最近在学习netty，看了几天的博客之后，打算自己写一个练手的程序。

这个程序很简单：客户端发送一个ping，服务端会相应地回复一个pong，当监测到服务端失去连接后，即断开。

整个代码分为client与server两部分，结构如下：

引入netty包：

<dependency>
	<groupId>io.netty</groupId>
	<artifactId>netty-all</artifactId>
	<version>4.1.15.Final</version>
</dependency>

client端：

PingClient.java

java">package org.attempt.netty4.demo001.client;

import io.netty.bootstrap.Bootstrap;
import io.netty.channel.Channel;
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.string.StringDecoder;
import io.netty.handler.codec.string.StringEncoder;

public class PingClient {

    /** 服务器IP地址 */
    private String host = "127.0.0.1";

    /** 服务器端口 */
    private int port = 8000;

    private EventLoopGroup group = null;
    private Bootstrap b = null;
    private Channel channel = null;

    public PingClient() throws Exception {
        group = new NioEventLoopGroup();
        b = new Bootstrap();
        b.group(group)
                .option(ChannelOption.SO_KEEPALIVE, true)
                .channel(NioSocketChannel.class)
                .handler(new ChannelInitializer<SocketChannel>() {
                    @Override
                    protected void initChannel(SocketChannel channel) throws Exception {
                        channel.pipeline()
                                //字符串解码和编码
                                .addLast(new StringDecoder())
                                .addLast(new StringEncoder())
                                //客户端的逻辑
                                .addLast(new PingClientHandler());
                    }
                });
    }

    public Channel getChannel() throws Exception {
        if(null == channel || !channel.isActive()) {
            channel = b.connect(host, port).sync().channel();
        }
        return channel;
    }

    public static void main(String[] args) throws Exception {
        PingClient client = null;
        try {
            client = new PingClient();
            Channel channel = client.getChannel();

            while(true) {
                //输出channel的状态，对应于close()
                System.out.println(channel.isOpen());
                //判断连接状态
                if(channel.isActive()) {
                    channel.writeAndFlush("ping");
                } else {
                    System.out.println("失去连接，关闭客户端");
                    channel.close();
                    break;
                }
                Thread.sleep(5000);
            }
        } finally {
            if(null != client) {
                client.stop();
            }
        }
    }

    public void stop() {
        if(null != group) {
            //优雅退出，释放线程池资源
            group.shutdownGracefully();
        }
    }

}

PingClientHandler.java

java">package org.attempt.netty4.demo001.client;

import io.netty.channel.ChannelHandlerContext;
import io.netty.channel.SimpleChannelInboundHandler;

public class PingClientHandler extends SimpleChannelInboundHandler {

    @Override
    protected void channelRead0(ChannelHandlerContext ctx, Object msg) throws Exception {
        System.out.println("receive from server: " + msg.toString());
    }

}

server端： PongServer.java

java">package org.attempt.netty4.demo001.server;

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.string.StringDecoder;
import io.netty.handler.codec.string.StringEncoder;

public class PongServer {

    public static void main(String[] args) throws Exception {
        int port = 8000;
        if (args != null && args.length > 0) {
            try {
                port = Integer.valueOf(args[0]);
            } catch (NumberFormatException e) {
                //采用默认值
            }
        }
        new PongServer().bind(port);
    }

    public void bind(int port) throws Exception {
        //配置服务端的NIO线程组
        EventLoopGroup bossGroup = new NioEventLoopGroup();
        EventLoopGroup workerGroup = new NioEventLoopGroup();
        try {
            ServerBootstrap b = new ServerBootstrap();
            b.group(bossGroup, workerGroup)
                    .channel(NioServerSocketChannel.class)
                    .option(ChannelOption.SO_BACKLOG, 1024)
                    .childHandler(new ChannelInitializer<SocketChannel>() {
                        @Override
                        protected void initChannel(SocketChannel channel) throws Exception {
                            channel.pipeline()
                            //字符串解码和编码
                            .addLast(new StringDecoder())
                            .addLast(new StringEncoder())
                            //服务器的逻辑
                            .addLast(new PongServerHandler());
                        }
                    });

            //绑定端口，同步等待成功
            ChannelFuture f = b.bind(port).sync();

            //等待服务器监听端口关闭
            f.channel().closeFuture().sync();
        } finally {
            //优雅退出，释放线程池资源
            bossGroup.shutdownGracefully();
            workerGroup.shutdownGracefully();
        }
    }

}

PongServerHandler.java

java">package org.attempt.netty4.demo001.server;

import io.netty.channel.ChannelHandlerContext;
import io.netty.channel.SimpleChannelInboundHandler;

/**
 * @author admin
 * @date 2018-09-06 22:48
 */
public class PongServerHandler extends SimpleChannelInboundHandler {

    @Override
    protected void channelRead0(ChannelHandlerContext ctx, Object msg) throws Exception {
        System.out.println("receive from client: " + msg.toString());
        //返回客户端消息
        if(msg.toString().equals("ping")) {
            ctx.writeAndFlush("pong");
        } else {
            ctx.writeAndFlush("UNKONWN");
        }
    }

}

运行，先启动server端，再启动client端，结果如下：

• server端：

receive from client: ping
receive from client: ping
receive from client: ping
receive from client: ping
receive from client: ping

Process finished with exit code -1

• client端：

true
receive from server: pong
true
receive from server: pong
true
receive from server: pong
true
receive from server: pong
true
receive from server: pong
false
失去连接，关闭客户端

代码中的一些类及用法，后面再说。