TCP粘包与拆包问题

0 概述

熟悉TCP编程的读者可能都知道，无论是服务端还是客户端，当我们读取或者发送消息的时候，都需要考虑TCP粘包与拆包问题机制。

1 TCP粘包与拆包问题

TCP是个”流协议”，所谓流，就是没有界限的一串数据。TCP底层并不了解上层业务的具体含义，它会根据TCP缓冲区的实际情况进行包的划分，所以在上层业务认为，一个完整的包可能被TCP拆分成多个包进行发送，也可能把多个小的包封装成一个大包发送，这就是TCP拆包与粘包问题。

下图中客户端向服务端先后发送了Packet1和Packet2两个包。



实例分析：

服务端code：

import io.netty.bootstrap.ServerBootstrap;
import io.netty.channel.ChannelFuture;
import io.netty.channel.ChannelInitializer;
import io.netty.channel.ChannelOption;
import io.netty.channel.EventLoopGroup;

import io.netty.channel.nio.NioEventLoopGroup;
import io.netty.channel.socket.SocketChannel;
import io.netty.channel.socket.nio.NioServerSocketChannel;

/**
* Created by apple on 17/9/4.
*/
public class NettyServer {
public static void main(String[] args) {
/**
* parentGroup 用于accept
*/
EventLoopGroup parentGroup = new NioEventLoopGroup(1);
/**
*  默认线程数 cup*2 ,用于处理
*/
EventLoopGroup childGroup = new NioEventLoopGroup();
try {
ServerBootstrap serverBootstrap = new ServerBootstrap();
serverBootstrap.group(parentGroup, childGroup);

serverBootstrap.option(ChannelOption.SO_BACKLOG, 128)
.channel(NioServerSocketChannel.class)
.option(ChannelOption.SO_KEEPALIVE, true)
.childHandler(new ChannelInitializer<SocketChannel>() {
protected void initChannel(SocketChannel ch) throws Exception {
ch.pipeline().addLast(new SimpleHandler());

}
});
ChannelFuture future = serverBootstrap.bind(8080).sync();
future.channel().closeFuture().sync();
} catch (Exception ex) {
ex.printStackTrace();
} finally {
parentGroup.shutdownGracefully();
childGroup.shutdownGracefully();
}

}
}

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

/**
* Created by apple on 17/9/29.
*/
public class SimpleHandler extends ChannelInboundHandlerAdapter {
@Override
public void channelRead(ChannelHandlerContext ctx, Object msg) throws Exception {
ByteBuf buf=(ByteBuf)msg;
byte [] data=new byte[buf.readableBytes()];
buf.readBytes(data);
System.out.println(new String(data));
String responseMsg="收到"+System.getProperty("line.separator");
ByteBuf sendMsg= Unpooled.copiedBuffer(responseMsg.getBytes());
ctx.writeAndFlush(sendMsg);
}
}

客户端

import io.netty.bootstrap.Bootstrap;
import io.netty.channel.*;
import io.netty.channel.nio.NioEventLoopGroup;
import io.netty.channel.socket.nio.NioSocketChannel;

/**
* Created by apple on 17/10/1.
*/
public class NettyClient {

public static void main(String[] args) {
startClient();
}

private static void startClient() {
EventLoopGroup eventLoopGroup = new NioEventLoopGroup();

Bootstrap bootstrap = new Bootstrap();
try {
bootstrap.channel(NioSocketChannel.class)
.group(eventLoopGroup)
.option(ChannelOption.SO_KEEPALIVE, true)
.handler(new ChannelInitializer<NioSocketChannel>() {
protected void initChannel(NioSocketChannel ch) throws Exception {
ch.pipeline().addLast(new ClientHandler());
}
});
//发起异步连接操作
ChannelFuture future= bootstrap.connect("127.0.0.1", 8080).sync();
//等待客户端链路关闭
future.channel().closeFuture().sync();
} catch (Exception ex) {
ex.printStackTrace();
} finally {
eventLoopGroup.shutdownGracefully();
}
}
}

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

/**
* Created by apple on 17/10/1.
*/
public class ClientHandler extends ChannelInboundHandlerAdapter {
@Override
public void channelRead(ChannelHandlerContext ctx, Object msg) throws Exception {
System.out.println("客户端开始读数据");
ByteBuf buf = (ByteBuf) msg;
byte[] data = new byte[buf.readableBytes()];
buf.readBytes(data);
System.out.println(new String(data));
}

@Override
public void channelActive(ChannelHandlerContext ctx) throws Exception {
//向服务端发送100 个hello world
for (int i = 0; i < 100; i++) {
ByteBuf buf = Unpooled.copiedBuffer("hello world".getBytes());
ctx.writeAndFlush(buf);
}

}
}

运行结果：

希望达到效果是服务端收到100条消息同时恢复100条消息，结果只收到了两条消息，同时也只回复了两条消息。



解决办法：

既然业务层知道需要用什么分割，那么就可以指定相应的解码器。

LineBasedFrameDecoder 是以换行符为结束标志的解码器，当然也可以自定义解码器实现私有协议。

import io.netty.bootstrap.ServerBootstrap;
import io.netty.channel.ChannelFuture;
import io.netty.channel.ChannelInitializer;
import io.netty.channel.ChannelOption;
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.LineBasedFrameDecoder;

/**
* Created by apple on 17/9/4.
*/
public class NettyServer {
public static void main(String[] args) {
/**
* parentGroup 用于accept
*/
EventLoopGroup parentGroup = new NioEventLoopGroup(1);
/**
*  默认线程数 cup*2 ,用于处理
*/
EventLoopGroup childGroup = new NioEventLoopGroup();
try {
ServerBootstrap serverBootstrap = new ServerBootstrap();
serverBootstrap.group(parentGroup, childGroup);

serverBootstrap.option(ChannelOption.SO_BACKLOG, 128)
.channel(NioServerSocketChannel.class)
.option(ChannelOption.SO_KEEPALIVE, true)
.childHandler(new ChannelInitializer<SocketChannel>() {
protected void initChannel(SocketChannel ch) throws Exception {
ch.pipeline().addLast(new LineBasedFrameDecoder(1024));
ch.pipeline().addLast(new SimpleHandler());

}
});
ChannelFuture future = serverBootstrap.bind(8080).sync();
future.channel().closeFuture().sync();
} catch (Exception ex) {
ex.printStackTrace();
} finally {
parentGroup.shutdownGracefully();
childGroup.shutdownGracefully();
}

}
}

客户端发送时候需要带上换行符：

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

/**
* Created by apple on 17/10/1.
*/
public class ClientHandler extends ChannelInboundHandlerAdapter {
@Override
public void channelRead(ChannelHandlerContext ctx, Object msg) throws Exception {
System.out.println("客户端开始读数据");
ByteBuf buf = (ByteBuf) msg;
byte[] data = new byte[buf.readableBytes()];
buf.readBytes(data);
System.out.println(new String(data));
}

@Override
public void channelActive(ChannelHandlerContext ctx) throws Exception {
//向服务端发送100 个hello world
String message="hello world"+System.getProperty("line.separator");
for (int i = 0; i < 100; i++) {

ByteBuf buf = Unpooled.copiedBuffer(message.getBytes());
ctx.writeAndFlush(buf);
}

}
}

参考文献

[1] Netty 权威指南（第二版），李林峰著