Netty入门例子

新建maven项目，添加依赖

<!-- https://mvnrepository.com/artifact/io.netty/netty-all -->
    <dependency>
        <groupId>io.netty</groupId>
        <artifactId>netty-all</artifactId>
        <version>5.0.0.Alpha1</version>
    </dependency>

TimeServer

package com.zhen.netty1129;

import java.awt.Event;
import java.net.Socket;

import io.netty.bootstrap.ServerBootstrap;
import io.netty.channel.ChannelFuture;
import io.netty.channel.ChannelInitializer;
import io.netty.channel.ChannelOption;
import io.netty.channel.EventLoop;
import io.netty.channel.EventLoopGroup;
import io.netty.channel.nio.NioEventLoopGroup;
import io.netty.channel.socket.SocketChannel;
import io.netty.channel.socket.nio.NioServerSocketChannel;

public class TimeServer {

    public void bind(int port) throws Exception{
        //配置服务端的NIO线程组
        //NioEventLoopGroup是个线程组，它包含了一组NIO线程，专门用于网络事件的处理，实际上它们就是Reactor线程组
        //bossGroup用于服务端接受客户端的连接
        EventLoopGroup bossGroup = new NioEventLoopGroup();
        //workerGroup进行SocketChannel的网络读写
        EventLoopGroup workerGroup = new NioEventLoopGroup();
        try {
            //Netty用于启动NIO服务端的辅助启动类，目的是降低服务端的开发复杂度
            ServerBootstrap bootstrap = new ServerBootstrap();
            //将两个NIO线程组当作入参传递到ServerBootstrap
            bootstrap.group(bossGroup, workerGroup)
                //设置创建的Channel为NioServerSocketChannel,它的功能对应于JDK NIO类库中的ServerSocketChannel类。
                .channel(NioServerSocketChannel.class)
                //配置NioServerSocketChannel的TCP参数，此处将它的backlog设置为1024
                .option(ChannelOption.SO_BACKLOG, 1024)
                //绑定I/O事件的处理类ChildChannelHandler，它的作用类似于Reactor模式中的Handler类，主要用于处理网络I/O事件，例如记录日志、对消息进行编解码等
                .childHandler(new ChildChannelHandler());
            //调用bind方法绑定监听端口，随后，调用它的同步阻塞方法sync等待绑定操作完成。
            //完成之后Netty会返回一个ChannelFuture，它的功能类似于JDK的java.util.concurrent.Future，主要用于异步操作的通知回调
            ChannelFuture future = bootstrap.bind(port).sync();
            //等待服务端监听端口关闭，等待服务端链路关闭之后main函数才退出
            future.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 ch) throws Exception {
            ch.pipeline().addLast(new TimeServerHandler());
        }
    }
    
    public static void main(String[] args) throws Exception {
        int port = 9090;
        if (args != null && args.length > 0) {
            try {
                port = Integer.valueOf(args[0]);
            } catch (Exception e) {
                e.printStackTrace();
            }
        }
        new TimeServer().bind(port);
    }
}

TimeServerHandler

package com.zhen.netty1129;

import java.util.Date;

import io.netty.buffer.ByteBuf;
import io.netty.buffer.Unpooled;
import io.netty.channel.ChannelHandlerAdapter;
import io.netty.channel.ChannelHandlerContext;

//TimeServerHandler 继承自ChannelHandlerAdapter，它用于对网络事件进行读写操作
public class TimeServerHandler extends ChannelHandlerAdapter{
    
    @Override
    public void channelRead(ChannelHandlerContext ctx, Object msg) throws Exception {
        //将msg转换成Netty的ByteBuf对象。ByteBuf类似于jdk中的java.nio.ByteBuffer对象，不过它提供了更加强大和灵活的功能
        ByteBuf buf = (ByteBuf) msg;
        //通过ByteBuf的readableBytes方法可以获取缓冲区可读的字节数，根据可读的字节数创建byte数组
        byte[] req = new byte[buf.readableBytes()];
        //通过ByteBuf的readBytes方法将缓冲区中的字节数据复制到新建的byte数组中
        buf.readBytes(req);
        //通过new String构造函数获取请求消息
        String body = new String(req, "UTF-8");
        System.out.println("The time server receive order : " + body);
        //对请求消息进行判断，如果是QUERY TIME ORDER则创建应答消息
        String currentTime = "QUERY TIME ORDER".equalsIgnoreCase(body) ? 
                new Date(System.currentTimeMillis()).toString() : "BAD ORDER";
        ByteBuf resp = Unpooled.copiedBuffer(currentTime.getBytes());
        //通过ChannelHandlerContext的write方法异步发送应答消息给客户端
        ctx.write(resp);
    }
    
    @Override
    public void channelReadComplete(ChannelHandlerContext ctx) throws Exception {
        //将消息发送队列中的消息写入到SocketChannel中发送给对方
        //从性能角度考虑，为了防止频繁的唤醒Selector进行消息发送，Netty的write方法并不直接将消息写入SocketChannel中
        //调用write方法只是把待发送的消息放到发送缓冲数组中，再通过调用flush方法，将发送缓冲区中的消息全部写到SocketChannel中。
        ctx.flush();
    }
    
    @Override
    public void exceptionCaught(ChannelHandlerContext ctx, Throwable cause) throws Exception {
        //当发生异常时，关闭ChannelHandlerContext，释放和ChannelHandlerContext相关联的句柄等资源
        ctx.close();
    }
}

TimeClient

package com.zhen.netty1129;

import io.netty.bootstrap.Bootstrap;
import io.netty.channel.Channel;
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.nio.NioSocketChannel;

public class TimeClient {

    public void connect(int port,String host) throws Exception{
        //配置客户端NIO线程组,客户端处理I/O读写的NioEventLoopGroup线程组
        EventLoopGroup group = new NioEventLoopGroup();
        try {
            //客户端辅助启动类Bootstrap
            Bootstrap bootstrap = new Bootstrap();
            //设置线程组
            bootstrap.group(group)
                //与服务端不同的是，它的channel需要设置为NioSocketChannel
                .channel(NioSocketChannel.class)
                .option(ChannelOption.TCP_NODELAY, true)
                //然后为其添加Handler,此处为了简单直接创建匿名内部类，实现initChannel方法
                //作用是当创建NioSocketChannel成功之后，在进行初始化时，将它的ChannelHandler设置到ChannelPipeline中，用于处理网络I/O事件
                .handler(new ChannelInitializer<Channel>() {
                    @Override
                    protected void initChannel(Channel ch) throws Exception {
                        ch.pipeline().addLast(new TimeClientHandler());
                    }
                });
            //调用connect发起异步连接操作，然后调用sync同步方法等待连接成功。
            ChannelFuture future = bootstrap.connect(host, port).sync();
            //等待客户端链路关闭，当客户端连接关闭之后，客户端主函数退出，退出之前释放NIO线程组的资源
            future.channel().closeFuture().sync();
        } finally {
            //优雅退出，释放NIO线程组
            group.shutdownGracefully();
        }
    }
    
    public static void main(String[] args) throws Exception{
        int port = 9090;
        String host = "127.0.0.1";
        if (args != null && args.length > 0) {
            try {
                port = Integer.valueOf(args[0]);
            } catch (Exception e) {
                e.printStackTrace();
            }
        }
        new TimeClient().connect(port, host);
    }
    
}

TimeClientHandler

package com.zhen.netty1129;

import java.util.logging.Logger;

import io.netty.buffer.ByteBuf;
import io.netty.buffer.Unpooled;
import io.netty.channel.ChannelHandlerAdapter;
import io.netty.channel.ChannelHandlerContext;

public class TimeClientHandler extends ChannelHandlerAdapter{
    
    private static final Logger logger = Logger.getLogger(TimeClientHandler.class.getName());
    
    private final ByteBuf firstMessage;
    
    public TimeClientHandler(){
        byte[] req = "QUERY TIME ORDER".getBytes();
        firstMessage = Unpooled.buffer(req.length);
        firstMessage.writeBytes(req);
    }
    
    //当客户端和服务端TCP链路建立成功之后，Netty的NIO线程会调用channelActive方法，发送查询时间的指令给服务端
    //调用ChannelHandlerContext的writeAndFlush方法将请求消息发送给客户端
    @Override
    public void channelActive(ChannelHandlerContext ctx) throws Exception {
        ctx.writeAndFlush(firstMessage);
    }
    
    //当客户端返回应答消息，channelRead方法被调用
    @Override
    public void channelRead(ChannelHandlerContext ctx, Object msg) throws Exception {
        ByteBuf buf = (ByteBuf) msg;
        byte[] req = new byte[buf.readableBytes()];
        buf.readBytes(req);
        String body = new String(req, "UTF-8");
        System.out.println("Now is :" + body);
    }
    
    //发生异常时，释放客户端资源
    @Override
    public void exceptionCaught(ChannelHandlerContext ctx, Throwable cause) throws Exception {
        logger.warning("Unexpected exception from downstream : " + cause.getMessage());
        ctx.close();
    }
    
}

先运行TimeServer,再运行TimeClient