Netty服务端和客户端搭建（入门）

本文根据Netty权威指南（第二版）中的案例编写，有些案例中的类在我使用的jar中找不到，所以有所修改。本文使用的版本是netty-all-4.0.46.Final.jar，可以自行去官网下载。

一、服务端
①TimeServer

package netty.server;

import io.netty.bootstrap.ServerBootstrap;
import io.netty.channel.*;
import io.netty.channel.nio.NioEventLoopGroup;
import io.netty.channel.socket.nio.NioServerSocketChannel;

public class TimeServer {


    public static void main(String[] args) {
        int port = 8080;
        new TimeServer().bind(port);
    }

    public void bind(int port) {

        //配置服务端的NIO线程组
        EventLoopGroup bossGroup = new NioEventLoopGroup();
        EventLoopGroup workerGroup = new NioEventLoopGroup();

        ServerBootstrap b = new ServerBootstrap();
        b.group(bossGroup, workerGroup)
                .channel(NioServerSocketChannel.class)
                .option(ChannelOption.SO_BACKLOG, 1024)
                .childHandler(new ChildChannelHandler());

        try {
            //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();
        } catch (InterruptedException e) {
            e.printStackTrace();
        } finally {
            //优雅退出，释放线程池资源
            bossGroup.shutdownGracefully();
            workerGroup.shutdownGracefully();
        }
    }
}

②ChildChannelHandler

package netty.server;

import io.netty.channel.ChannelInitializer;
import io.netty.channel.socket.SocketChannel;

public class ChildChannelHandler extends ChannelInitializer<SocketChannel> {
    @Override
    protected void initChannel(SocketChannel socketChannel) throws Exception {
        socketChannel.pipeline().addLast(new TimeServerHandler());
    }
}

③TimeServerHandler

package netty.server;

import io.netty.buffer.ByteBuf;
import io.netty.buffer.Unpooled;
import io.netty.channel.ChannelHandlerContext;
import io.netty.channel.ChannelInboundHandlerAdapter;
import java.util.Date;

public class TimeServerHandler extends ChannelInboundHandlerAdapter {

    @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("The time server receive order : " + body);
        String currentTime = "Query time order".equalsIgnoreCase(body) ? new Date(System.currentTimeMillis()).toString() : "Bad Order";

        ByteBuf resp = Unpooled.copiedBuffer(currentTime.getBytes());
        ctx.write(resp);
    }

    @Override
    public void channelReadComplete(ChannelHandlerContext ctx) throws Exception {
        ctx.flush();
    }

    @Override
    public void exceptionCaught(ChannelHandlerContext ctx, Throwable cause) throws Exception {
        ctx.close();
    }
}

二、客户端
①TimeClient

package netty.client;

import io.netty.bootstrap.Bootstrap;
import io.netty.channel.ChannelFuture;
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 static void main(String[] args) {
        int port = 8080;
        new TimeClient().connect(port, "127.0.0.1");
    }

    public void connect(int port, String host) {
        //配置客户端的NIO线程组
        EventLoopGroup group = new NioEventLoopGroup();
        Bootstrap b = new Bootstrap();
        b.group(group)
                .channel(NioSocketChannel.class)
                .option(ChannelOption.TCP_NODELAY, true)
                .handler(new ChildChannelHandler());


        try {
            //发生异步连接操作
            ChannelFuture f = b.connect(host, port).sync();

            //等待客户端链路关闭
            f.channel().closeFuture().sync();
        } catch (InterruptedException e) {
            e.printStackTrace();
        } finally {
            //优雅退出，释放NIO线程组
            group.shutdownGracefully();
        }
    }
}

②ChildChannelHandler

package netty.client;

import io.netty.channel.ChannelInitializer;
import io.netty.channel.socket.SocketChannel;

public class ChildChannelHandler extends ChannelInitializer<SocketChannel> {
    @Override
    protected void initChannel(SocketChannel ch) throws Exception {
        ch.pipeline().addLast(new TimeClientHandler());
    }
}

③TimeClientHandler

package netty.client;

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

import java.util.logging.Logger;

public class TimeClientHandler extends ChannelInboundHandlerAdapter {


    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);
    }

    @Override
    public void channelActive(ChannelHandlerContext ctx) throws Exception {
        ctx.writeAndFlush(firstMessage);
    }

    @Override
    public void exceptionCaught(ChannelHandlerContext ctx, Throwable cause) throws Exception {
        logger.warning("Unexpected exception from downstream:" + cause.getMessage());
        ctx.close();
    }

    @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);
    }
}

服务端：
NioEventLoopGroup是个线程组，它包含了一组NIO线程，专门用于网络事件处理，实际上他们就是Reactor线程组。 bossGroup用于服务端接收客户端的连接，workerGroup用于进行SocketChannel的网络读写

ServerBootstrap对象用于启动NIO服务端的辅助启动类（目的是降低服务端的开发复杂度）

group()方法将两个NIO线程组当做参数传递到ServerBootstrap中，接着设置创建的Channel为NioServerSocketChannel（对应JDK的NIO类库中的ServerSocketChannel），然后配置NioServerSocketChannel的TCP参数，最后绑定I/O事件的处理类ChildChannelHandler（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）

服务端启动辅助类配置完成之后，调用它的bind方法绑定监听端口，随后，调用它的同步阻塞方法sync等待绑定事件操作完成。 完成后Netty会返回一个ChannelFuture，它的作用类似于JDK中java.util.concurrent.Future，主要用于异步操作的通知回调

f.channel().closeFuture().sync()方法进行阻塞，等待服务端链路关闭之后main函数才退出。
NIO线程组的shutdownGracefully进行优雅退出，释放跟shutdownGracefully相关的资源

ChannelHandlerContext的write()只是把待发送的消息放到缓冲数组中，在通过调用flush()方法，将缓冲区中的消息全部写到SocketChannel中

客户端：首先创建客户端处理I/O读写的NioEventLoopGroup线程组，
然后创建客户端辅助启动类Bootstrap，随后对其进行配置