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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：
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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<SocketChannel>() { ???????????????????????????????? @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 );???? }} |
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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();???? }} |
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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();???? }?
} |
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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：
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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;?
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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);???? }} |
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下面是客户端?
六、HelloClient：
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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<SocketChannel>() {???????????????? @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：
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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();???? }} |
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。
原文：https://blog.51cto.com/iteyer/3237664