Netty5源码解析
今天让我来总结下netty5的服务端代码。
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服务端（ServerBootstrap）
示例代码如下：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; /** * Created by yaojiafeng on 16/1/17. */ public class SimpleServer { public void bind(int port) throws Exception { // 配置服务端的NIO线程组 EventLoopGroup bossGroup = new NioEventLoopGroup(1); EventLoopGroup workerGroup = new NioEventLoopGroup(1); try { ServerBootstrap b = new ServerBootstrap(); b.group(bossGroup, workerGroup) .channel(NioServerSocketChannel.class) .option(ChannelOption.SO_BACKLOG, 1024) .childHandler(new ChildChannelHandler()); // 绑定端口，同步等待成功 ChannelFuture f = b.bind(port).sync(); // 等待服务端监听端口关闭 f.channel().closeFuture().sync(); } finally { // 优雅退出，释放线程池资源 bossGroup.shutdownGracefully(); workerGroup.shutdownGracefully(); } } private class ChildChannelHandler extends ChannelInitializer<SocketChannel> { @Override protected void initChannel(SocketChannel arg0) throws Exception { arg0.pipeline().addLast(new SimpleServerHandler()); } } /** * @param args * @throws Exception */ public static void main(String[] args) throws Exception { int port = 8081; if (args != null && args.length > 0) { try { port = Integer.valueOf(args[0]); } catch (NumberFormatException e) { // 采用默认值 } } new SimpleServer().bind(port); } }1.1. 设置EventLoopGroup
首先创建2个EventLoopGroup，一个parentGroup（用于接受新连接），childGroup（用于执行读写事件），NioEventLoopGroup内部根据设置的nEventLoops参数创建对应大小的NioEventLoop数组，并且每个NioEventLoop默认使用ForkJoinPool的一个线程，所以NioEventLoop称为单线程事件循环。1.2. 构造ServerBootstrap
构造ServerBootstrap对象，并设置EventLoopGroup，channel（NioServerSocketChannel服务端套接字），一些option例如ChannelOption.SO_BACKLOG，childHandler（客户端连接后在管道链设置的ChannelHandler）1.3. 同步绑定端口
b.bind(port).sync()1.3.1 validate方法
validate方法验证parentGroup和channelFactory不能为null1.3.2 initAndRegister方法
刚方法内部使用channelFactory通过反射构造NioServerSocketChannel的实例对象，NioServerSocketChannel实例对象构造内部主要包含ServerSocketChannel，DefaultChannelId（标识唯一性），Unsafe（所有IO操作都在这个类里）,DefaultChannelPipeline(通道处理器管道链,自定义的ChannelHandler都在这里),NioServerSocketChannelConfig(一些配置信息)。
构造完调用init初始化NioServerSocketChannel，包括设置自定义的ChannelHandler，ServerBootstrapAcceptor（专门用于接受客户端新连接时，初始化NioSocketChannel并注册进childGroup进行读写监听）。ChannelFuture regFuture = group().register(channel)异步注册NioServerSocketChannel到parentGroup里的NioEventLoop。
因为注册过程是在NioEventLoop异步执行的，这里直接先分析register方法1.3.3 异步register
channel.unsafe().register(this, promise)注册的时候会调用以上方法，委派给Unsafe的register方法，内部会给NioServerSocketChannel的字段eventLoop初始化（NioServerSocketChannel关联唯一的一个NioEventLoop），然后会调用
eventLoop.execute(new OneTimeTask() { @Override public void run() { register0(promise); } });这个会开启NioEventLoop的事件循环线程，并放task到taskQueue里，作为异步执行register0方法。
register0方法会调用外部类（NioServerSocketChannel）的doRegister方法,
protected void doRegister() throws Exception { boolean selected = false; for (;;) { try { selectionKey = javaChannel().register(((NioEventLoop) eventLoop().unwrap()).selector, 0, this); return; } catch (CancelledKeyException e) { if (!selected) { // Force the Selector to select now as the "canceled" SelectionKey may still be // cached and not removed because no Select.select(..) operation was called yet. ((NioEventLoop) eventLoop().unwrap()).selectNow(); selected = true; } else { // We forced a select operation on the selector before but the SelectionKey is still cached // for whatever reason. JDK bug ? throw e; } } } }这里使用了NIO的API，把NioServerSocketChannel里的ServerSocketChannel注册到NioServerSocketChannel关联的NioEventLoop里的selector。
接下来的safeSetSuccess会把Main线程设置的监听器，设置bind任务。1.3.4 执行bind操作
private static void doBind0( final ChannelFuture regFuture, final Channel channel, final SocketAddress localAddress, final ChannelPromise promise) { // This method is invoked before channelRegistered() is triggered. Give user handlers a chance to set up // the pipeline in its channelRegistered() implementation. channel.eventLoop().execute(new Runnable() { @Override public void run() { if (regFuture.isSuccess()) { channel.bind(localAddress, promise).addListener(ChannelFutureListener.CLOSE_ON_FAILURE); } else { promise.setFailure(regFuture.cause()); } } }); }注册成功的情况下，执行bind操作（NioServerSocketChannel的bind方法），一路追踪到
unsafe.bind(localAddress, promise);unsafe的bind方法，内部调用NioServerSocketChannel的doBind方法
protected void doBind(SocketAddress localAddress) throws Exception { javaChannel().socket().bind(localAddress, config.getBacklog()); }并且设置pipeline.fireChannelActive()任务，fireChannelActive任务会调用channel.read()方法，内部会调用到unsafe.beginRead()方法，最终调用的是NioServerSocketChannel的doBeginRead方法，重新设置SelectionKey的感兴趣的事件readInterestOp（NioServerSocketChannel构造的时候确定的为SelectionKey.OP_ACCEPT），开始接收新连接。
protected void doBeginRead() throws Exception { // Channel.read() or ChannelHandlerContext.read() was called if (inputShutdown) { return; } final SelectionKey selectionKey = this.selectionKey; if (!selectionKey.isValid()) { return; } readPending = true; final int interestOps = selectionKey.interestOps(); if ((interestOps & readInterestOp) == 0) { selectionKey.interestOps(interestOps | readInterestOp); } }1.4. NioEventLoop事件循环接受新连接
NioEventLoop不停的通过ForkJoinPool执行它的asRunnable任务（通过每次执行任务将要完成时，重新把asRunnable设置到ForkJoinPool里）。
从asRunnable的run方法开始，内部先执行selector的select操作，然后先调用processSelectedKeys()方法，获取到激活的selectedKeys数组，这里如果有新连接进来，那么就有一个SelectionKey，获取它的attachment（NioServerSocketChannel）,然后调用processSelectedKey方法。private static void processSelectedKey(SelectionKey k, AbstractNioChannel ch) { final AbstractNioChannel.NioUnsafe unsafe = ch.unsafe(); if (!k.isValid()) { // close the channel if the key is not valid anymore unsafe.close(unsafe.voidPromise()); return; } try { int readyOps = k.readyOps(); // Also check for readOps of 0 to workaround possible JDK bug which may otherwise lead // to a spin loop if ((readyOps & (SelectionKey.OP_READ | SelectionKey.OP_ACCEPT)) != 0 || readyOps == 0) { unsafe.read(); if (!ch.isOpen()) { // Connection already closed - no need to handle write. return; } } if ((readyOps & SelectionKey.OP_WRITE) != 0) { // Call forceFlush which will also take care of clear the OP_WRITE once there is nothing left to write ch.unsafe().forceFlush(); } if ((readyOps & SelectionKey.OP_CONNECT) != 0) { // remove OP_CONNECT as otherwise Selector.select(..) will always return without blocking // See https://github.com/netty/netty/issues/924 int ops = k.interestOps(); ops &= ~SelectionKey.OP_CONNECT; k.interestOps(ops); unsafe.finishConnect(); } } catch (CancelledKeyException ignored) { unsafe.close(unsafe.voidPromise()); } }1.4.1 执行获取新连接方法
if ((readyOps & (SelectionKey.OP_READ | SelectionKey.OP_ACCEPT)) != 0 || readyOps == 0) { unsafe.read(); if (!ch.isOpen()) { // Connection already closed - no need to handle write. return; } }当readyOps等于SelectionKey.OP_ACCEPT调用unsafe.read()，这里调用到了AbstractNioMessageChannel的内部类NioMessageUnsafe的read方法。
read方法会循环接受新连接，一次默认能接受16个连接，具体调用doReadMessages方法。protected int doReadMessages(List<Object> buf) throws Exception { SocketChannel ch = javaChannel().accept(); try { if (ch != null) { buf.add(new NioSocketChannel(this, ch)); return 1; } } catch (Throwable t) { logger.warn("Failed to create a new channel from an accepted socket.", t); try { ch.close(); } catch (Throwable t2) { logger.warn("Failed to close a socket.", t2); } } return 0; }这里调用NIO的API，accept方法获取SocketChannel，并封装成NioSocketChannel（NioSocketChannel的构造字段和NioServerSocketChannel类似，只是NioSocketChannel默认的感兴趣事件为SelectionKey.OP_READ）。
接受连接完成，循环调用pipeline.fireChannelRead()方法。1.4.2 ServerBootstrapAcceptor的channelRead方法
上面的管道调用fireChannelRead方法，通过责任链方式依次调用ChannelHandler的channelRead方法，最重要的就是ServerBootstrapAcceptor的channelRead方法。
它这个方法设置了childHandler到NioSocketChannel（新连接）的管道链里，然后又是异步注册NioSocketChannel到childGroup里的NioEventLoop里，注册过程和前面1.3章节的大体一致，也是启动了childGroup里的NioEventLoop的事件循环异步注册。只是因为是NioSocketChannel一些实现的方法不一样，执行的代码有点差别，最终注册完成也会调用pipeline的fireChannelActive()方法。1.4.3 fireChannelActive方法
public ChannelPipeline fireChannelActive() { head.fireChannelActive(); if (channel.config().isAutoRead()) { channel.read(); } return this; }channel的read方法最终委派调用到unsafe.beginRead()方法，然后又是NioSocketChannel的doBeginRead方法，重新设置SelectionKey的感兴趣事件为SelectionKey.OP_READ（NioSocketChannel的默认值）。到这里连接已经建立，并且开启了客户端连接读事件的监听。
PS：上面的SimpleServerHandler代码如下：
import io.netty.buffer.ByteBuf;
import io.netty.buffer.Unpooled;
import io.netty.channel.ChannelHandlerAdapter;
import io.netty.channel.ChannelHandlerContext;
/**
* Created by yaojiafeng on 16/1/17.
*/
public class SimpleServerHandler extends ChannelHandlerAdapter {
@Override
public void channelRead(ChannelHandlerContext ctx, Object msg)
throws Exception {
ByteBuf body = (ByteBuf) msg;
byte[] bytes = new byte[body.readableBytes()];
body.readBytes(bytes);
System.out.println(new String(bytes));
ByteBuf resp = Unpooled.copiedBuffer(bytes);
ctx.writeAndFlush(resp);
}
@Override
public void exceptionCaught(ChannelHandlerContext ctx, Throwable cause) {
ctx.close();
}
}