netty 學習 （2）Handler的執行順序

Handler在netty中，無疑佔據着很是重要的地位。Handler與Servlet中的filter很像，經過Handler能夠完成通信報文的解碼編碼、攔截指定的報文、統一對日誌錯誤進行處理、統一對請求進行計數、控制Handler執行與否。一句話，沒有它作不到的只有你想不到的。

Netty中的全部handler都實現自ChannelHandler接口。按照輸出輸出來分，分爲ChannelInboundHandler、ChannelOutboundHandler兩大類。ChannelInboundHandler對從客戶端發往服務器的報文進行處理，通常用來執行解碼、讀取客戶端數據、進行業務處理等；ChannelOutboundHandler對從服務器發往客戶端的報文進行處理，通常用來進行編碼、發送報文到客戶端。

Netty中，能夠註冊多個handler。ChannelInboundHandler按照註冊的前後順序執行；ChannelOutboundHandler按照註冊的前後順序逆序執行，以下圖所示，按照註冊的前後順序對Handler進行排序，request進入Netty後的執行順序爲：

下面例子涉及的類包括：

1、HelloServer：

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 {
									// 註冊兩個OutboundHandler，執行順序爲註冊順序的逆序，因此應該是OutboundHandler2 OutboundHandler1
									ch.pipeline().addLast(new OutboundHandler1());
									ch.pipeline().addLast(new OutboundHandler2());
									// 註冊兩個InboundHandler，執行順序爲註冊順序，因此應該是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.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);
		
		// 通知執行下一個InboundHandler
		//ctx.fireChannelRead(msg);
	}

	@Override
	public void channelReadComplete(ChannelHandlerContext ctx) throws Exception {
		logger.info("InboundHandler1.channelReadComplete");
		ctx.flush();
	}
}

3、InboundHandler2：

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
	// 讀取Client發送的信息，並打印出來
	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.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發送消息
	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.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;


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");
		// 執行下一個OutboundHandler
		super.write(ctx, msg, promise);
	}
}

下面是客戶端

6、HelloClient：

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);
	}
}

7、HelloClientIntHandler：

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
	// 讀取服務端的信息
	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
	// 當鏈接創建的時候向服務端發送消息 ，channelActive 事件當鏈接創建的時候會觸發
	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();
	}
}

8、總結：

在使用Handler的過程當中，須要注意：
一、ChannelInboundHandler之間的傳遞，經過調用 ctx.fireChannelRead(msg) 實現；調用ctx.write(msg) 將傳遞到ChannelOutboundHandler。
二、ctx.write()方法執行後，須要調用flush()方法才能令它當即執行。
三、ChannelOutboundHandler 在註冊的時候須要放在最後一個ChannelInboundHandler以前，不然將沒法傳遞到ChannelOutboundHandler。
四、Handler的消費處理放在最後一個處理。