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() {
                        @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() {
                        @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。