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固定字符串的长度的方式主要是依赖于这个类，先看下jdk中的解释

A decoder that splits the received ByteBufs by the fixed number of bytes. For example, if you received the following four fragmented packets:

+---+----+------+----+ | A | BC | DEFG | HI | +---+----+------+----+

+-----+-----+-----+ | ABC | DEF | GHI | +-----+-----+-----+

翻译过来就是：

这个类是一个将接收的byteebuf缓冲对象按照固定的字节长度分割的解码器，例如，如果你收到的是以下片段，

+---+----+------+----+ | A | BC | DEFG | HI | +---+----+------+----+

一个按照固定3个字节拆分的解码器将会分割成：

+-----+-----+-----+ | ABC | DEF | GHI | +-----+-----+-----+

我们看下代码：

import io.netty.bootstrap.ServerBootstrap;import io.netty.channel.ChannelFuture;import io.netty.channel.ChannelInitializer;import io.netty.channel.ChannelOption;import io.netty.channel.EventLoopGroup;import io.netty.channel.nio.NioEventLoopGroup;import io.netty.channel.socket.SocketChannel;import io.netty.channel.socket.nio.NioServerSocketChannel;import io.netty.handler.codec.FixedLengthFrameDecoder;import io.netty.handler.codec.string.StringDecoder;public class Server {	public static void main(String[] args) throws Exception{		//1 创建2个线程，一个是负责接收客户端的连接。一个是负责进行数据传输的		EventLoopGroup pGroup = new NioEventLoopGroup();		EventLoopGroup cGroup = new NioEventLoopGroup();				//2 创建服务器辅助类		ServerBootstrap b = new ServerBootstrap();		b.group(pGroup, cGroup)		 .channel(NioServerSocketChannel.class)		 .option(ChannelOption.SO_BACKLOG, 1024)		 .option(ChannelOption.SO_SNDBUF, 32*1024)		 .option(ChannelOption.SO_RCVBUF, 32*1024)		 .childHandler(new ChannelInitializer
   
    () {			@Override			protected void initChannel(SocketChannel sc) throws Exception {				//设置定长字符串接收				sc.pipeline().addLast(new FixedLengthFrameDecoder(5));				//设置字符串形式的解码				sc.pipeline().addLast(new StringDecoder());				sc.pipeline().addLast(new ServerHandler());			}		});				//4 绑定连接		ChannelFuture cf = b.bind(8765).sync();				//等待服务器监听端口关闭		cf.channel().closeFuture().sync();		pGroup.shutdownGracefully();		cGroup.shutdownGracefully();			}	}
   

ServerHandler

import io.netty.buffer.Unpooled;import io.netty.channel.ChannelHandlerAdapter;import io.netty.channel.ChannelHandlerContext;public class ServerHandler extends ChannelHandlerAdapter {	@Override	public void channelActive(ChannelHandlerContext ctx) throws Exception {		System.out.println(" server channel active... ");	}	@Override	public void channelRead(ChannelHandlerContext ctx, Object msg) throws Exception {		String request = (String)msg;		System.out.println("Server :" + msg);		String response =  request ;		ctx.writeAndFlush(Unpooled.copiedBuffer(response.getBytes()));	}	@Override	public void channelReadComplete(ChannelHandlerContext ctx) throws Exception {			}	@Override	public void exceptionCaught(ChannelHandlerContext ctx, Throwable t) throws Exception {	}}

Client

import io.netty.bootstrap.Bootstrap;import io.netty.buffer.Unpooled;import io.netty.channel.ChannelFuture;import io.netty.channel.ChannelInitializer;import io.netty.channel.EventLoopGroup;import io.netty.channel.nio.NioEventLoopGroup;import io.netty.channel.socket.SocketChannel;import io.netty.channel.socket.nio.NioSocketChannel;import io.netty.handler.codec.FixedLengthFrameDecoder;import io.netty.handler.codec.string.StringDecoder;public class Client {	public static void main(String[] args) throws Exception {				EventLoopGroup group = new NioEventLoopGroup();				Bootstrap b = new Bootstrap();		b.group(group)		 .channel(NioSocketChannel.class)		 .handler(new ChannelInitializer
   
    () {			@Override			protected void initChannel(SocketChannel sc) throws Exception {				sc.pipeline().addLast(new FixedLengthFrameDecoder(5));				sc.pipeline().addLast(new StringDecoder());				sc.pipeline().addLast(new ClientHandler());			}		});				ChannelFuture cf = b.connect("127.0.0.1", 8765).sync();				cf.channel().writeAndFlush(Unpooled.wrappedBuffer("aaaaabbbbb".getBytes()));		cf.channel().writeAndFlush(Unpooled.copiedBuffer("ccccccc".getBytes()));				//等待客户端端口关闭		cf.channel().closeFuture().sync();		group.shutdownGracefully();			}}
   

ClientHandler

import io.netty.channel.ChannelHandlerAdapter;import io.netty.channel.ChannelHandlerContext;public class ClientHandler extends ChannelHandlerAdapter{	@Override	public void channelActive(ChannelHandlerContext ctx) throws Exception {		System.out.println("client channel active... ");	}	@Override	public void channelRead(ChannelHandlerContext ctx, Object msg) throws Exception {		String response = (String)msg;		System.out.println("Client: " + response);	}	@Override	public void channelReadComplete(ChannelHandlerContext ctx) throws Exception {	}	@Override	public void exceptionCaught(ChannelHandlerContext ctx, Throwable cause) throws Exception {	}}

打印：

client

client channel active... Client: aaaaaClient: bbbbbClient: ccccc

Server

server channel active... Server :aaaaaServer :bbbbbServer :ccccc

可以看到客户端发送aaaaabbbbb，被完美分割成aaaaa和bbbbb两部分，但ccccccc含有7个字符，服务器端只能收到ccccc

缺失了2个字符，这是因为使用定长字符串方式只会发送符合长度的字符串，多余的不会被发送，解决办法是可以补齐缺少的字s符，比如给多的cc补三个空格。这样server将会收到：

server channel active... Server :aaaaaServer :bbbbbServer :cccccServer :cc

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