使用JDK NIO类库时开发NIO的异步服务端时，需要用到：多路复用器Selector，ServerSocketChannel,SocketChanel,ByteBuffer,SelectionKey等。如果用源生的JAVA NIO搭建服务端，无疑是十分复杂的，这不仅拖慢了项目的进度，而且开发出来的项目可能还不稳定。那么Netty是如何降低其复杂度的呢？Netty开发简单异步服务端不超过9步，下面分步骤来讲解用Netty来开发异步服务端：

第一步：创建EventLoopGroup

//第一步 创建EventLoopGroup EventLoopGroup bossGroup = new NioEventLoopGroup();EventLoopGroup workerGroup = new NioEventLoopGroup();

讲解：EventLoopGroup 是EventLoop的数组，EventLoop的职责是处理所有注册到本线程多路复用器Selector上的Channel，Selector的轮询操作由EventLoop线程的run方法驱动。

第二步：创建ServerBootstrap

ServerBootstrap b = new ServerBootstrap();

讲解：ServerBootstrap是Netty服务端启动的辅助类，从代码可以看出，我们在创建ServerBootstrap时，用的是一个无参构造器创建的，你也会惊讶的发现，ServerBootstrap只有一个无参构造器，因为这个辅助类需要的参数太多了，而且有些参数是用户可以选择性添加的，所以ServerBootstrap引入了Builder模式 让用户动态添加参数：

ServerBootstrap b = new ServerBootstrap();//动态添加参数，支持链式添加b.group(bossGroup, workerGroup).channel(NioServerSocketChannel.class).handler(new LoggingHandler()).childHandler(new HttpServerHandler());

第三步：设置并绑定服务端Channel（开始绑定ServerSocketChannel）

b.channel(NioServerSocketChannel.class)

讲解：这里只需要传入对应的.class类即可，ServerBootstrap会通过工厂类，利用反射帮我们创建对应的Class对象。

第四步：创建并初始化处理网络时间的职责链 ChannelPipeline

ch.pipeline().addLast(new LoggingHandler());		//请求 http解码		ch.pipeline().addLast("http-decoder",new HttpRequestDecoder());		//将多个消息转换为单一的FullHttpRequest		ch.pipeline().addLast("http-aggregator",new HttpObjectAggregator(65536));		//应答http编码		ch.pipeline().addLast("http-encoder",new HttpResponseEncoder());		//连接异步发送请求，防止内存溢出		ch.pipeline().addLast("http-chunked",new ChunkedWriteHandler());		ch.pipeline().addLast("httpServerHandlerAdapter",new HttpServerHandlerAdapter());

讲解：ChannelPipeline并不是NIO服务端必须的，它本质就是一个处理网络事件的职责链，负责管理和执行ChannelHandler，我们可以在其中添加服务端接收请求的解码和发送请求的编码，以及粘包\拆包的处理等。

第五步：添加并设置ChannelHandler

ChannelHandler是Netty提供给我们定制和扩展的关键接口，利用ChannelHandler我们可以完成大多数功能的定制，例如消息编解码，心跳安全认证等，下面是我定义的Handler：

ServerBootstrap b = new ServerBootstrap();			b.group(bossGroup, workerGroup)			.channel(NioServerSocketChannel.class)			.handler(new LoggingHandler())						//加入自定义的handler			.childHandler(new HttpServerHandler());//我定义的handlerpublic class HttpServerHandler extends ChannelInitializer
    
     {	@Override	protected void initChannel(SocketChannel ch) throws Exception {		ch.pipeline().addLast(new LoggingHandler());		//请求 http解码		ch.pipeline().addLast("http-decoder",new HttpRequestDecoder());		//将多个消息转换为单一的FullHttpRequest		ch.pipeline().addLast("http-aggregator",new HttpObjectAggregator(65536));		//应答http编码		ch.pipeline().addLast("http-encoder",new HttpResponseEncoder());		//连接异步发送请求，防止内存溢出		ch.pipeline().addLast("http-chunked",new ChunkedWriteHandler());		ch.pipeline().addLast("httpServerHandlerAdapter",new HttpServerHandlerAdapter());					}//第二层对消息处理public class HttpServerHandlerAdapter extends SimpleChannelInboundHandler
     
      {	@Override	public boolean acceptInboundMessage(Object msg) throws Exception {		System.out.println(msg);		return super.acceptInboundMessage(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();	}	@Override	protected void channelRead0(ChannelHandlerContext ctx, FullHttpRequest msg) throws Exception {		System.out.println(msg);	}	}
     
    

第六步：绑定并启动监听端口：

ChannelFuture f = b.bind(8080).sync();			System.out.println("Http服务器启动完成，监听端口为："+port);			f.channel().closeFuture().sync();

讲解：在监听端口之前，Netty会做一系列的初始化和检测工作，然后启动监听端口，同时，会将ServerSocketChannel注册到多路复用器Selector上，监听客户端的连接。

第七步：Selector轮询：

讲解：Selector的轮询是由Reactor线程NioEventLoop负责调度的，然后将准备就绪的Channel放到一个集合中；

第八步：Reactor线程NioEventLoop将准备就绪的Channel去执行我们定义好的ChannelPipeline方法，并执行系统定义的ChannelHandler；

第九步：最终，执行我们根据具体业务逻辑定义好的的ChannelHandler。

完整源码如下:

HttpServer:

package kaoqin.server;import io.netty.bootstrap.ServerBootstrap;import io.netty.channel.ChannelFuture;import io.netty.channel.EventLoopGroup;import io.netty.channel.nio.NioEventLoopGroup;import io.netty.channel.socket.nio.NioServerSocketChannel;import io.netty.handler.logging.LoggingHandler;public class HttpServer {		private void run(int port) throws InterruptedException{		//第一步 创建EventLoopGroup 		EventLoopGroup bossGroup = new NioEventLoopGroup();		EventLoopGroup workerGroup = new NioEventLoopGroup();		try{			ServerBootstrap b = new ServerBootstrap();			b.group(bossGroup, workerGroup)			.channel(NioServerSocketChannel.class)			.handler(new LoggingHandler())						//加入自定义的handler			.childHandler(new HttpServerHandler());			//.option(ChannelOption.AUTO_READ,true);			ChannelFuture f = b.bind(port).sync();			System.out.println("Http服务器启动完成，监听端口为："+port);			f.channel().closeFuture().sync();		}finally{			bossGroup.shutdownGracefully();			workerGroup.shutdownGracefully();		}	}	public static void main(String[] args) throws InterruptedException {		new HttpServer().run(9999);	}}

HttpServerHandler:

package kaoqin.server;import io.netty.channel.ChannelInitializer;import io.netty.channel.socket.SocketChannel;import io.netty.handler.codec.http.HttpObjectAggregator;import io.netty.handler.codec.http.HttpRequestDecoder;import io.netty.handler.codec.http.HttpResponseEncoder;import io.netty.handler.logging.LoggingHandler;import io.netty.handler.stream.ChunkedWriteHandler;public class HttpServerHandler extends ChannelInitializer
    
     {	@Override	protected void initChannel(SocketChannel ch) throws Exception {		ch.pipeline().addLast(new LoggingHandler());		//请求 http解码		ch.pipeline().addLast("http-decoder",new HttpRequestDecoder());		//将多个消息转换为单一的FullHttpRequest		ch.pipeline().addLast("http-aggregator",new HttpObjectAggregator(65536));		//应答http编码		ch.pipeline().addLast("http-encoder",new HttpResponseEncoder());		//连接异步发送请求，防止内存溢出		ch.pipeline().addLast("http-chunked",new ChunkedWriteHandler());		ch.pipeline().addLast("httpServerHandlerAdapter",new HttpServerHandlerAdapter());					}}
    

HttpServerHandlerAdapter:

package kaoqin.server;import io.netty.channel.ChannelHandlerContext;import io.netty.channel.SimpleChannelInboundHandler;import io.netty.handler.codec.http.FullHttpRequest;public class HttpServerHandlerAdapter extends SimpleChannelInboundHandler
    
     {	@Override	public boolean acceptInboundMessage(Object msg) throws Exception {		System.out.println(msg);		return super.acceptInboundMessage(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();	}	@Override	protected void channelRead0(ChannelHandlerContext ctx, FullHttpRequest msg) throws Exception {		System.out.println(msg);	}	}