本文是学习Netty的第一篇文章，主要对Netty的Server和Client间的通讯机制进行验证。
Server与Client建立连接后，会执行以下的步骤：
1、Client向Server发送消息：Are you ok?
2、Server接收客户端发送的消息，并打印出来。
3、Server端向客户端发送消息：I am ok!
4、Client接收Server端发送的消息，并打印出来，通讯结束。
涉及到的类有4个：
1、HelloServer ：server类，启动Netty server
2、HelloServerInHandler：server的handler，，接收客户端消息，并向客户端发送消息
3、HelloClient：client类，建立于Netty server的连接
4、HelloClientIntHandler：client的handler，接收server端的消息，并向服务端发送消息
1、HelloServer代码如下：
package com.guowl.testserver;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;public class HelloServer {public void start(int port) throws Exception {EventLoopGroup bossGroup = new NioEventLoopGroup();EventLoopGroup workerGroup = new NioEventLoopGroup();try {ServerBootstrap b = new ServerBootstrap();b.group(bossGroup, workerGroup).channel(NioServerSocketChannel.class).childHandler(new ChannelInitializer<SocketChannel>() {@Overridepublic void initChannel(SocketChannel ch)throws Exception {// 注册handlerch.pipeline().addLast(new HelloServerInHandler());}}).option(ChannelOption.SO_BACKLOG, 128).childOption(ChannelOption.SO_KEEPALIVE, true);ChannelFuture f = b.bind(port).sync();f.channel().closeFuture().sync();} finally {workerGroup.shutdownGracefully();bossGroup.shutdownGracefully();}}public static void main(String[] args) throws Exception {HelloServer server = new HelloServer();server.start(8000);}}2、HelloServerInHandler代码如下：package com.guowl.testserver;import io.netty.buffer.ByteBuf;import io.netty.channel.ChannelHandlerContext;import io.netty.channel.ChannelInboundHandlerAdapter;import org.slf4j.Logger;import org.slf4j.LoggerFactory;// 该handler是InboundHandler类型public class HelloServerInHandler extends ChannelInboundHandlerAdapter {private static Logger logger = LoggerFactory.getLogger(HelloServerInHandler.class);@Overridepublic void channelRead(ChannelHandlerContext ctx, Object msg)throws Exception {logger.info("HelloServerInHandler.channelRead");ByteBuf result = (ByteBuf) msg;byte[] result1 = new byte[result.readableBytes()];// msg中存储的是ByteBuf类型的数据，把数据读取到byte[]中result.readBytes(result1);String resultStr = new String(result1);// 接收并打印客户端的信息System.out.println("Client said:" + resultStr);// 释放资源，这行很关键result.release();// 向客户端发送消息String response = "I am ok!";// 在当前场景下，发送的数据必须转换成ByteBuf数组ByteBuf encoded = ctx.alloc().buffer(4 * response.length());encoded.writeBytes(response.getBytes());ctx.write(encoded);ctx.flush();}@Overridepublic void channelReadComplete(ChannelHandlerContext ctx) throws Exception {ctx.flush();}}3、HelloClient代码如下：package com.guowl.testserver;import io.netty.bootstrap.Bootstrap;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;public class HelloClient {public void connect(String host, int port) throws Exception {EventLoopGroup workerGroup = new NioEventLoopGroup();try {Bootstrap b = new Bootstrap();b.group(workerGroup);b.channel(NioSocketChannel.class);b.option(ChannelOption.SO_KEEPALIVE, true);b.handler(new ChannelInitializer<SocketChannel>() {@Overridepublic void initChannel(SocketChannel ch) throws Exception {ch.pipeline().addLast(new HelloClientIntHandler());}});// Start the client.ChannelFuture f = b.connect(host, port).sync();// Wait until the connection is closed.f.channel().closeFuture().sync();} finally {workerGroup.shutdownGracefully();}}public static void main(String[] args) throws Exception {HelloClient client = new HelloClient();client.connect("127.0.0.1", 8000);}}4、HelloClientIntHandler代码如下：package com.guowl.testserver;import io.netty.buffer.ByteBuf;import io.netty.channel.ChannelHandlerContext;import io.netty.channel.ChannelInboundHandlerAdapter;import org.slf4j.Logger;import org.slf4j.LoggerFactory;public class HelloClientIntHandler extends ChannelInboundHandlerAdapter {private static Logger logger = LoggerFactory.getLogger(HelloClientIntHandler.class);// 接收server端的消息，并打印出来@Overridepublic void channelRead(ChannelHandlerContext ctx, Object msg) throws Exception {logger.info("HelloClientIntHandler.channelRead");ByteBuf result = (ByteBuf) msg;byte[] result1 = new byte[result.readableBytes()];result.readBytes(result1);System.out.println("Server said:" + new String(result1));result.release();}// 连接成功后，向server发送消息@Overridepublic void channelActive(ChannelHandlerContext ctx) throws Exception {logger.info("HelloClientIntHandler.channelActive");String msg = "Are you ok?";ByteBuf encoded = ctx.alloc().buffer(4 * msg.length());encoded.writeBytes(msg.getBytes());ctx.write(encoded);ctx.flush();}}通过上面简单的实例可以发现：
1、在没有任何encoder、decoder的情况下，Netty发送接收数据都是按照ByteBuf的形式，其它形式都是不合法的。
2、接收发送数据操作都是通过handler实现的，handler在netty中占据了非常重要的位置。
3、netty的handler是基于事件触发的，例如当client连接server成功后，client中的HelloClientIntHandler的channelActive方法会自动调用。
妩媚动人，让我感受到了大自然的神奇。
