Netty3：分隔符和定长解码器

2018-06-27 09:58:24来源：未知阅读 ()

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"￥_"：

 1 public class EchoServer {
 2 
 3     public void bind(int port) throws Exception {
 4         // NIO线程组
 5         EventLoopGroup bossGroup = new NioEventLoopGroup();
 6         EventLoopGroup workerGroup = new NioEventLoopGroup();
 7         
 8         try {
 9             ServerBootstrap b = new ServerBootstrap();
10             b.group(bossGroup, workerGroup)
11                 .channel(NioServerSocketChannel.class)
12                 .option(ChannelOption.SO_BACKLOG, 1024)
13                 .childHandler(new ChildChannelHandler());
14             
15             // 绑定端口，同步等待成功
16             ChannelFuture f = b.bind(port).sync();
17             // 等待服务端监听端口关闭
18             f.channel().closeFuture().sync();
19         } finally {
20             // 优雅退出，释放线程池资源
21             bossGroup.shutdownGracefully();
22             workerGroup.shutdownGracefully();
23         }
24     }
25     
26     private class ChildChannelHandler extends ChannelInitializer<SocketChannel> {
27         @Override
28         protected void initChannel(SocketChannel arg0) throws Exception {
29             ByteBuf delimiter = Unpooled.copiedBuffer("$_".getBytes());
30             
31             arg0.pipeline().addLast(new DelimiterBasedFrameDecoder(1024, delimiter));
32             arg0.pipeline().addLast(new StringDecoder());
33             arg0.pipeline().addLast(new EchoServerHandler());
34         }
35     }
36     
37 }

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：

 1 public class EchoServerHandler extends ChannelHandlerAdapter {
 2 
 3     private int counter = 0;
 4     
 5     @Override
 6     public void channelRead(ChannelHandlerContext ctx, Object msg) throws Exception {
 7         String body = (String)msg;
 8         System.out.println("This is " + ++counter + " times receive client:[" + body + "]");
 9         
10         body += "$_";
11         ByteBuf echo = Unpooled.copiedBuffer(body.getBytes());
12         ctx.writeAndFlush(echo);
13     }
14     
15     @Override
16     public void exceptionCaught(ChannelHandlerContext ctx, Throwable cause) throws Exception {
17         cause.printStackTrace();
18         ctx.close();
19     }
20     
21 }

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：

 1 public class EchoClient {
 2 
 3     public void connect(int port, String host) throws Exception {
 4         EventLoopGroup group = new NioEventLoopGroup();
 5         try {
 6             Bootstrap b = new Bootstrap();
 7             
 8             b.group(group)
 9                 .channel(NioSocketChannel.class)
10                 .option(ChannelOption.TCP_NODELAY, true)
11                 .handler(new ChannelInitializer<SocketChannel>() {
12                     protected void initChannel(SocketChannel ch) throws Exception {
13                         ByteBuf delimiter = Unpooled.copiedBuffer("$_".getBytes());
14                         
15                         ch.pipeline().addLast(new DelimiterBasedFrameDecoder(1024, delimiter));
16                         ch.pipeline().addLast(new StringDecoder());
17                         ch.pipeline().addLast(new EchoClientHandler());
18                     };
19                 });
20             
21             // 发起异步连接操作
22             ChannelFuture f = b.connect(host, port).sync();
23             // 等待客户端连接关闭
24             f.channel().closeFuture().sync();
25         } finally {
26             // 优雅退出，释放NIO线程组
27             group.shutdownGracefully();
28         }
29     }
30     
31 }

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

 1 public class EchoClientHandler extends ChannelHandlerAdapter {
 2 
 3     private int counter;
 4     
 5     private static final String ECHO_REQ = "Hi, RickyXu, Welcome to Netty.$_";
 6     
 7     public EchoClientHandler() {
 8         
 9     }
10     
11     @Override
12     public void channelActive(ChannelHandlerContext ctx) throws Exception {
13         for (int i = 0; i < 10; i++) {
14             ctx.writeAndFlush(Unpooled.copiedBuffer(ECHO_REQ.getBytes()));
15         }
16     }
17     
18     @Override
19     public void channelRead(ChannelHandlerContext ctx, Object msg) throws Exception {
20         System.out.println("This is" + ++counter + " times receive server:[" + msg + "]");
21     }
22     
23     @Override
24     public void channelReadComplete(ChannelHandlerContext ctx) throws Exception {
25         ctx.flush();
26     }
27     
28     @Override
29     public void exceptionCaught(ChannelHandlerContext ctx, Throwable cause) throws Exception {
30         cause.printStackTrace();
31         ctx.close();
32     }
33     
34 }

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();
    }
    
}