessHandler extends SimpleChannelInboundHandler<ByteBuf> {
    int count = 1;

    @Override
    protected void channelRead0(ChannelHandlerContext ctx, ByteBuf msg) throws Exception {
        byte[] bytes = new byte[msg.readableBytes()];
        msg.readBytes(bytes);
        System.out.println("从服务器端接收到的信息: " + new String(bytes, Charset.forName("UTF-8")));
        System.out.println("从服务器端读到的请求的个数: " + count++);
    }

    @Override
    public void channelActive(ChannelHandlerContext ctx) throws Exception {
        for (int i = 0; i < 10; i++) {
            ctx.writeAndFlush(Unpooled.copiedBuffer("中国移动".getBytes()));
        }
        ctx.writeAndFlush(Unpooled.copiedBuffer("jiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyiping".getBytes()));
    }
}

Client:

import io.netty.bootstrap.Bootstrap;
import io.netty.channel.Channel;
import io.netty.channel.ChannelInitializer;
import io.netty.channel.EventLoopGroup;
import io.netty.channel.nio.NioEventLoopGroup;
import io.netty.channel.socket.SocketChannel;
import io.netty.channel.socket.nio.NioSocketChannel;


public class Client {
    public static void main(String[] args) throws InterruptedException {
        EventLoopGroup eventExecutors = new NioEventLoopGroup();
        Bootstrap bootstrap = new Bootstrap();
        bootstrap.group(eventExecutors)
                .channel(NioSocketChannel.class)
                .handler(new ChannelInitializer<SocketChannel>() {
                    @Override
                    protected void initChannel(SocketChannel ch) throws Exception {
                        ch.pipeline().addLast(new ClientBusinessHandler());
                    }
                });
        Channel channel = bootstrap.connect("127.0.0.1", 8899).channel();
        channel.closeFuture().sync();

    }
}    

分别运行服务器端和客户端,我们看到服务器端的输出:

相应的,服务器端也只给客户端了两个返回,而不是我们期待的11个

我们从客户端传入的十个 "中国移动"和第二次发送的长的字符串的前一部分发生了粘包,而第二个长字符串的后一部分则被拆分到第二个数据包里了

粘包和拆包的情况都出现了

那怎么解决粘包和拆包的问题呢? 一种方法是在字符串中使用特定的分隔符来分隔,另外一种方法是,我们在发送数据包的时候在前边附加上数据包的长度

netty为我们提供了多种的解码器,比如定长解码器和基于分隔符的解码器等,这里,我么使用 LengthFieldBasedFrameDecoder这个解码器来解决粘包和拆包的问题,关于这个解码器的详细信息,可以参考这个类的java doc文档,上边讲述的很详细,简单来说,就是我们需要在请求中添加一些关于数据字段长度的信息(以下简称length),LengthFieldBasedFrameDecoder的构造方法中,设置一下关于length所占的字节数和要跳过的字节数等信息(我们只需要读取数据内容的话,可以跳过length的信息) 这样的话,我们从客户端发送的每个数据包,都可以正确地解析

首先,我们增加一个编码器,为数据包附件上lenght:

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


//本类作用,加上四个字节的消息头(int类型),表示数据包长度
public class LengthEncoder extends MessageToByteEncoder<ByteBuf> {
    @Override
    protected void encode(ChannelHandlerContext ctx, ByteBuf msg, ByteBuf out) throws Exception {
        int length = msg.readableBytes();
        byte[] bytes = new byte[length];
        msg.readBytes(bytes);
        out.writeInt(length);
        out.writeBytes(bytes);
    }
}

然后,将我们的编码器和对应的 LengthFieldBasedFrameDecoder添加到服务器端和客户端的ChannelInitializer里边去

修改之后的服务器端代码:

import io.netty.bootstrap.ServerBootstrap;
import io.netty.channel.ChannelInitializer;
import io.netty.channel.EventLoopGroup;
import io.netty.channel.nio.NioEventLoopGroup;
import io.netty.channel.socket.SocketChannel;
import io.netty.channel.socket.nio.NioServerSocketChannel;
import io.netty.handler.codec.LengthFieldBasedFrameDecoder;
import java.net.InetSocketAddress;


public class Server {
    public static void main(String[] args) throws InterruptedException {
        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 {
                            ch.pipeline().addLast(new LengthFieldBasedFrameDecoder(4096,0,4,0,4));
                            ch.pipeline().addLast(new LengthEncoder());
                            ch.pipeline().addLast(new ServerBusinessHanler());
                        }
                    });
            serverBootstrap.bind(new InetSocketAddress(8899)).sync().channel().closeFuture().sync();

        } finally {
            bossGroup.shutdownGracefully();
            workerGroup.shutdownGracefully();
        }

    }
}   

new LengthFieldBasedFrameDecoder(4096,0,4,0,4));的意思是:使用LengthFieldBasedFrameDecoder的最大的数据包大小是4096个,其中length占四个字节,读取的时候跳过4个字节(也就是最终解码的时候,忽略掉length字段,只返回真正的数据内容)

修改之后的客户端代码:

import io.netty.bootstrap.Bootstrap;
import io.netty.channel.Channel;
import io.netty.channel.ChannelInitializer;
import io.netty.channel.EventLoopGroup;
import io.netty.channel.nio.NioEventLoopGroup;
import io.netty.channel.socket.SocketChannel;
import io.netty.channel.socket.nio.NioSocketChannel;
import io.netty.handler.codec.LengthFieldBasedFrameDecoder;


public class Client {
    public static void main(String[] args) throws InterruptedException {
        EventLoopGroup eventExecutors = new NioEventLoopGroup();
        Bootstrap bootstrap = new Bootstrap();
        bootstrap.group(eventExecutors)
                .channel(NioSocketChannel.class)
                .handler(new ChannelInitializer<SocketChannel>() {
                    @Override
                    protected void initChannel(SocketChannel ch) throws Exception {
                        ch.pipeline().addLast(new LengthFieldBasedFrameDecoder(4096,0,4,0,4));
                        ch.pipeline().addLast(new LengthEncoder());
                        ch.pipeline().addLast(new ClientBusinessHandler());
                    }
                });
        Channel channel = bootstrap.connect("127.0.0.1", 8899).channel();
        channel.closeFuture().sync();

    }
}  

分别运行服务器端和客户端之后我们得到的结果:
服务器端输出:

客户端输出:

这样就解决了粘包和拆包导致的问题