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(); } }
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：
- 建立与服务器的TCP连接
- 从键盘上接收输入的字符
- 把输入的字符变成标准格式并发送给服务器
- 从服务器接收输出的信息
- 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。