[toc]


Netty中LineBasedFrameDecoder解码器使用与分析:解决TCP粘包问题

上一篇文章《Netty中TCP粘包问题代码示例与分析》演示了使用了时间服务器的例子演示了TCP的粘包问题,这里使用LineBasedFrameDecoder就是用来解决这个问题的。

不过需要注意的是,LineBasedFrameDecoder见名知其义,可见其是与行相关的,而在前面演示TCP粘包问题时,作者是有意在发送的消息中都加入了换行符,目的也是为了在后面去讲解LineBasedFrameDecoder的使用,当然也是给出一种简单解决TCP粘包问题的方法,或者是通过一个简单案例来让我们这些学习者有更直观的理解。

代码来自于《Netty权威指南》第4章,不过我依然是做了部分的修改。

服务端代码

TimeServer.java

package cn.xpleaf.netty02;

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;
import io.netty.handler.codec.string.StringDecoder;

public class TimeServer {

    public void bind(int port) throws Exception {
        // 配置服务端的NIO线程组
        EventLoopGroup bossGroup = new NioEventLoopGroup();
        EventLoopGroup workerGroup = new NioEventLoopGroup();

        try {
            ServerBootstrap b = new ServerBootstrap();
            b.group(bossGroup, workerGroup)
                .channel(NioServerSocketChannel.class)
                .option(ChannelOption.SO_BACKLOG, 1024)
                .childHandler(new ChildChannelHandler());

            // 绑定端口,同步等待成功
            ChannelFuture f = b.bind(port).sync();

            // 等待服务端监听端口关闭
            f.channel().closeFuture().sync();
        } finally {
            // 优雅退出,释放线程池资源
            bossGroup.shutdownGracefully();
            workerGroup.shutdownGracefully();
        }
    }

    private class ChildChannelHandler extends ChannelInitializer<SocketChannel> {

        @Override
        protected void initChannel(SocketChannel ch) throws Exception {
            // 核心在下面两行,加入了LineBasedFrameDecoder和StringDecoder两个解码器
            // 所以当消息到达我们的业务处理handler即TimerServerHandler,所看到的消息
            // 都是前面两个解码器经过处理之后的结果
            ch.pipeline().addLast(new LineBasedFrameDecoder(1024));
            ch.pipeline().addLast(new StringDecoder());
            ch.pipeline().addLast(new TimeServerHandler());
        }

    }

    public static void main(String[] args) throws Exception {
        int port = 8080;
        if(args != null && args.length > 0) {
            try {
                port = Integer.valueOf(port);
            } catch (NumberFormatException e) {
                // TODO: handle exception
            }
        }
        new TimeServer().bind(port);
    }

}

TimeServerHandler.java

package cn.xpleaf.netty02;

import java.sql.Date;

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

public class TimeServerHandler extends ChannelInboundHandlerAdapter {

    private int counter = 0;

    @Override
    public void channelRead(ChannelHandlerContext ctx, Object msg) throws Exception {
        String body = (String) msg;
        // counter的作用是标记这是第几次收到客户端的请求
        System.out.println("The time server receive order : " + body + " ; the counter is : " + ++counter);
        String currentTime = "QUERY TIME ORDER".equalsIgnoreCase(body) ? 
                new Date(System.currentTimeMillis()).toString() : "BAD ORDER";
        currentTime = currentTime + System.getProperty("line.separator");
        ByteBuf resp = Unpooled.copiedBuffer(currentTime.getBytes());
        ctx.write(resp);
    }

    @Override
    public void channelReadComplete(ChannelHandlerContext ctx) throws Exception {
        ctx.flush();
    }

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

客户端代码

TimeClient.java

package cn.xpleaf.netty02;

import io.netty.bootstrap.Bootstrap;
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.NioSocketChannel;
import io.netty.handler.codec.LineBasedFrameDecoder;
import io.netty.handler.codec.string.StringDecoder;

public class TimeClient {

    public void connect(int port, String host) throws Exception {
        // 配置客户端NIO线程组
        EventLoopGroup group = new NioEventLoopGroup();
        try {
            Bootstrap b = new Bootstrap();
            b.group(group).channel(NioSocketChannel.class)
                .option(ChannelOption.TCP_NODELAY, true)
                .handler(new ChannelInitializer<SocketChannel>() {

                    @Override
                    protected void initChannel(SocketChannel ch) throws Exception {
                        // 核心在下面两行,加入了LineBasedFrameDecoder和StringDecoder两个解码器
                        // 所以当消息到达我们的业务处理handler即TimerServerHandler,所看到的消息
                        // 都是前面两个解码器经过处理之后的结果
                        ch.pipeline().addLast(new LineBasedFrameDecoder(1024));
                        ch.pipeline().addLast(new StringDecoder());
                        ch.pipeline().addLast(new TimeClientHandler());
                    }
                });
            // 发起异步连接操作
            ChannelFuture f = b.connect(host, port).sync();

            // 等待客户端链路关闭
            f.channel().closeFuture().sync();
        } finally {
            // 优雅退出,释放NIO线程组
            group.shutdownGracefully();
        }
    }

    public static void main(String[] args) throws Exception {
        int port = 8080;
        if(args != null && args.length > 0) {
            try {
                port = Integer.valueOf(port);
            } catch (NumberFormatException e) {
                // 采用默认值
            }
        }
        new TimeClient().connect(port, "localhost");
    }
}

TimeClientHandler.java

package cn.xpleaf.netty02;

import java.util.logging.Logger;

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

public class TimeClientHandler extends ChannelInboundHandlerAdapter {

    private static final Logger logger = Logger.getLogger(TimeServerHandler.class.getName());

    private int counter;

    private byte[] req;

    public TimeClientHandler() {
        req = ("QUERY TIME ORDER" + System.getProperty("line.separator")).getBytes();
    }

    @Override
    public void channelActive(ChannelHandlerContext ctx) {
        ByteBuf message = null;
        for(int i = 0; i < 100; i++) {
            message = Unpooled.buffer(req.length);
            message.writeBytes(req);
            ctx.writeAndFlush(message);
        }
    }

    @Override
    public void channelRead(ChannelHandlerContext ctx, Object msg) throws Exception {
        String body = (String) msg;
        // counter的作用是标记这是第几次收到客户端的请求
        System.out.println("Now is : " + body + " ; the counter is : " + ++counter);
    }

    @Override
    public void exceptionCaught(ChannelHandlerContext ctx, Throwable cause) throws Exception {
        logger.warning("Unexpected exception from downstream : ");
        ctx.close();
    }

}

测试

服务端运行结果:

The time server receive order : QUERY TIME ORDER ; the counter is : 1
The time server receive order : QUERY TIME ORDER ; the counter is : 2
The time server receive order : QUERY TIME ORDER ; the counter is : 3
...省略多行...
The time server receive order : QUERY TIME ORDER ; the counter is : 98
The time server receive order : QUERY TIME ORDER ; the counter is : 99
The time server receive order : QUERY TIME ORDER ; the counter is : 100

客户端运行结果:

Now is : 2018-02-11 ; the counter is : 1
Now is : 2018-02-11 ; the counter is : 2
Now is : 2018-02-11 ; the counter is : 3
...省略多行...
Now is : 2018-02-11 ; the counter is : 98
Now is : 2018-02-11 ; the counter is : 99
Now is : 2018-02-11 ; the counter is : 100

从输出结果可以看出,这确实解决了TCP的粘包问题。

原理分析

下面的分析来自于书本上,这里也贴出来:
Netty中LineBasedFrameDecoder解码器使用与分析:解决TCP粘包问题

当然,对应到上面这个例子,具体是如何解决TCP粘包问题的呢,下面也来简单分析一下。

首先需要明确的是,TCP依然会有粘包的情况,即TCP的机制依然没有变,而且也不可能变,因为这是TCP/IP协议栈的规定,并且由操作系统实现。

所以不管应用层,我们的程序代码如何改变,本质上,TCP还是会按照上一篇文章中所说的方式去进行包的发送,即客户端中,TCP依然会把netty的多个请求发送到服务端。

只是这时不同的是,发送的消息不是直接先到达我们写的TimeServerHandler处理器,而是先达到LineBasedFrameDecoder解码器,因为该解码器就是以行作为分隔符来解析客户端的请求的,所以当它分析客户端的请求时,一旦发现有换行符,它就认为当初Netty的客户端在进行消息的发送时,是以换行符为分隔符的去发送请求的,只是TCP把它的多个请求合并发送了而已,因此它就把每一行解析为客户端的一个请求。那么这时,解析出每行是一个请求后,它就会以一个请求一次的方式去调用下面的Handler,所以就会多次调用我们的业务处理handler,在TimeServerHandler看起来这就相当于是多个请求,而当对数据进行回写的时候,原理也是一样的。

所以可以知道,TCP粘包问题的解决是在应用层的解决,需要服务端和客户端事先约定一种规范,比如这里以换行符作为每个请求的分隔符。