Handler在netty中，无疑占据着非常重要的地位。Handler与Servlet中的filter很像，通过Handler可以完成通讯报文的解码编码、拦截指定的报文、统一对日志错误进行处理、统一对请求进行计数、控制Handler执行与否。一句话，没有它做不到的只有你想不到的。

Netty中的所有handler都实现自ChannelHandler接口。按照输出输出来分，分为ChannelInboundHandler、ChannelOutboundHandler两大类。ChannelInboundHandler对从客户端发往服务器的报文进行处理，一般用来执行解码、读取客户端数据、进行业务处理等；ChannelOutboundHandler对从服务器发往客户端的报文进行处理，一般用来进行编码、发送报文到客户端。

Netty中，可以注册多个handler。ChannelInboundHandler按照注册的先后顺序执行；ChannelOutboundHandler按照注册的先后顺序逆序执行，如下图所示，按照注册的先后顺序对Handler进行排序，request进入Netty后的执行顺序为：

下面例子涉及的类包括：

一、HelloServer：

package com.yao.nettyhandler;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
    
     () { 								@Override								public void initChannel(SocketChannel ch) throws Exception {									// 注册两个OutboundHandler，执行顺序为注册顺序的逆序，所以应该是OutboundHandler2 OutboundHandler1									ch.pipeline().addLast(new OutboundHandler1());									ch.pipeline().addLast(new OutboundHandler2());									// 注册两个InboundHandler，执行顺序为注册顺序，所以应该是InboundHandler1 InboundHandler2									ch.pipeline().addLast(new InboundHandler1());									ch.pipeline().addLast(new InboundHandler2());								}							}).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);	}}
    

二、InboundHandler1：

package com.yao.nettyhandler;import io.netty.channel.ChannelHandlerContext;import io.netty.channel.ChannelInboundHandlerAdapter;import org.apache.commons.logging.Log;import org.apache.commons.logging.LogFactory;public class InboundHandler1 extends ChannelInboundHandlerAdapter {	private static Log logger = LogFactory.getLog(InboundHandler1.class);	@Override	public void channelRead(ChannelHandlerContext ctx, Object msg) throws Exception {		logger.info("InboundHandler1.channelRead: ctx :" + ctx);				// 通知执行下一个InboundHandler		//ctx.fireChannelRead(msg);	}	@Override	public void channelReadComplete(ChannelHandlerContext ctx) throws Exception {		logger.info("InboundHandler1.channelReadComplete");		ctx.flush();	}}

三、InboundHandler2：

package com.yao.nettyhandler;import io.netty.buffer.ByteBuf;import io.netty.channel.ChannelHandlerContext;import io.netty.channel.ChannelInboundHandlerAdapter;import org.apache.commons.logging.Log;import org.apache.commons.logging.LogFactory;public class InboundHandler2 extends ChannelInboundHandlerAdapter {	private static Log	logger	= LogFactory.getLog(InboundHandler2.class);	@Override	// 读取Client发送的信息，并打印出来	public void channelRead(ChannelHandlerContext ctx, Object msg) throws Exception {		logger.info("InboundHandler2.channelRead: ctx :" + ctx);		ByteBuf result = (ByteBuf) msg;		byte[] result1 = new byte[result.readableBytes()];		result.readBytes(result1);		String resultStr = new String(result1);		System.out.println("Client said:" + resultStr);		result.release();		ctx.write(msg);	}	@Override	public void channelReadComplete(ChannelHandlerContext ctx) throws Exception {		logger.info("InboundHandler2.channelReadComplete");		ctx.flush();	}}

四、OutboundHandler1 ：

package com.yao.nettyhandler;import io.netty.buffer.ByteBuf;import io.netty.channel.ChannelHandlerContext;import io.netty.channel.ChannelOutboundHandlerAdapter;import io.netty.channel.ChannelPromise;import org.apache.commons.logging.Log;import org.apache.commons.logging.LogFactory;public class OutboundHandler1 extends ChannelOutboundHandlerAdapter {	private static Log	logger	= LogFactory.getLog(OutboundHandler1.class);	@Override	// 向client发送消息	public void write(ChannelHandlerContext ctx, Object msg, ChannelPromise promise) throws Exception {		logger.info("OutboundHandler1.write");		String response = "I am ok!";		ByteBuf encoded = ctx.alloc().buffer(4 * response.length());		encoded.writeBytes(response.getBytes());		ctx.write(encoded);		ctx.flush();	}		}

五、OutboundHandler2：

package com.yao.nettyhandler;import io.netty.channel.ChannelHandlerContext;import io.netty.channel.ChannelOutboundHandlerAdapter;import io.netty.channel.ChannelPromise;import org.apache.commons.logging.Log;import org.apache.commons.logging.LogFactory;public class OutboundHandler2 extends ChannelOutboundHandlerAdapter {	private static Log	logger	= LogFactory.getLog(OutboundHandler2.class);		@Override	public void write(ChannelHandlerContext ctx, Object msg, ChannelPromise promise) throws Exception {		logger.info("OutboundHandler2.write");		// 执行下一个OutboundHandler		super.write(ctx, msg, promise);	}}

下面是客户端

六、HelloClient：

package com.yao.nettyhandler;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
    
     () {				@Override				public void initChannel(SocketChannel ch) throws Exception {					ch.pipeline().addLast(new HelloClientIntHandler());				}			});			// Start the client.			ChannelFuture f = b.connect(host, port).sync();			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);	}}
    

七、HelloClientIntHandler：

package com.yao.nettyhandler;import io.netty.buffer.ByteBuf;import io.netty.channel.ChannelHandlerContext;import io.netty.channel.ChannelInboundHandlerAdapter;import org.apache.commons.logging.Log;import org.apache.commons.logging.LogFactory;public class HelloClientIntHandler extends ChannelInboundHandlerAdapter {	private static Log	logger	= LogFactory.getLog(HelloClientIntHandler.class);	@Override	// 读取服务端的信息	public 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);		result.release();		ctx.close();		System.out.println("Server said:" + new String(result1));	}	@Override	// 当连接建立的时候向服务端发送消息 ，channelActive 事件当连接建立的时候会触发	public 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();	}}

八、总结：

在使用Handler的过程中，需要注意：

1、ChannelInboundHandler之间的传递，通过调用 ctx.fireChannelRead(msg) 实现；调用ctx.write(msg) 将传递到ChannelOutboundHandler。 2、ctx.write()方法执行后，需要调用flush()方法才能令它立即执行。 3、ChannelOutboundHandler 在注册的时候需要放在最后一个ChannelInboundHandler之前，否则将无法传递到ChannelOutboundHandler。 4、Handler的消费处理放在最后一个处理。