1. Netty解决Tcp粘包拆包

一. TCP粘包问题

1. 实际发送的消息, 可能会被TCP拆分成很多数据包发送, 也可能把很多消息组合成一个数据包发送

2. 粘包拆包发生的原因

   (1) 应用程序一次写的字节大小超过socket发送缓冲区大小
   (2) 数据长度超多MSS大小进行分片

 > MSS :  Maximum Segment Size 最大报文段长度, 是TCP数据包数据段的最大长度  
                   MSS值等于收发双方提供的MSS值的最小值, 等于TCP报文长度-TCP首部长度  

  (3) 以太网帧的payload大于MTU进行IP分片
		MTU : 硬件线路上可以传输的最大字节数

二. Netty的解决方法

1. LineBasedFrameDecoder:
   依次遍历ByteBuf中的可读字节, 发现有"\n"或者"\r\n" , 就把可读位置到该位置的字节看做一条消息.他是用换行符作为分隔符的解码器. 支持配置单行消息最大长度, 若达到最大长度还没出现换行符, 会抛出异常, 并忽略之前的异常数据

2. StringDecoder: 把接收到的数据流按照编码格式转换成字符串

   public class TimeServer4 {
   	public static void main(String[] args) {
   		new TimeServer4().bind(8112);
   	}
   
   	public void bind(int port){
   		// 配置服务端线程组
   		EventLoopGroup bossGroup = new NioEventLoopGroup();     // 接受线程组
   		EventLoopGroup workerGroup = new NioEventLoopGroup();   // 处理请求的线程组
   		try {
   			ServerBootstrap sbs = new ServerBootstrap();
   			sbs.group(bossGroup,workerGroup)
   					.channel(NioServerSocketChannel.class)
   					.option(ChannelOption.SO_BACKLOG,1024)  // 链接的缓冲队列
   					.childHandler(new ChannelInitializer<SocketChannel>() {
   						@Override
   						protected void initChannel(SocketChannel ch) throws Exception {
   							// 若在1024个字节内没有发现\n或\r\n报错,然后忽略之前的数据流
   							ch.pipeline().addLast(new LineBasedFrameDecoder(1024));   
   							ch.pipeline().addLast(new StringDecoder(Charset.forName("utf-8")));
   							ch.pipeline().addLast(new TimeServerHandler());
   						}
   					});
   
   			//绑定端口,同步等待成功
   			ChannelFuture f = sbs.bind(port).sync();
   			f.channel().closeFuture().sync();
   
   		} catch (InterruptedException e) {
   			e.printStackTrace();
   		} finally {
   			bossGroup.shutdownGracefully();
   			workerGroup.shutdownGracefully();
   		}
   	}
   
   	private class TimeServerHandler extends ChannelHandlerAdapter{
   		private int counter;
   
   		@Override
   		public void channelActive(ChannelHandlerContext ctx) throws Exception {
   			System.out.println("链接开启");
   		}
   
   		@Override
   		public void channelRead(ChannelHandlerContext ctx, Object msg) throws 
   	   /*   不加StringDecoder,需要手动转化ByteBuf写到字节数组
   			ByteBuf buf = (ByteBuf) msg;
   			byte[] bytes = new byte[buf.readableBytes()]; 
   			buf.readBytes(bytes);
   			String body = new String(bytes); */
   			String body = (String)msg;    
   			System.out.println("recieve order:"+body+"; counter is:"+ ++counter);
   			if("query time".equalsIgnoreCase(body)){
   				//客户端也注册了LineNasedFrameDecoder,所以服务端发送的消息也要以"\n"或"\r\n"结尾 
   				String nowTime = new SimpleDateFormat("yyyy-MM-dd").format(new Date()) + System.getProperty("line.separator");
   				ByteBuf byteBuf = Unpooled.copiedBuffer(nowTime.getBytes());
   				ctx.writeAndFlush(byteBuf);
   			}else{
   				ByteBuf byteBuf = Unpooled.copiedBuffer("bad order".getBytes());
   				ctx.writeAndFlush(byteBuf);
   			}
   		}
   
   		@Override
   		public void exceptionCaught(ChannelHandlerContext ctx, Throwable cause) throws Exception {
   			cause.printStackTrace();
   			ctx.close();
   		}
   	}
   }
   

   public class TimeClient {
   
   	public static void main(String[] args) {
   		new TimeClient().connect(8112,"localhost");
   	}
   
   	public void connect(int port,String host){
   		EventLoopGroup group = new NioEventLoopGroup();
   		try{
   			Bootstrap bs = new Bootstrap();
   			bs.group(group).channel(NioSocketChannel.class)
   					.option(ChannelOption.TCP_NODELAY,true)
   					.handler(new ChannelInitializer<SocketChannel>() {
   						@Override
   						protected void initChannel(SocketChannel ch) throws Exception {
   							ch.pipeline().addLast(new LineBasedFrameDecoder(1024));
   							ch.pipeline().addLast(new StringDecoder());
   							ch.pipeline().addLast(new TimeClientHandler());
   						}
   					});
   			//发起异步操作链接
   			ChannelFuture f = bs.connect(host,port).sync();
   			f.channel().closeFuture().sync();
   		} catch (InterruptedException e) {
   			e.printStackTrace();
   		} finally {
   			group.shutdownGracefully();
   		}
   	}
   
   	private class TimeClientHandler extends ChannelHandlerAdapter{
   		private int counter;
   		// 发出的请求报文必须带有"\n"或"\r\n",否则服务端的LineBasedFrameDecoder无法解析
   		private byte[] req = ("query time"+ System.getProperty("line.separator")).getBytes();
   
   		@Override
   		public void channelRead(ChannelHandlerContext ctx, Object msg) throws Exception {
   			String body = (String) msg;
   			System.out.println("Now : "+ body + "the counter is "+ ++counter);
   		}
   
   		@Override
   		public void channelActive(ChannelHandlerContext ctx) throws Exception {
   			ByteBuf msg = null;
   			for (int i = 0; i < 100; i++) {
   				msg = Unpooled.buffer(req.length);  // 创建指定长度的buf
   				msg.writeBytes(req);
   				ctx.writeAndFlush(msg);
   			}
   		}
   
   		@Override
   		public void exceptionCaught(ChannelHandlerContext ctx, Throwable cause) throws Exception {
   			cause.printStackTrace();
   			ctx.close();
   		}
   	}
   }