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
1 import io.netty.buffer.ByteBuf; 2 import io.netty.channel.*; 3 import io.netty.channel.nio.NioEventLoopGroup; 4 import io.netty.channel.socket.nio.NioServerSocketChannel; 5 6 import java.net.InetSocketAddress; 7 import java.nio.charset.Charset; 8 9 public class TcpServer {10 private NioEventLoopGroup group = new NioEventLoopGroup();11 12 public static void main(String[] argc){13 TcpServer server = new TcpServer();14 server.start();15 16 while(true){17 try{18 Thread.sleep(1000);19 }catch (Exception e){20 break;21 }22 }23 24 server.stop();25 }26 27 public void start(){28 NioServerSocketChannel server = new NioServerSocketChannel();29 30 ChannelPipeline pipleline = server.pipeline();31 pipleline.addFirst(new ServerHandler());32 33 group.register(server).addListener(new ChannelFutureListener() {34 @Override35 public void operationComplete(ChannelFuture future) throws Exception {36 server.bind(new InetSocketAddress(9001));37 System.out.println("server listen add:"+9001);38 }39 });40 }41 public void stop(){42 group.shutdownGracefully();43 }44 45 private class ServerHandler extends ChannelInboundHandlerAdapter{46 47 @Override48 public void channelRead(ChannelHandlerContext ctx, Object msg) throws Exception{49 Channel child = (Channel)msg;50 51 child.pipeline().addLast(new ChildHandler());52 53 group.register(child);54 55 }56 }57 58 private class ChildHandler extends ChannelInboundHandlerAdapter{59 60 @Override61 public void channelActive(ChannelHandlerContext ctx) throws Exception {62 System.out.println("connected");63 }64 65 @Override66 public void channelInactive(ChannelHandlerContext ctx) throws Exception {67 System.out.println("closed");68 }69 70 @Override71 public void channelRead(ChannelHandlerContext ctx, Object msg) throws Exception{72 Channel chnl = ctx.channel();73 ByteBuf data = (ByteBuf)msg;74 75 System.out.println("recv: "+data.toString(Charset.forName("utf-8")));76 chnl.write(msg);77 }78 79 @Override80 public void channelReadComplete(ChannelHandlerContext ctx) throws Exception {81 ctx.channel().flush();82 }83 84 }85 }
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。
原文出处：https://www.cnblogs.com/brandonli/p/10210592.html
作者：自带buff
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