TCP拆包粘包之分隔符解码器

TCP以流的方式进行数据传输，上层的应用协议为了对消息进行区分，往往采用如下4种方式。

（1）消息长度固定，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报；

（2）将回车换行符作为消息结束符，例如FTP协议，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对上面四种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。

两种实用的解码器——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应用开发

演示程序以经典的Echo服务为例。EchoServer接收到EchoClient的请求消息后，将其打印出来，然后将原始消息返回给客户端，消息以“$_”作为分隔符。

服务端示例：

import io.netty.bootstrap.ServerBootstrap;
import io.netty.buffer.ByteBuf;
import io.netty.buffer.Unpooled;
import io.netty.channel.*;
import io.netty.channel.nio.NioEventLoopGroup;
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 EchoServer {

    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, 100)
                    .handler(new LoggingHandler(LogLevel.INFO))
                    .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 {
        @Override
        protected void initChannel(Channel arg0) throws Exception {
            //首先创建分隔符缓冲对象ByteBuf，本例程中使用“$_”作为分隔符。
            ByteBuf delimiter = Unpooled.copiedBuffer("$_".getBytes());
            //创建DelimiterBasedFrameDecoder对象，将其加入到ChannelPipeline中。
            //DelimiterBasedFrameDecoder有多个构造方法，这里我们传递两个参数，
            //第一个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TooLongFrame Exception异常，防止由于异常码流缺失分隔符导致的内存溢出，
            //这是Netty解码器的可靠性保护；第二个参数就是分隔符缓冲对象。
            arg0.pipeline().addLast(new DelimiterBasedFrameDecoder(1024,delimiter));
            arg0.pipeline().addLast(new StringDecoder());
            arg0.pipeline().addLast(new EchoServerHandler());
        }
    }

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

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

@ChannelHandler.Sharable
public class EchoServerHandler extends ChannelHandlerAdapter {

    int counter = 0;

    @Override
    public void channelRead(ChannelHandlerContext ctx, Object msg)
            throws Exception {
        //直接将接收的消息打印出来，由于DelimiterBasedFrameDecoder自动对请求消息进行了解码
        //后续的ChannelHandler接收到的msg对象就是个完整的消息包；
        //第二个ChannelHandler是StringDecoder，它将ByteBuf解码成字符串对象
        //第三个EchoServerHandler接收到的msg消息就是解码后的字符串对象。
        String body = (String) msg;
        System.out.println("This is " + ++counter + " times receive client : ["+ body + "]");
        body += "$_";
        //由于我们设置DelimiterBasedFrameDecoder过滤掉了分隔符，
        //所以，返回给客户端时需要在请求消息尾部拼接分隔符“$_”，
        //最后创建ByteBuf，将原始消息重新返回给客户端。
        ByteBuf echo = Unpooled.copiedBuffer(body.getBytes());
        ctx.writeAndFlush(echo);
    }

    @Override
    public void exceptionCaught(ChannelHandlerContext ctx, Throwable cause) {
        cause.printStackTrace();
        ctx.close();// 发生异常，关闭链路
    }
}

客户端示例：

import io.netty.bootstrap.Bootstrap;
import io.netty.buffer.ByteBuf;
import io.netty.buffer.Unpooled;
import io.netty.channel.*;
import io.netty.channel.nio.NioEventLoopGroup;
import io.netty.channel.socket.nio.NioSocketChannel;
import io.netty.handler.codec.DelimiterBasedFrameDecoder;
import io.netty.handler.codec.string.StringDecoder;

public class EchoClient {

    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() {
                        @Override
                        public void initChannel(Channel ch)
                                throws Exception {
                            ByteBuf delimiter = Unpooled.copiedBuffer("$_".getBytes());
                            ch.pipeline().addLast(new DelimiterBasedFrameDecoder(1024,delimiter));
                            ch.pipeline().addLast(new StringDecoder());
                            ch.pipeline().addLast(new EchoClientHandler());
                        }
                    });

            // 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(args[0]);
            } catch (NumberFormatException e) {
                // 采用默认值
            }
        }
        new EchoClient().connect(port, "127.0.0.1");
    }
}

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

public class EchoClientHandler extends ChannelHandlerAdapter {

    private int counter;

    static final String ECHO_REQ = "Hi, Netty. Welcome to Netty.$_";

    public EchoClientHandler() {
    }

    @Override
    public void channelActive(ChannelHandlerContext ctx) {
        for (int i = 0; i < 10; i++) {
            ctx.writeAndFlush(Unpooled.copiedBuffer(ECHO_REQ.getBytes()));
        }
    }

    @Override
    public void channelRead(ChannelHandlerContext ctx, Object msg)
            throws Exception {
        System.out.println("This is " + ++counter + " times receive server : ["
                + msg + "]");
    }

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

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

运行结果：

服务端

This is 1 times receive client : [Hi, Netty. Welcome to Netty.]
...............................
This is 10 times receive client : [Hi, Netty. Welcome to Netty.]

客户端

This is 1 times receive client : [Hi, Netty. Welcome to Netty.]
...............................
This is 10 times receive client : [Hi, Netty. Welcome to Netty.]

FixedLengthFrameDecoder应用开发

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，开发者不需要考虑TCP的粘包/拆包问题，非常实用。

利用FixedLengthFrameDecoder解码器，无论一次接收到多少数据报，它都会按照构造函数中设置的固定长度进行解码，如果是半包消息，FixedLengthFrameDecoder会缓存半包消息并等待下个包到达后进行拼包，直到读取到一个完整的包。

服务端示例：

在服务端的ChannelPipeline中新增FixedLengthFrameDecoder，长度设置为20，然后再依次增加字符串解码器和EchoServerHandler

import io.netty.bootstrap.ServerBootstrap;
import io.netty.channel.*;
import io.netty.channel.nio.NioEventLoopGroup;
import io.netty.channel.socket.nio.NioServerSocketChannel;
import io.netty.handler.codec.FixedLengthFrameDecoder;
import io.netty.handler.codec.string.StringDecoder;
import io.netty.handler.logging.LogLevel;
import io.netty.handler.logging.LoggingHandler;

public class EchoServer {
    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, 100)
                    .handler(new LoggingHandler(LogLevel.INFO))
                    .childHandler(new ChannelInitializer() {
                        @Override
                        public void initChannel(Channel ch)throws Exception {
                            ch.pipeline().addLast( new FixedLengthFrameDecoder(20));
                            ch.pipeline().addLast(new StringDecoder());
                            ch.pipeline().addLast(new EchoServerHandler());
                        }
                    });

            // 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();
        }
    }

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

import io.netty.channel.ChannelHandler;
import io.netty.channel.ChannelHandlerAdapter;
import io.netty.channel.ChannelHandlerContext;

@ChannelHandler.Sharable
public class EchoServerHandler extends ChannelHandlerAdapter {

    @Override
    public void channelRead(ChannelHandlerContext ctx, Object msg) throws Exception {
        System.out.println("Receive client : [" + msg + "]");
    }

    @Override
    public void exceptionCaught(ChannelHandlerContext ctx, Throwable cause) {
        cause.printStackTrace();
        ctx.close();// 发生异常，关闭链路
    }
}

客户端示例:

1. telnet localhost 8080
2. 通过set localecho命令打开本地回显功能

➜  zcy-fixed git:(feature/transaction) ✗ telnet localhost 8080
Trying ::1...
Connected to localhost.
Escape character is '^]'.
12
123456789012345678

//连接日志
10:10:59.755 [nioEventLoopGroup-2-1] INFO  i.n.handler.logging.LoggingHandler - [id: 0x979fc7a5, /0:0:0:0:0:0:0:0:8080] RECEIVED: [id: 0x79dc7a78, /0:0:0:0:0:0:0:1:50749 => /0:0:0:0:0:0:0:1:8080]
Receive client : []
Receive client : [12
1234567890123456]