Netty3：分隔符和定长解码器

回顾TCP粘包/拆包问题解决方案

上文详细说了TCP粘包/拆包问题产生的原因及解决方式，并以LineBasedFrameDecoder为例演示了粘包/拆包问题的实际解决方案，本文再介绍两种粘包/拆包问题的解决方案：分隔符和定长解码器。在开始本文之前，先回顾一下解决粘包/拆包问题的几个方式：

• 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，将计数器重置，重新读取下一个消息
• 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
• 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
• 通过在消息头中定义长度字段来标志消息的总长度

Netty对上面4种方式做了统一的抽象，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。

基于DelimiterBasedFrameDecoder的TCP粘包/拆包解决方案

使用DelimiterBasedFrameDecoder，我们可以自动完成以分隔符作为码流结束标志的消息的解码，下面通过例子来学习下DelimiterBasedFrameDecoder的使用。

首先写一个EchoServer，和TimeServer差不多，主要是加上了DelimiterBasedFrameDecoder，分隔符定为"￥_"：

 public class EchoServer {

     public void bind(int port) throws Exception {
         // 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 arg0) throws Exception {
             ByteBuf delimiter = Unpooled.copiedBuffer("$_".getBytes());

             arg0.pipeline().addLast(new DelimiterBasedFrameDecoder(1024, delimiter));
             arg0.pipeline().addLast(new StringDecoder());
             arg0.pipeline().addLast(new EchoServerHandler());
         }
     }

 }

接着是EchoServerHandler，将接收到的消息打印出来，并记录收到的消息次数：

 public class EchoServerHandler extends ChannelHandlerAdapter {

     private int counter = 0;

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

         body += "$_";
         ByteBuf echo = Unpooled.copiedBuffer(body.getBytes());
         ctx.writeAndFlush(echo);
     }

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

 }

收到消息之后，将消息加上"$_"回给客户端。看下客户端EchoClient的写法，也是一样，加上DelimiterBasedFrameDecoder：

 public class EchoClient {

     public void connect(int port, String host) throws Exception {
         EventLoopGroup group = new NioEventLoopGroup();
         try {
             Bootstrap b = new Bootstrap();

             b.group(group)
                 .channel(NioSocketChannel.class)
                 .option(ChannelOption.TCP_NODELAY, true)
                 .handler(new ChannelInitializer<SocketChannel>() {
                     protected void initChannel(SocketChannel 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();
             // 等待客户端连接关闭
             f.channel().closeFuture().sync();
         } finally {
             // 优雅退出，释放NIO线程组
             group.shutdownGracefully();
         }
     }

 }

写一个EchoClientHandler，发送10条消息到Server并记录从Server回来的数据：

 public class EchoClientHandler extends ChannelHandlerAdapter {

     private int counter;

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

     public EchoClientHandler() {

     }

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

 }

先运行服务端代码再运行客户端代码，看下服务端收到的数据为：

This is 1 times receive client:[Hi, RickyXu, Welcome to Netty.]
This is 2 times receive client:[Hi, RickyXu, Welcome to Netty.]
This is 3 times receive client:[Hi, RickyXu, Welcome to Netty.]
This is 4 times receive client:[Hi, RickyXu, Welcome to Netty.]
This is 5 times receive client:[Hi, RickyXu, Welcome to Netty.]
This is 6 times receive client:[Hi, RickyXu, Welcome to Netty.]
This is 7 times receive client:[Hi, RickyXu, Welcome to Netty.]
This is 8 times receive client:[Hi, RickyXu, Welcome to Netty.]
This is 9 times receive client:[Hi, RickyXu, Welcome to Netty.]
This is 10 times receive client:[Hi, RickyXu, Welcome to Netty.]

一模一样收到10条且末尾的分隔符被忽略，客户端收到的响应为：

This is 1 times receive server:[Hi, RickyXu, Welcome to Netty.]
This is 2 times receive server:[Hi, RickyXu, Welcome to Netty.]
This is 3 times receive server:[Hi, RickyXu, Welcome to Netty.]
This is 4 times receive server:[Hi, RickyXu, Welcome to Netty.]
This is 5 times receive server:[Hi, RickyXu, Welcome to Netty.]
This is 6 times receive server:[Hi, RickyXu, Welcome to Netty.]
This is 7 times receive server:[Hi, RickyXu, Welcome to Netty.]
This is 8 times receive server:[Hi, RickyXu, Welcome to Netty.]
This is 9 times receive server:[Hi, RickyXu, Welcome to Netty.]
This is 10 times receive server:[Hi, RickyXu, Welcome to Netty.]

同样收到了10条数据且忽略了末尾的换行符。

这样我们就通过一个示例演示了使用DelimiterBasedFrameDecoder解决TCP粘包/拆包，下面看一下使用FixedLengthFrameDecoder解决TCP粘包/拆包的示例。

基于FixedLengthFrameDecoder的TCP粘包/拆包解决方案

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，

同样看一下例子，先写一个EchoServer，加入FixedLengthFrameDecoder：

public class EchoServer {

    public void bind(int port) throws Exception {
        // 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 arg0) throws Exception {
            arg0.pipeline().addLast(new FixedLengthFrameDecoder(20));
            arg0.pipeline().addLast(new StringDecoder());
            arg0.pipeline().addLast(new EchoServerHandler());
        }
    }

}

接着下一下EchoServerHandler，EchoServerHandler比较简单：

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) throws Exception {
        cause.printStackTrace();
        ctx.close();
    }

}

这里只打印接收到的数据，因为我们使用telnet来模拟发送请求而不是写一个EchoClient。首先telnet一下localhost 8080：

接着使用"Ctrl+]"：

输入回车，这样就变成了回显模式，即输入什么看到什么，简单说一下telnet原理：

1. 建立与服务器的TCP连接
2. 从键盘上接收输入的字符
3. 把输入的字符变成标准格式并发送给服务器
4. 从服务器接收输出的信息
5. 输出的信息显示在屏幕/控制台上

注意一下第2点+第3点，这里是键盘上输入一个字符就会发送这个字符到服务端的，对这点有疑问的可以在FixedLengthFrameDecoder的decode方法上打断点，就可以看到每输入一个字符，断点就会进入一次。

接着我们"0123456789"这样一直循环输入，FixedLengthFrameDecoder设置的Length是20，按照上面的解释，这样的话应该输入两轮"0123456789"即第二个9输入之后，控制台上就会打印了：

看一下控制台：

Receive client:[01234567890123456789]

没毛病，证明了FixedLengthFrameDecoder确实是按照定长接收包的，如果收到的包是半包消息，FixedLengthFrameDecoder会缓存半包消息并等待下个包到达之后进行拼包，直到读取到一个完整的包。