netty源码解解析(4.0)-11 Channel NIO实现-概览

　  结构设计

　　Channel的NIO实现位于io.netty.channel.nio包和io.netty.channel.socket.nio包中，其中io.netty.channel.nio是抽象实现，io.netty.channel.socket.nio最终实现。下面是Channel NIO相关类的派生图:

　　NIO实现最终派生出3个类型NioServerSocketChannel实现了tcp server, NioSocketChannel实现了tcp client, NioDatagramChannel实现了udp socket。

　　整个NIO实现分为三个层次:

　　

　　AbstractNioChannel抽象层

　　对channel进行基本的初始化工作，把channel设置成非阻塞模式。

　　实现Channel.Unsafe的connect方法框架，提供给了doConnection, doFinishConnect两个抽象方法，把真正的连接操作交给子类实现。

　　覆盖了AbstractChannel的doRegister，doDeregister方法，正两个方法实现了Channel的selectionKey的注册和注销。

　　实现AbstractChannel的doClose, 这个方法并没有真正关闭channel动作。

　　形如doXXX的方法是，AbstractChannel提供的扩展点，在<<netty源码解解析(4.0)-3 Channel的抽象实现>>的末尾，给出了这些扩展点的详细列表。

　　

　　AbstractNioByteChannel, AbstractNioMessageChannel抽象层

　　这两个类主要实现read和write的框架，它们的实现大致相同AbstractNioByteChannel读写的是byte array，而AbstractNioMessageChannel读的时候会把byte array转换成结构化的对象，写的时候把结构化对象序列化成byte array。

　　AbstractNioByteChannel定义了3个抽象方法用来实现真正的读写操作: doReadBytes, doWriteBytes, doWriteFileRegion。

　　AbstractNioMessageChannel第了两个2个抽象方法用来实现真正的结构化数据类型的读写: doReadMessages, doWriteMessage。

　　NioServerSocketChannel, NioSocketChannel, NioDatagramChannel最终实现

　　封装NIO API调用，真正的I/O操操作和socket相关的api调用都在这一层实现。　　

　　使用方式

　　使用过netty的人都知道，netty提供了ServerBootstrap和Bootstrap类帮助用户方便地创建服务器端和客户端应用，但这不是必须的。仅仅使用NioServerSocketChannel, NioSocketChannel, NioDatagramChannel和NioEventLoopGroup就可以用开发tcp的server和client, 及udp应用。

　　为了能让读者能够更清晰地理解NioEventLoopGroup和Channel直接的关系，下面给出了最原始的使用使用netty框架的代码。

　　tcp server实现

 import io.netty.buffer.ByteBuf;
 import io.netty.channel.*;
 import io.netty.channel.nio.NioEventLoopGroup;
 import io.netty.channel.socket.nio.NioServerSocketChannel;

 import java.net.InetSocketAddress;
 import java.nio.charset.Charset;

 public class TcpServer {
     private NioEventLoopGroup group = new NioEventLoopGroup();

     public static void main(String[] argc){
         TcpServer server = new TcpServer();
         server.start();

         while(true){
             try{
                 Thread.sleep(1000);
             }catch (Exception e){
                 break;
             }
         }

         server.stop();
     }

     public void start(){
         NioServerSocketChannel server = new NioServerSocketChannel();

         ChannelPipeline pipleline = server.pipeline();
         pipleline.addFirst(new ServerHandler());

         group.register(server).addListener(new ChannelFutureListener() {
             @Override
             public void operationComplete(ChannelFuture future) throws Exception {
                 server.bind(new InetSocketAddress(9001));
                 System.out.println("server listen add:"+9001);
             }
         });
     }
     public void stop(){
         group.shutdownGracefully();
     }

     private class ServerHandler extends ChannelInboundHandlerAdapter{

         @Override
         public void channelRead(ChannelHandlerContext ctx, Object msg) throws Exception{
             Channel child = (Channel)msg;

             child.pipeline().addLast(new ChildHandler());

             group.register(child);

         }
     }

     private class ChildHandler extends ChannelInboundHandlerAdapter{

         @Override
         public void channelActive(ChannelHandlerContext ctx) throws Exception {
             System.out.println("connected");
         }

         @Override
         public void channelInactive(ChannelHandlerContext ctx) throws Exception {
             System.out.println("closed");
         }

         @Override
         public void channelRead(ChannelHandlerContext ctx, Object msg) throws Exception{
             Channel chnl = ctx.channel();
             ByteBuf data = (ByteBuf)msg;

             System.out.println("recv: "+data.toString(Charset.forName("utf-8")));
             chnl.write(msg);
         }

         @Override
         public void channelReadComplete(ChannelHandlerContext ctx) throws Exception {
             ctx.channel().flush();
         }

     }
 }

　　一个channel创建之后，首先要做的事就是向pipleline中添加handler，然后才是把它注册到NioEventLoopGroup中(第31，33行)。这个顺序不能错，否则，handler的handlerAdded，channelRegistered和channelActive将不会被调用。当NioServerSocketChannel收到一个连接时，ServerHandler的的channelRead方法将会被调用，的新建好的连接当成参数传递进来，第49-53行是对新连接的初始化代码。

　　tcp client实现

import io.netty.buffer.ByteBuf;
import io.netty.channel.*;
import io.netty.channel.nio.NioEventLoopGroup;
import io.netty.channel.socket.nio.NioSocketChannel;

import java.net.InetSocketAddress;
import java.nio.charset.Charset;

public class TcpClient {

    public static void main(String[] args){
        NioEventLoopGroup group = new NioEventLoopGroup();

        NioSocketChannel client = new NioSocketChannel();
        client.pipeline().addLast(new ClientInboundHandler());

        group.register(client);

        client.connect(new InetSocketAddress(9001));

        try{
            Thread.sleep(3000);
        }catch (Exception e){

        }
        group.shutdownGracefully();
    }

    private static class ClientInboundHandler extends ChannelInboundHandlerAdapter{
        @Override
        public void channelActive(ChannelHandlerContext ctx) throws Exception {
            System.out.println("connected");
            Channel chnl = ctx.channel();
            ByteBuf buf = chnl.alloc().buffer();
            buf.writeBytes( "this is test".getBytes());
            chnl.writeAndFlush(buf);
        }

        @Override
        public void channelInactive(ChannelHandlerContext ctx) throws Exception {
            System.out.println("closed");
        }

        @Override
        public void channelRead(ChannelHandlerContext ctx, Object msg) throws Exception{
            ByteBuf data = (ByteBuf)msg;
            System.out.println("recv: "+data.toString(Charset.forName("utf-8")));
            ctx.channel().close();
        }
    }

}

　　client的实现比server实现相对简单，添加handler，register顺序和server一致。只有把一个channel注册到gruop中之后才能调用它的方法，应为channel的大多数方法都需要通过pipleline调用，而pipleline需要在eventLoop中执行。

　　udp没有server和client的区别，这里为了使代码更加清晰，把server和client代码区分开来。

　　udp server

import io.netty.buffer.ByteBuf;
import io.netty.channel.*;
import io.netty.channel.nio.NioEventLoopGroup;
import io.netty.channel.socket.DatagramPacket;
import io.netty.channel.socket.nio.NioDatagramChannel;

import java.net.InetSocketAddress;
import java.nio.charset.Charset;

public class UdpServer {

    public static void main(String[] args){
        NioEventLoopGroup group = new NioEventLoopGroup();

        NioDatagramChannel chnl = new NioDatagramChannel();
        chnl.pipeline().addLast(new UdpHandler());

        group.register(chnl).addListener(new ChannelFutureListener() {
            @Override
            public void operationComplete(ChannelFuture future) throws Exception {
                chnl.bind(new InetSocketAddress(9002));
                System.out.println("udp bind at:"+9002);
            }
        });

        while(true){
            try{
                Thread.sleep(1000);
            }catch (Exception e){
                break;
            }
        }
        group.shutdownGracefully();
    }
    private static class UdpHandler extends ChannelInboundHandlerAdapter {

        @Override
        public void channelActive(ChannelHandlerContext ctx) throws Exception {
            System.out.print("udp channel active");
        }

        @Override
        public void channelRead(ChannelHandlerContext ctx, Object msg) throws Exception{
            Channel chnl = ctx.channel();
            DatagramPacket pkg = (DatagramPacket)msg;
            ByteBuf content = pkg.content();
            InetSocketAddress from = pkg.sender();
            System.out.println("recv: "+content.toString(Charset.forName("utf-8"))+" from:"+from.toString());
            pkg = new DatagramPacket(content, from);
            chnl.write(pkg);
        }

        @Override
        public void channelReadComplete(ChannelHandlerContext ctx) throws Exception {
            ctx.channel().flush();
        }
    }
}

　　udp client

import io.netty.buffer.ByteBuf;
import io.netty.channel.*;
import io.netty.channel.nio.NioEventLoopGroup;
import io.netty.channel.socket.DatagramPacket;
import io.netty.channel.socket.nio.NioDatagramChannel;

import java.net.InetSocketAddress;
import java.nio.charset.Charset;

public class UdpClient {

    public static void main(String[] args){
        NioEventLoopGroup group = new NioEventLoopGroup();

        NioDatagramChannel chnl = new NioDatagramChannel();
        chnl.pipeline().addLast(new UdpHandler());

        group.register(chnl).addListener(new ChannelFutureListener() {
            @Override
            public void operationComplete(ChannelFuture future) throws Exception {
                chnl.bind(new InetSocketAddress(0));
            }
        });

        try{
            Thread.sleep(3000);
        }catch (Exception e){
        }
        group.shutdownGracefully();
    }

    private static class UdpHandler extends ChannelInboundHandlerAdapter {

        @Override
        public void channelActive(ChannelHandlerContext ctx) throws Exception {
            System.out.print("udp channel active");
            Channel chnl = ctx.channel();
            ByteBuf content = chnl.alloc().buffer();
            content.writeBytes("udp message".getBytes());
            chnl.writeAndFlush(new DatagramPacket(content, new InetSocketAddress("127.0.0.1", 9002)));
            System.out.println("send message");
        }

        @Override
        public void channelRead(ChannelHandlerContext ctx, Object msg) throws Exception{
            DatagramPacket pkg = (DatagramPacket)msg;
            ByteBuf content = pkg.content();
            InetSocketAddress from = pkg.sender();
            System.out.println("recv: "+content.toString(Charset.forName("utf-8"))+" from:"+from.toString());
        }
    }
}

　　NioDatagramChannel和NioSocketChannel的初始化过程大致相同。它们的不同点是，NioSocketChannel在connect之后处于active状态，NioDatagramChannel是在bind之后处于才处于active状态。