Netty4.0学习笔记系列之二：Handler的执行顺序

朱雀 2022-04-11 09:10 180阅读 0赞

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：

![978a5c11-b2c9-324f-b4ef-760ad1c7276c.jpg][]  
        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.java、InboundHandler1.java、InboundHandler2.java、OutboundHandler1.java、OutboundHandler2.java

1.HelloServer.java

package com.bijian.netty.server;
    
    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.java

package com.bijian.netty.server;
    
    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.java

package com.bijian.netty.server;
    
    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.java

package com.bijian.netty.server;
    
    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.java

package com.bijian.netty.server;
    
    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.java、HelloClientIntHandler.java

1.HelloClient.java

package com.bijian.netty.client;
    
    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.java

package com.bijian.netty.client;
    
    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：

![7abc52f0-70c4-3093-9d07-6061d706adad.png][]  
        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。

文章来源：[http://blog.csdn.net/u013252773/article/details/21195593][http_blog.csdn.net_u013252773_article_details_21195593]

[978a5c11-b2c9-324f-b4ef-760ad1c7276c.jpg]: /images/20220411/e67d2073fd2b442da70212989fa70901.png
[7abc52f0-70c4-3093-9d07-6061d706adad.png]: /images/20220411/a50c263377594058a2d48dcb80f0812a.png
[http_blog.csdn.net_u013252773_article_details_21195593]: http://blog.csdn.net/u013252773/article/details/21195593