1.Netty中handler的执行顺序
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。按照输入输出来分，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：
2.Netty中handler执行顺序代码示例：
客户端代码同上
服务端代码及业务逻辑类：
import io.netty.bootstrap.ServerBootstrap;import io.netty.channel.Channel;import io.netty.channel.ChannelFuture;import io.netty.channel.ChannelInitializer;import io.netty.channel.EventLoopGroup;import io.netty.channel.nio.NioEventLoopGroup;import io.netty.channel.socket.nio.NioServerSocketChannel;/** * ? 配置服务器功能，如线程、端口 ? 实现服务器处理程序，它包含业务逻辑，决定当有一个请求连接或接收数据时该做什么 */public class EchoServer { ???private final int port; ???public EchoServer(int port) { ???????this.port = port; ???} ???public void start() throws Exception { ???????EventLoopGroup eventLoopGroup = null; ???????try { ???????????//server端引导类 ???????????ServerBootstrap serverBootstrap = new ServerBootstrap(); ???????????//连接池处理数据 ???????????eventLoopGroup = new NioEventLoopGroup(); ???????????serverBootstrap.group(eventLoopGroup) ???????????.channel(NioServerSocketChannel.class)//指定通道类型为NioServerSocketChannel，一种异步模式，OIO阻塞模式为OioServerSocketChannel ???????????.localAddress("localhost",port)//设置InetSocketAddress让服务器监听某个端口已等待客户端连接。 ???????????.childHandler(new ChannelInitializer<Channel>() {//设置childHandler执行所有的连接请求 ???????????????@Override ???????????????protected void initChannel(Channel ch) throws Exception { ???????????????????// 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 EchoOutHandler1()); ???????????????????ch.pipeline().addLast(new EchoOutHandler2()); ????????????????????// 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 EchoInHandler1()); ???????????????????ch.pipeline().addLast(new EchoInHandler2()); ???????????????} ???????????????????}); ???????????// 最后绑定服务器等待直到绑定完成，调用sync()方法会阻塞直到服务器完成绑定,然后服务器等待通道关闭，因为使用sync()，所以关闭操作也会被阻塞。 ???????????ChannelFuture channelFuture = serverBootstrap.bind().sync(); ???????????System.out.println("开始监听，端口为：" + channelFuture.channel().localAddress()); ???????????channelFuture.channel().closeFuture().sync(); ???????} finally { ???????????eventLoopGroup.shutdownGracefully().sync(); ???????} ???} ???public static void main(String[] args) throws Exception { ???????new EchoServer(20000).start(); ???}}业务类EchoInHandler1：
import io.netty.buffer.ByteBuf;import io.netty.buffer.Unpooled;import io.netty.channel.ChannelHandlerContext;import io.netty.channel.ChannelInboundHandlerAdapter;import java.util.Date;import cn.itcast_03_netty.sendobject.bean.Person;public class EchoInHandler1 extends ChannelInboundHandlerAdapter { ???@Override ???public void channelRead(ChannelHandlerContext ctx, Object msg) ???????????throws Exception { ???????System.out.println("in1"); ????????// 通知执行下一个InboundHandler ???????ctx.fireChannelRead(msg);//ChannelInboundHandler之间的传递，通过调用 ctx.fireChannelRead(msg) 实现 ???} ???@Override ???public void channelReadComplete(ChannelHandlerContext ctx) throws Exception { ???????ctx.flush();//刷新后才将数据发出到SocketChannel ???} ???@Override ???public void exceptionCaught(ChannelHandlerContext ctx, Throwable cause) ???????????throws Exception { ???????cause.printStackTrace(); ???????ctx.close(); ???}}业务类 EchoInHandler2：
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 EchoInHandler2 extends ChannelInboundHandlerAdapter { ???@Override ???public void channelRead(ChannelHandlerContext ctx, Object msg) throws Exception { ???????System.out.println("in2"); ???????ByteBuf buf = (ByteBuf) msg; ???????byte[] req = new byte[buf.readableBytes()]; ???????buf.readBytes(req); ???????String body = new String(req, "UTF-8"); ???????System.out.println("接收客户端数据:" + body); ???????//向客户端写数据 ???????System.out.println("server向client发送数据"); ???????String currentTime = new Date(System.currentTimeMillis()).toString(); ???????ByteBuf resp = Unpooled.copiedBuffer(currentTime.getBytes()); ???????ctx.write(resp); //用ctx.write(msg) 将传递到 ChannelOutboundHandler ???} ???@Override ???public void channelReadComplete(ChannelHandlerContext ctx) throws Exception { ???????ctx.flush();//刷新后才将数据发出到SocketChannel ???} ???@Override ???public void exceptionCaught(ChannelHandlerContext ctx, Throwable cause) ???????????throws Exception { ???????cause.printStackTrace(); ???????ctx.close(); ???}}业务逻辑类 EchoOutHandler2：
import io.netty.channel.ChannelHandlerContext;import io.netty.channel.ChannelOutboundHandlerAdapter;import io.netty.channel.ChannelPromise;public class EchoOutHandler2 extends ChannelOutboundHandlerAdapter { ????@Override ???????public void write(ChannelHandlerContext ctx, Object msg, ChannelPromise promise) throws Exception { ???????????System.out.println("out2"); ???????????// 执行下一个OutboundHandler ???????????super.write(ctx, msg, promise); ???????}}业务逻辑类 EchoOutHandler1：
import java.util.Date;import io.netty.buffer.ByteBuf;import io.netty.buffer.Unpooled;import io.netty.channel.ChannelHandlerContext;import io.netty.channel.ChannelOutboundHandlerAdapter;import io.netty.channel.ChannelPromise;public class EchoOutHandler1 extends ChannelOutboundHandlerAdapter { ???@Override ???// 向client发送消息 ???public void write(ChannelHandlerContext ctx, Object msg, ChannelPromise promise) throws Exception { ???????System.out.println("out1"); ???????String currentTime = new Date(System.currentTimeMillis()).toString(); ???????ByteBuf resp = Unpooled.copiedBuffer(currentTime.getBytes()); ???????ctx.write(resp); ???????ctx.flush(); //ctx.write()方法执行后，需要调用flush()方法才能令它立即执行 ??????}}运行结果：
开始监听，端口为：/127.0.0.1:20000in1in2接收客户端数据:QUERY TIME ORDERserver向client发送数据out2out1
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
（流水线pipeline中outhander不能放到最后，否则不生效）
4、Handler的消费处理放在最后一个处理。
7.Netty中 handler 的执行顺序
原文地址：https://www.cnblogs.com/yaboya/p/9176703.html