八、分隔符和定长解码器的应用(2)

下面主要是使用FixedLengthFrameDecoder应用开发
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，我们不需要去考虑TCP的粘包和拆包问题。同样上代码实例：
服务端代码：

package FixedLengthFrameDecoder;import NettyFrameDecoder.EchoServerHandler;import io.netty.bootstrap.ServerBootstrap;import io.netty.buffer.ByteBuf;import io.netty.buffer.Unpooled;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.DelimiterBasedFrameDecoder;import io.netty.handler.codec.FixedLengthFrameDecoder;import io.netty.handler.codec.string.StringDecoder;import io.netty.handler.logging.LoggingHandler;import sun.rmi.log.LogHandler;/** * Created by L_kanglin on 2017/6/12. */public class EchoServerFixed {    public void bind(int port) throws Exception {        //配置服务端的线程组        EventLoopGroup bossGroup=new NioEventLoopGroup();        EventLoopGroup workGroup=new NioEventLoopGroup();        try {            ServerBootstrap b = new ServerBootstrap();            b.group(bossGroup,workGroup)                    .channel(NioServerSocketChannel.class)                    .option(ChannelOption.SO_BACKLOG,100)                    .handler(new LoggingHandler(LogHandler.class))                    .childHandler(new ChannelInitializer<SocketChannel>() {                        @Override                        protected void initChannel(SocketChannel ch) throws Exception {                            //创建FixedLengthFrameDecoder对象，长度设置为20                            ch.pipeline().addLast(new FixedLengthFrameDecoder(20));                            //将接收到的消息转化为字符串，将ByteBuf解码成字符串对象                            ch.pipeline().addLast((new StringDecoder()));                            //接收到的msg消息就是解码后的字符串对象                            ch.pipeline().addLast(new EchoServerHandlerFixed());                        }                    });            //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{            bossGroup.shutdownGracefully();            workGroup.shutdownGracefully();        }    }    public static void main(String[] args) throws Exception {        int port=9030;        if(args !=null && args.length>0){            port=Integer.valueOf(args[0]);        }        new EchoServerFixed().bind(port);    }}

package FixedLengthFrameDecoder;import io.netty.buffer.ByteBuf;import io.netty.buffer.Unpooled;import io.netty.channel.ChannelHandlerAdapter;import io.netty.channel.ChannelHandlerContext;/** * Created by L_kanglin on 2017/6/12. */public class EchoServerHandlerFixed extends ChannelHandlerAdapter {    @Override    public void channelRead(ChannelHandlerContext ctx, Object msg) throws Exception {        System.out.println("Receive client : [" +msg+ "]");        String body = (String) msg;        ByteBuf echo= Unpooled.copiedBuffer(body.getBytes());        ctx.writeAndFlush(echo);    }    @Override    public void exceptionCaught(ChannelHandlerContext ctx, Throwable cause) throws Exception {        cause.printStackTrace();        ctx.close();    }}

客户端代码如下：

package FixedLengthFrameDecoderClient;import NettyFrameDecoderClient.EchoClientHandler;import io.netty.bootstrap.Bootstrap;import io.netty.buffer.ByteBuf;import io.netty.buffer.Unpooled;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.NioSocketChannel;import io.netty.handler.codec.DelimiterBasedFrameDecoder;import io.netty.handler.codec.FixedLengthFrameDecoder;import io.netty.handler.codec.string.StringDecoder;/** * Created by L_kanglin on 2017/6/12. */public class EchoClientFixed {    public void connnect(int port,String host) throws InterruptedException {        EventLoopGroup group = new NioEventLoopGroup();        try {            Bootstrap b =new Bootstrap();            b.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 FixedLengthFrameDecoder(20));                            ch.pipeline().addLast(new StringDecoder());                            ch.pipeline().addLast(new EchoClientHandlerFixed());                        }                    });            //发起异步连接操作            ChannelFuture f =b.connect(host,port).sync();            //等待客户端链路关闭            f.channel().closeFuture().sync();        } finally{            //优雅退出，释放NIO线程组            group.shutdownGracefully();        }    }    public static void main(String[] args) throws InterruptedException {        int port = 9030;        if(args !=null && args.length>0){            port=Integer.valueOf(args[0]);        }        new EchoClientFixed().connnect(port,"127.0.0.1");    }}

package FixedLengthFrameDecoderClient;import io.netty.buffer.Unpooled;import io.netty.channel.ChannelHandlerAdapter;import io.netty.channel.ChannelHandlerContext;/** * Created by L_kanglin on 2017/6/12. */public class EchoClientHandlerFixed extends ChannelHandlerAdapter {    private int counter;    static final String ECHO_REQ="LiKanglin Welcome to Netty at WuHan";    public EchoClientHandlerFixed() {    }    //在TCP链路建立成功之后循环发送请求消息给服务端    @Override    public void channelActive(ChannelHandlerContext ctx) throws Exception {        //发送指定内容            ctx.writeAndFlush(Unpooled.copiedBuffer(ECHO_REQ.getBytes()));    }    //打印接收到的服务端应答消息同时进行计数    @Override    public void channelRead(ChannelHandlerContext ctx, Object msg) throws Exception {        System.out.println(" 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();    }}

服务端运行如下：

Receive client : [LiKanglin Welcome to]

客户端运行如下：

 receive server:{ LiKanglin Welcome to}

通过FixedLengthFrameDecoder解码器，每次会按照设置的固定长度进行解码，如果是半包消息，FixedLengthFrameDecoder会缓存半包消息并等待下个包到达后进行拼包，直到读取到一个完整的包。

总结：
通过熟练运用这两种解码器，即可完成很多消息的自动解码，并且不用考虑TCP的粘包和拆包导致的读半包问题，极大的提升了开发效率。
补充：重要的是记住什么情况下会发生TCP的粘包和拆包问题，主要是应用层、运输层、网络层，详情请看前面的部分。