Netty4.0学习笔记系列之二：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：

基本的概念就说到这，下面用一个例子来进行验证。该例子模拟Client与Server间的通讯，Server端注册了2个ChannelInboundHandler、2个ChannelOutboundHandler。当Client连接到Server后，会向Server发送一条消息。Server端通过ChannelInboundHandler 对Client发送的消息进行读取，通过ChannelOutboundHandler向client发送消息。最后Client把接收到的信息打印出来。

Server端一共有5个类：HelloServer InboundHandler1 InboundHandler2 OutboundHandler1 OutboundHandler2

1、HelloServer 代码如下

package com.guowl.testmultihandler;

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);
    }
}

2、InboundHandler1

package com.guowl.testmultihandler;

import io.netty.channel.ChannelHandlerContext;
import io.netty.channel.ChannelInboundHandlerAdapter;

import org.slf4j.Logger;
import org.slf4j.LoggerFactory;

public class InboundHandler1 extends ChannelInboundHandlerAdapter {
    private static Logger   logger  = LoggerFactory.getLogger(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();
    }
}

3、InboundHandler2

package com.guowl.testmultihandler;

import io.netty.buffer.ByteBuf;
import io.netty.channel.ChannelHandlerContext;
import io.netty.channel.ChannelInboundHandlerAdapter;

import org.slf4j.Logger;
import org.slf4j.LoggerFactory;

public class InboundHandler2 extends ChannelInboundHandlerAdapter {
    private static Logger   logger  = LoggerFactory.getLogger(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();
    }

}

4、OutboundHandler1

package com.guowl.testmultihandler;

import io.netty.buffer.ByteBuf;
import io.netty.channel.ChannelHandlerContext;
import io.netty.channel.ChannelOutboundHandlerAdapter;
import io.netty.channel.ChannelPromise;

import org.slf4j.Logger;
import org.slf4j.LoggerFactory;

public class OutboundHandler1 extends ChannelOutboundHandlerAdapter {
    private static Logger   logger  = LoggerFactory.getLogger(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();
    }


}

5、OutboundHandler2

package com.guowl.testmultihandler;

import io.netty.channel.ChannelHandlerContext;
import io.netty.channel.ChannelOutboundHandlerAdapter;
import io.netty.channel.ChannelPromise;

import org.slf4j.Logger;
import org.slf4j.LoggerFactory;

public class OutboundHandler2 extends ChannelOutboundHandlerAdapter {
    private static Logger   logger  = LoggerFactory.getLogger(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);
    }
}

Client端有两个类：HelloClient  HelloClientIntHandler

1、HelloClient  

package com.guowl.testmultihandler;

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);
    }
}

2、HelloClientIntHandler

package com.guowl.testmultihandler;

import io.netty.buffer.ByteBuf;
import io.netty.channel.ChannelHandlerContext;
import io.netty.channel.ChannelInboundHandlerAdapter;

import org.slf4j.Logger;
import org.slf4j.LoggerFactory;

public class HelloClientIntHandler extends ChannelInboundHandlerAdapter {
    private static Logger   logger  = LoggerFactory.getLogger(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();
    }
}

server端执行结果为：

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。

• 如果调用的是ctx.channel().write()是从尾开始执行，不会有博主说的问题，如果是直接用ctx.write()时则会有博主说的问题