《Netty权威指南 第2版》学习笔记（3）---分隔符和定长解码器

前言

前面介绍过上层应用协议为了对消息进行区分，解决策略一般采用如下4种方式。

1、消息定长，累计读取到长度总和为定长LEN的报文后，就认为读取到了一个完整的消息，将计数器置位，重新开始读取下一个数据包。
2、在包尾增加回车换行符进行分割，例如FTP，这种方式在文本协议中应用比较广泛。
3、将特殊的分隔符作为消息的结束标志，回车换行符就是一种特殊的结束分隔符。
4、通过在消息头中定义长度字段来标识消息的总长度。

Netty对上面应用做了统一的抽象，提供了4种解码器来解决对应的问题，使用起来非常方便。有了这些解码器，用户不需要自己对读取的报文进行人工解码，也不需要考虑TCP的粘包和拆包。

本文继续学习另外两种解码器：DelimiterBasedFrameDecoder和FixedLengthFrameDecoder，前者可以自动完成以分隔符做结束标志的消息的解码，后者可以自动完成对定长消息的解码，它们都能解决TCP粘包、拆包导致的读半包问题。

DelimiterBasedFrameDecoder解码器

服务端

import io.netty.bootstrap.ServerBootstrap;
import io.netty.buffer.ByteBuf;
import io.netty.buffer.Unpooled;
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.DelimiterBasedFrameDecoder;
import io.netty.handler.codec.string.StringDecoder;
import io.netty.handler.logging.LogLevel;
import io.netty.handler.logging.LoggingHandler;

public class TimeServer {
    
    
    public void bind(int port) throws Exception {
    
    
        EventLoopGroup boss = new NioEventLoopGroup();
        EventLoopGroup worker = new NioEventLoopGroup();
        try {
    
    
            ServerBootstrap bootstrap = new ServerBootstrap();
            bootstrap.group(boss, worker)
                    .channel(NioServerSocketChannel.class)
                    .option(ChannelOption.SO_BACKLOG, 100)
                    .childHandler(new ChannelInitializer<SocketChannel>() {
    
    
                        @Override
                        protected void initChannel(SocketChannel ch) throws Exception {
    
    
                            /*
                            使用DelimiterBasedFrameDecoder解码器，并指定按照"$_"作为分隔符，
                            1024表示单条消息的最大长度，当达到该长度后仍然没有查找到分隔符，就抛出TooLongFrameException异常，
                            防止由于异常码流缺失分隔符导致的内存溢出。
                             */
                            ByteBuf buf = Unpooled.copiedBuffer("$_".getBytes());
                            ch.pipeline().addLast(new DelimiterBasedFrameDecoder(1024, buf));
                            ch.pipeline().addLast(new StringDecoder());
                            ch.pipeline().addLast(new TimeServerHandler());
                        }
                    });
            ChannelFuture future = bootstrap.bind(port).sync();
            future.channel().closeFuture().sync();
        } finally {
    
    
            boss.shutdownGracefully();
            worker.shutdownGracefully();
        }
    }


    public static void main(String[] args) throws Exception {
    
    
        new TimeServer().bind(8080);
    }
}

TimeServerHandler

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

import java.util.Date;

public class TimeServerHandler extends ChannelInboundHandlerAdapter {
    
    
    private int counter;

    @Override
    public void channelRead(ChannelHandlerContext ctx, Object msg) throws Exception {
    
    
        String body = (String) msg;
        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";
        /*
        每行结尾新增"$_"
         */
        currentTime += "$_";
        ByteBuf resp = Unpooled.copiedBuffer(currentTime.getBytes());
        ctx.writeAndFlush(resp);
    }

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

客户端

import io.netty.bootstrap.Bootstrap;
import io.netty.buffer.ByteBuf;
import io.netty.buffer.Unpooled;
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.DelimiterBasedFrameDecoder;
import io.netty.handler.codec.string.StringDecoder;

public class TimeClient {
    
    

    public void connect(int port, String host) throws InterruptedException {
    
    
        EventLoopGroup group = new NioEventLoopGroup();
        try {
    
    
            Bootstrap bootstrap = new Bootstrap();
            bootstrap.group(group)
                    .channel(NioSocketChannel.class)
                    .option(ChannelOption.TCP_NODELAY, true)
                    .handler(new ChannelInitializer<SocketChannel>() {
    
    
                        @Override
                        protected void initChannel(SocketChannel ch) throws Exception {
    
    
                            /*
                            使用DelimiterBasedFrameDecoder解码器，并指定按照"$_"作为分隔符
                             */
                            ByteBuf buf = Unpooled.copiedBuffer("$_".getBytes());
                            ch.pipeline().addLast(new DelimiterBasedFrameDecoder(1024, buf));
                            ch.pipeline().addLast(new StringDecoder());
                            ch.pipeline().addLast(new TimeClientHandler());
                        }
                    });
            ChannelFuture future = bootstrap.connect(host, port).sync();
            future.channel().closeFuture().sync();
        } finally {
    
    
            group.shutdownGracefully();
        }
    }

    public static void main(String[] args) throws InterruptedException {
    
    
        int port = 8080;
        new TimeClient().connect(port, "127.0.0.1");
    }
}

TimeClientHandler

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

public class TimeClientHandler extends ChannelInboundHandlerAdapter {
    
    

    private int counter;

    private byte[] req;

    public TimeClientHandler() {
    
    
        /*
        每条请求的数据后面都拼上一个"$_"
         */
        req = ("QUERY TIME ORDER$_").getBytes();
    }

    @Override
    public void channelActive(ChannelHandlerContext ctx) throws Exception {
    
    
        ByteBuf message;
        for (int i = 0; i < 500; 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;
        System.out.println("Now is : " + body + " ; the counter is : " + ++counter);
    }

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

和之前使用LineBasedFrameDecoder解码器的代码几乎一样。

单条消息超过设定的最大长度

io.netty.handler.codec.TooLongFrameException: frame length exceeds 1: 16 - discarded

FixedLengthFrameDecoder解码器

FixedLengthFrameDecoder是固定长度解码器，它能够按照指定的长度对消息进行自动解码。

服务端

只需要替换FixedLengthFrameDecoder，这里固定长度为64个字节

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

public class TimeServer {
    
    
    public void bind(int port) throws Exception {
    
    
        EventLoopGroup boss = new NioEventLoopGroup();
        EventLoopGroup worker = new NioEventLoopGroup();
        try {
    
    
            ServerBootstrap bootstrap = new ServerBootstrap();
            bootstrap.group(boss, worker)
                    .channel(NioServerSocketChannel.class)
                    .option(ChannelOption.SO_BACKLOG, 100)
                    .childHandler(new ChannelInitializer<SocketChannel>() {
    
    
                        @Override
                        protected void initChannel(SocketChannel ch) throws Exception {
    
    
                            /*
                            使用FixedLengthFrameDecoder解码器，固定长度为64个字节
                             */
                            ch.pipeline().addLast(new FixedLengthFrameDecoder(64));
                            ch.pipeline().addLast(new StringDecoder());
                            ch.pipeline().addLast(new TimeServerHandler());
                        }
                    });
            ChannelFuture future = bootstrap.bind(port).sync();
            future.channel().closeFuture().sync();
        } finally {
    
    
            boss.shutdownGracefully();
            worker.shutdownGracefully();
        }
    }


    public static void main(String[] args) throws Exception {
    
    
        new TimeServer().bind(8080);
    }
}

TimeServerHandler

没有任何变化

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

import java.util.Date;

public class TimeServerHandler extends ChannelInboundHandlerAdapter {
    
    
    private int counter;

    @Override
    public void channelRead(ChannelHandlerContext ctx, Object msg) throws Exception {
    
    
        String body = (String) msg;
        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";
        ByteBuf resp = Unpooled.copiedBuffer(currentTime.getBytes());
        ctx.writeAndFlush(resp);
    }

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

TimeClient

与服务端一样替换成FixedLengthFrameDecoder，固定长度64个字节

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

public class TimeClient {
    
    

    public void connect(int port, String host) throws InterruptedException {
    
    
        EventLoopGroup group = new NioEventLoopGroup();
        try {
    
    
            Bootstrap bootstrap = new Bootstrap();
            bootstrap.group(group)
                    .channel(NioSocketChannel.class)
                    .option(ChannelOption.TCP_NODELAY, true)
                    .handler(new ChannelInitializer<SocketChannel>() {
    
    
                        @Override
                        protected void initChannel(SocketChannel ch) throws Exception {
    
    
                            /*
                            使用FixedLengthFrameDecoder解码器，固定长度为64个字节
                             */
                            ch.pipeline().addLast(new FixedLengthFrameDecoder(64));
                            ch.pipeline().addLast(new StringDecoder());
                            ch.pipeline().addLast(new TimeClientHandler());
                        }
                    });
            ChannelFuture future = bootstrap.connect(host, port).sync();
            future.channel().closeFuture().sync();
        } finally {
    
    
            group.shutdownGracefully();
        }
    }

    public static void main(String[] args) throws InterruptedException {
    
    
        int port = 8080;
        new TimeClient().connect(port, "127.0.0.1");
    }
}

TimeClientHandler

构造一个固定64个字节长度的字节数组，进行发送。

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

public class TimeClientHandler extends ChannelInboundHandlerAdapter {
    
    

    private int counter;

    private byte[] req;

    private byte[] fixReq;

    public TimeClientHandler() {
    
    
        req = ("QUERY TIME ORDER").getBytes();
        /*
        使用System.arraycopy技巧，保证发送内容为64个字节
        源数组如果不足64字节，则会以空字符表示
         */
        fixReq = new byte[64];
        System.arraycopy(req, 0, fixReq, 0, req.length);
    }

    @Override
    public void channelActive(ChannelHandlerContext ctx) throws Exception {
    
    
        ByteBuf message;
        for (int i = 0; i < 500; i++) {
    
    
            message = Unpooled.buffer(fixReq.length);
            message.writeBytes(fixReq);
            ctx.writeAndFlush(message);
        }
    }

    @Override
    public void channelRead(ChannelHandlerContext ctx, Object msg) throws Exception {
    
    
        String body = (String) msg;
        /*
        body.trim()去除头尾空格
         */
        System.out.println("Now is : " + body.trim() + " ; the counter is : " + ++counter);
    }

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

服务端打印

客户端打印

可以看出服务端打印没有问题，因为客户端已经按照固定长度发送的，所以FixedLengthFrameDecoder会处理粘包问题，但是客户端打印出现了粘包的问题，因为服务端在发送时并没有按照指定的字节长度进行发送。

TimeServerHandler

按照客户端方式修改服务端TimeServerHandler

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

import java.util.Date;

public class TimeServerHandler extends ChannelInboundHandlerAdapter {
    
    
    private int counter;

    @Override
    public void channelRead(ChannelHandlerContext ctx, Object msg) throws Exception {
    
    
        String body = (String) msg;
        //去除收尾空格
        body = body.trim();
        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";

        /*
        使用System.arraycopy技巧，保证发送内容为64个字节
        源数组如果不足64字节，则会以空字符表示
         */
        byte[] fixReq = new byte[64];
        System.arraycopy(currentTime.getBytes(), 0, fixReq, 0, currentTime.getBytes().length);

        ByteBuf resp = Unpooled.copiedBuffer(fixReq);
        ctx.writeAndFlush(resp);
    }

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

服务端打印

客户端打印

总结

DelimiterBasedFrameDecoder用于对使用分隔符结尾的消息进行自动解码，FixedLengthFrameDecoder用于对固定长度的消息进行自动解码，有了这两种解码器，可以轻松地完成对很多消息的自动解码，而且不需要考虑TCP粘包、拆包导致的读半包问题，极大的提升了开发效率。