看了很多讲解Netty中InboundHandler、OutboundHandler执行顺序的文章，很少有一篇能完全讲的全的、对的、没有歧义的，在此根据所学、亲自验证得出的结论给大家
举例：netty服务端是这个添加顺序
ch.pipeline().addLast(new InboundHandler1());
ch.pipeline().addLast(new InboundHandler2());
ch.pipeline().addLast(new OutboundHandler1());
ch.pipeline().addLast(new OutboundHandler2()); 链表中的顺序为head->in1->in2->out1->out2->tail
1、如果在InboundHandler2中执行的是ctx.channel().writeAndFlush，执行顺序是：
InboundHandler1
InboundHandler2
OutboundHandler2
OutboundHandler1结果证明：执行完InboundHandler1、InboundHandler2之后，由于InboundHandler2中执行的是ctx.channel().writeAndFlush，它会直接从tail开始往前找Outbound执行，链表中的顺序为head->in1->in2->out1->out2->tail，所以会执行out2，再执行out1
2、如果在InboundHandler2中执行的是ctx.writeAndFlush，执行顺序是：
InboundHandler1
InboundHandler2 结果证明，由于OutboundHandler1与OutboundHandler2的add在InboundHandler2之后，而InboundHandler2中执行的是ctx.writeAndFlush，它会从当前InboundHandler2的位置，往前找Outbound执行，根据链表中的顺序为head->in1->in2->out1->out2->tail，in2之前已经无Outbound，所以不会再有Outbound会执行
上面两个已经能说明问题，为了方便对netty的handler执行不是太熟悉的读者理解，再举个例子：
ch.pipeline().addLast(new OutboundHandler1());
ch.pipeline().addLast(new OutboundHandler2());
ch.pipeline().addLast(new InboundHandler1());
ch.pipeline().addLast(new InboundHandler2()); 链表中的顺序为head->out1->out2->in1->in2->tail
1、如果在InboundHandler2中执行的是ctx.channel().writeAndFlush，执行顺序是：
InboundHandler1
InboundHandler2
OutboundHandler2
OutboundHandler1 结果证明，执行完InboundHandler2后，由于InboundHandler2中执行的是ctx.channel().writeAndFlush，它会从tail往前找Outbound执行，而链表中的顺序为head->out1->out2->in1->in2->tail，所以会继续执行到out2、out1
2、如果在InboundHandler2中执行的是ctx.writeAndFlush，执行顺序是：
InboundHandler1
InboundHandler2
OutboundHandler2
OutboundHandler1结果证明，执行完InboundHandler2后，由于InboundHandler2中执行的是ctx.writeAndFlush，它会从InboundHandler2位置往前找Outbound执行，而链表中的顺序为head->out1->out2->in1->in2->tail，所以会继续执行到out2、out1
核心最容易让人误解的点总结：
1、ctx.writeAndFlush只会从当前的handler位置开始，往前找outbound执行
2、ctx.pipeline().writeAndFlush与ctx.channel().writeAndFlush会从tail的位置开始，往前找outbound执行
附录代码：
package net.xdclass.echo;
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 EchoServer {
private int port;
public EchoServer(int port){
this.port = port;
}
/**
* 启动流程
*/
public void run() throws InterruptedException {
//配置服务端线程组
EventLoopGroup bossGroup = new NioEventLoopGroup();
EventLoopGroup workGroup = new NioEventLoopGroup();
try{
ServerBootstrap serverBootstrap = new ServerBootstrap();
serverBootstrap.group(bossGroup, workGroup)
.channel(NioServerSocketChannel.class)
.option(ChannelOption.SO_BACKLOG,1024)
.childHandler(new ChannelInitializer<SocketChannel>() {
protected void initChannel(SocketChannel ch) throws Exception {
ch.pipeline().addLast(new OutboundHandler1());
ch.pipeline().addLast(new OutboundHandler2());
ch.pipeline().addLast(new InboundHandler1());
ch.pipeline().addLast(new InboundHandler2());
}
});
System.out.println("Echo 服务器启动ing");
//绑定端口，同步等待成功
ChannelFuture channelFuture = serverBootstrap.bind(port).sync();
//等待服务端监听端口关闭
channelFuture.channel().closeFuture().sync();
}finally {
//优雅退出，释放线程池
workGroup.shutdownGracefully();
bossGroup.shutdownGracefully();
}
}
public static void main(String [] args) throws InterruptedException {
int port = 8080;
if(args.length > 0){
port = Integer.parseInt(args[0]);
}
new EchoServer(port).run();
//服务启动后，如果没有客户端，想测试服务端功能，可以用telnet:
// telnet 127.0.0.1 8080 输入内容即可与服务端交互
}
}
package net.xdclass.echo;
import io.netty.buffer.ByteBuf;
import io.netty.buffer.Unpooled;
import io.netty.channel.ChannelHandlerContext;
import io.netty.channel.ChannelInboundHandlerAdapter;
import io.netty.util.CharsetUtil;
public class InboundHandler1 extends ChannelInboundHandlerAdapter {
@Override
public void channelRead(ChannelHandlerContext ctx, Object msg)
throws Exception {
ByteBuf data = (ByteBuf) msg;
System.out.println("InboundHandler1 channelRead 服务端收到数据：" + data.toString(CharsetUtil.UTF_8));
// 执行下一个InboundHandler
ctx.fireChannelRead(Unpooled.copiedBuffer("InboundHandler1 "+data.toString(CharsetUtil.UTF_8), CharsetUtil.UTF_8));
}
@Override
public void channelReadComplete(ChannelHandlerContext ctx) throws Exception {
ctx.flush();
}
@Override
public void exceptionCaught(ChannelHandlerContext ctx, Throwable cause)
throws Exception {
cause.printStackTrace();
ctx.close();
}
}
package net.xdclass.echo;
import io.netty.buffer.ByteBuf;
import io.netty.buffer.Unpooled;
import io.netty.channel.ChannelHandlerContext;
import io.netty.channel.ChannelInboundHandlerAdapter;
import io.netty.util.CharsetUtil;
public class InboundHandler2 extends ChannelInboundHandlerAdapter {
@Override
public void channelRead(ChannelHandlerContext ctx, Object msg)
throws Exception {
ByteBuf data = (ByteBuf) msg;
System.out.println("InboundHandler2 channelRead 服务端收到数据：" + data.toString(CharsetUtil.UTF_8));
//调用pipeline().writeAndFlush 或者 channel().writeAndFlush，还需要顺着经过Outbound了，再返回给客户端
//ctx.pipeline().writeAndFlush(Unpooled.copiedBuffer("InboundHandler2 "+data.toString(CharsetUtil.UTF_8), CharsetUtil.UTF_8));
//ctx.channel().writeAndFlush(Unpooled.copiedBuffer("InboundHandler2 "+data.toString(CharsetUtil.UTF_8), CharsetUtil.UTF_8));
//调用ChannelHandlerContext.writeAndFlush，若InboundHandler2不是最后一个addLast，则OutboundHandler1、OutboundHandler1不会只想
ctx.writeAndFlush(Unpooled.copiedBuffer("InboundHandler2 "+data.toString(CharsetUtil.UTF_8), CharsetUtil.UTF_8));
}
@Override
public void channelReadComplete(ChannelHandlerContext ctx) throws Exception {
ctx.flush();
}
@Override
public void exceptionCaught(ChannelHandlerContext ctx, Throwable cause)
throws Exception {
cause.printStackTrace();
ctx.close();
}
}
package net.xdclass.echo;
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;
import io.netty.util.CharsetUtil;
public class OutboundHandler1 extends ChannelOutboundHandlerAdapter {
@Override
public void write(ChannelHandlerContext ctx, Object msg, ChannelPromise promise) throws Exception {
ByteBuf data = (ByteBuf) msg;
System.out.println("OutboundHandler1 write : "+data.toString(CharsetUtil.UTF_8));
ctx.write(Unpooled.copiedBuffer("OutboundHandler1 "+data.toString(CharsetUtil.UTF_8), CharsetUtil.UTF_8));
ctx.flush();
}
}
package net.xdclass.echo;
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;
import io.netty.util.CharsetUtil;
public class OutboundHandler2 extends ChannelOutboundHandlerAdapter {
@Override
public void write(ChannelHandlerContext ctx, Object msg, ChannelPromise promise) throws Exception {
ByteBuf data = (ByteBuf) msg;
System.out.println("OutboundHandler2 write : "+data.toString(CharsetUtil.UTF_8));
ctx.write(Unpooled.copiedBuffer(" "+data.toString(CharsetUtil.UTF_8), CharsetUtil.UTF_8));
ctx.flush();
}
}
