Netty采用的是主从Reactor多线程的模型，参考Scalable IO in Java，但netty的subReactor为一个组

一、从FileServer服务器示例入手 

public final class FileServer {static final boolean SSL = System.getProperty("ssl") != null;// Use the same default port with the telnet example so that we can use the telnet client example to access it.static final int PORT = Integer.parseInt(System.getProperty("port", SSL? "8992" : "8023"));public static void main(String[] args) throws Exception {// Configure SSL.final SslContext sslCtx = ServerUtil.buildSslContext();// Configure the server.主从Reactor线程组EventLoopGroup bossGroup = new NioEventLoopGroup(1);EventLoopGroup workerGroup = new NioEventLoopGroup();try {ServerBootstrap b = new ServerBootstrap();b.group(bossGroup, workerGroup)//配置主Reactor中的channel类型.channel(NioServerSocketChannel.class)// 设置主Reactor中channel的option选项，设置底层JDK NIO Socket的一些选项.option(ChannelOption.SO_BACKLOG, 100)//设置主Reactor中Channel->pipline->handler.handler(new LoggingHandler(LogLevel.INFO))//设置 SocketChannel 对应的 Handler；.childHandler(new ChannelInitializer<SocketChannel>() {@Overridepublic void initChannel(SocketChannel ch) throws Exception {ChannelPipeline p = ch.pipeline();if (sslCtx != null) {p.addLast(sslCtx.newHandler(ch.alloc()));}p.addLast(new StringEncoder(CharsetUtil.UTF_8),new LineBasedFrameDecoder(8192),new StringDecoder(CharsetUtil.UTF_8),new ChunkedWriteHandler(),new FileServerHandler());}});// Start the server.ChannelFuture f = b.bind(PORT).sync();// Wait until the server socket is closed.f.channel().closeFuture().sync();} finally {// Shut down all event loops to terminate all threads.bossGroup.shutdownGracefully();workerGroup.shutdownGracefully();}}
}

1.1  netty的主从模式    

       首先大致了解netty的主从模式中：bossGroup 中的MainReactor管理的Channel类型为NioServerSocketChannel，用来监听端口，接收客户端连接，为客户端创建初始化NioSocketChannel，然后采用round-robin轮询的方式从workerGroup中选择一个SubReactor与该客户端NioSocketChannel进行绑定。一个SubReactor线程负责处理多个NioSocketChannel上的IO事件

1.2 NioServerSocketChannel

 包含了JDK原生的ServerSocketChannel属性

1.2.1 channel(NioServerSocketChannel.class)

在执行channel的时候，返回的是channelFactory属性，如下：

return channelFactory(new ReflectiveChannelFactory<C>(ObjectUtil.checkNotNull(channelClass, "channelClass")// ReflectiveChannelFactory通过泛型，反射，工厂的方式灵活创建不同类型的channel
public class ReflectiveChannelFactory<T extends Channel> implements ChannelFactory<T> {private final Constructor<? extends T> constructor;public ReflectiveChannelFactory(Class<? extends T> clazz) {ObjectUtil.checkNotNull(clazz, "clazz");try {this.constructor = clazz.getConstructor();} catch (NoSuchMethodException e) {throw new IllegalArgumentException("Class " + StringUtil.simpleClassName(clazz) +" does not have a public non-arg constructor", e);}}@Overridepublic T newChannel() {try {return constructor.newInstance();} catch (Throwable t) {throw new ChannelException("Unable to create Channel from class " + constructor.getDeclaringClass(), t);}}@Overridepublic String toString() {return StringUtil.simpleClassName(ReflectiveChannelFactory.class) +'(' + StringUtil.simpleClassName(constructor.getDeclaringClass()) + ".class)";}
}

1.3 ChannelInitializer的作用

      Pipeline添加ChannelHandler：1、显式添加的方式是由用户在main线程中通过ServerBootstrap#handler的方式添加。2、如果需要添加多个ChannelHandler，则可以通过ChannelInitializer向pipeline中进行添加。

1.3.1 childHandler(new ChannelInitializer<SocketChannel>() {}使用的原因：

      NioSocketChannel是在服务端accept连接后，在服务端NioServerSocketChannel中被创建出来的。但是此时我们正处于配置ServerBootStrap阶段，服务端还没有启动，更没有客户端连接上来，此时客户端NioSocketChannel还没有被创建出来，所以也就没办法向客户端NioSocketChannel的pipeline中添加ChannelHandler。 以及客户端NioSocketChannel中Pipeline里可以添加任意多个ChannelHandler，但是Netty框架无法预知用户到底需要添加多少个ChannelHandler，所以Netty框架提供了回调函数ChannelInitializer#initChannel，使用户可以自定义ChannelHandler的添加行为。

二、服务端启动全过程

    public ChannelFuture bind(int inetPort) {return bind(new InetSocketAddress(inetPort));}public ChannelFuture bind(SocketAddress localAddress) {//校验Netty核心组件是否配置齐全validate();//服务端开始启动，绑定端口地址，接收客户端连接return doBind(ObjectUtil.checkNotNull(localAddress, "localAddress"));}private ChannelFuture doBind(final SocketAddress localAddress) {//异步创建，初始化，注册ServerSocketChannel到main reactor上final ChannelFuture regFuture = initAndRegister();final Channel channel = regFuture.channel();if (regFuture.cause() != null) {return regFuture;}if (regFuture.isDone()) {   ........serverSocketChannel向Main Reactor注册成功后开始绑定端口....,               } else {//如果此时注册操作没有完成，则向regFuture添加operationComplete回调函数，注册成功后回调。regFuture.addListener(new ChannelFutureListener() {@Overridepublic void operationComplete(ChannelFuture future) throws Exception {........serverSocketChannel向Main Reactor注册成功后开始绑定端口...., });return promise;}}

2.1 初始化并注册channel

    final ChannelFuture initAndRegister() {Channel channel = null;try {// io.netty.channel.ReflectiveChannelFactory.newChannelchannel = channelFactory.newChannel();// 初始化channelinit(channel);} catch (Throwable t) {if (channel != null) {// channel can be null if newChannel crashed (eg SocketException("too many open files"))channel.unsafe().closeForcibly();// as the Channel is not registered yet we need to force the usage of the GlobalEventExecutorreturn new DefaultChannelPromise(channel, GlobalEventExecutor.INSTANCE).setFailure(t);}// as the Channel is not registered yet we need to force the usage of the GlobalEventExecutorreturn new DefaultChannelPromise(new FailedChannel(), GlobalEventExecutor.INSTANCE).setFailure(t);}ChannelFuture regFuture = config().group().register(channel);if (regFuture.cause() != null) {if (channel.isRegistered()) {channel.close();} else {channel.unsafe().closeForcibly();}}// If we are here and the promise is not failed, it's one of the following cases:// 1) If we attempted registration from the event loop, the registration has been completed at this point.//    i.e. It's safe to attempt bind() or connect() now because the channel has been registered.// 2) If we attempted registration from the other thread, the registration request has been successfully//    added to the event loop's task queue for later execution.//    i.e. It's safe to attempt bind() or connect() now://         because bind() or connect() will be executed *after* the scheduled registration task is executed//         because register(), bind(), and connect() are all bound to the same thread.return regFuture;}

2.1.1 channelFactory.newChannel();

    根据1.2.1 可以知道，实际就是调用return constructor.newInstance();也就是实例化NioServerSocketChannel

public class NioServerSocketChannel extends AbstractNioMessageChannelimplements io.netty.channel.socket.ServerSocketChannel {//SelectorProvider(用于创建Selector和Selectable Channels)private static final SelectorProvider DEFAULT_SELECTOR_PROVIDER = SelectorProvider.provider();/*** Create a new instance*/public NioServerSocketChannel() {this(DEFAULT_SELECTOR_PROVIDER);}/*** Create a new instance using the given {@link SelectorProvider}.*/public NioServerSocketChannel(SelectorProvider provider) {this(provider, null);}/*** Create a new instance using the given {@link SelectorProvider} and protocol family (supported only since JDK 15).*/public NioServerSocketChannel(SelectorProvider provider, InternetProtocolFamily family) {this(newChannel(provider, family));}/*** Create a new instance using the given {@link ServerSocketChannel}.*/public NioServerSocketChannel(ServerSocketChannel channel) {super(null, channel, SelectionKey.OP_ACCEPT);config = new NioServerSocketChannelConfig(this, javaChannel().socket());}}

2.1.1.1 SelectorProvider选择器和可选择通道的服务提供者类

    public static SelectorProvider provider() {synchronized (lock) {if (provider != null)return provider;return AccessController.doPrivileged(new PrivilegedAction<SelectorProvider>() {public SelectorProvider run() {if (loadProviderFromProperty())return provider;if (loadProviderAsService())return provider;provider = sun.nio.ch.DefaultSelectorProvider.create();return provider;}});}}// 支持根据系统属性名进行实例化。private static boolean loadProviderFromProperty() {String cn = System.getProperty("java.nio.channels.spi.SelectorProvider");if (cn == null)return false;try {Class<?> c = Class.forName(cn, true,ClassLoader.getSystemClassLoader());provider = (SelectorProvider)c.newInstance();return true;} catch (ClassNotFoundException x) {throw new ServiceConfigurationError(null, x);} catch (IllegalAccessException x) {throw new ServiceConfigurationError(null, x);} catch (InstantiationException x) {throw new ServiceConfigurationError(null, x);} catch (SecurityException x) {throw new ServiceConfigurationError(null, x);}}// 根据spi进行实例化，即META-INF/services/下的定义名为java.nio.channels.spi.SelectorProvider的SPI文件，文件中第一个定义的SelectorProvider实现类全限定名就会被加载。private static boolean loadProviderAsService() {ServiceLoader<SelectorProvider> sl =ServiceLoader.load(SelectorProvider.class,ClassLoader.getSystemClassLoader());Iterator<SelectorProvider> i = sl.iterator();for (;;) {try {if (!i.hasNext())return false;provider = i.next();return true;} catch (ServiceConfigurationError sce) {if (sce.getCause() instanceof SecurityException) {// Ignore the security exception, try the next providercontinue;}throw sce;}}}//因为是windows
public class DefaultSelectorProvider {private DefaultSelectorProvider() {}public static SelectorProvider create() {return new WindowsSelectorProvider();}
}

nio中的channel注册selector 

2.1.1.2 newChannel(provider, family) 

    private static ServerSocketChannel newChannel(SelectorProvider provider, InternetProtocolFamily family) {try {// family为空时 SelectorProviderUtil.newChannel 返回nullServerSocketChannel channel =SelectorProviderUtil.newChannel(OPEN_SERVER_SOCKET_CHANNEL_WITH_FAMILY, provider, family);// 创建 JDK 底层的 ServerSocketChannelreturn channel == null ? provider.openServerSocketChannel() : channel;} catch (IOException e) {throw new ChannelException("Failed to open a socket.", e);}}

因为初始化的时候family为null，所以调用的是JDK底层的openServerSocketChannel

2.1.1.3  NioServerSocketChannel构造

        //设置的是SelectionKey.OP_ACCEPT事件super(null, channel, SelectionKey.OP_ACCEPT);// 创建Channel的配置类NioServerSocketChannelConfig，在配置类中封装了对Channel底层的一些配置行为，以及JDK中的ServerSocket。以及创建NioServerSocketChannel接收数据用的Buffer分配器AdaptiveRecvByteBufAllocatorconfig = new NioServerSocketChannelConfig(this, javaChannel().socket());protected AbstractNioChannel(Channel parent, SelectableChannel ch, int readInterestOp) {super(parent);this.ch = ch;this.readInterestOp = readInterestOp;try {// 设置 Channel 为非阻塞模式。ch.configureBlocking(false);} catch (IOException e) {try {ch.close();} catch (IOException e2) {logger.warn("Failed to close a partially initialized socket.", e2);}throw new ChannelException("Failed to enter non-blocking mode.", e);}}protected AbstractChannel(Channel parent) {this.parent = parent;id = newId(); // 全局唯一idunsafe = newUnsafe(); // unsafe 操作底层读写pipeline = newChannelPipeline(); // pipeline 负责业务处理器编排}protected DefaultChannelPipeline newChannelPipeline() {return new DefaultChannelPipeline(this);}protected DefaultChannelPipeline(Channel channel) {this.channel = ObjectUtil.checkNotNull(channel, "channel");succeededFuture = new SucceededChannelFuture(channel, null);voidPromise =  new VoidChannelPromise(channel, true);tail = new TailContext(this);head = new HeadContext(this);head.next = tail;tail.prev = head;}

此时channel的pipeline只有head和tail两个节点；

2.1.2 init(channel);初始化

    @Overridevoid init(Channel channel) {setChannelOptions(channel, newOptionsArray(), logger);setAttributes(channel, newAttributesArray());ChannelPipeline p = channel.pipeline();final EventLoopGroup currentChildGroup = childGroup;final ChannelHandler currentChildHandler = childHandler;final Entry<ChannelOption<?>, Object>[] currentChildOptions = newOptionsArray(childOptions);final Entry<AttributeKey<?>, Object>[] currentChildAttrs = newAttributesArray(childAttrs);//  ChannelInitializer 实现的 initChannel() 方法用于添加 ServerSocketChannel 对应的 Handlerp.addLast(new ChannelInitializer<Channel>() {@Overridepublic void initChannel(final Channel ch) {final ChannelPipeline pipeline = ch.pipeline();// 将handler(new LoggingHandler(LogLevel.INFO)) 中的handler加入pipeLineChannelHandler handler = config.handler();if (handler != null) {pipeline.addLast(handler);}//  然后 Netty 通过异步 task 的方式又向 Pipeline 一个处理器 ServerBootstrapAcceptor，这是一个连接接入器，专门用于接收新的连接，然后把事件分发给 EventLoop 执行ch.eventLoop().execute(new Runnable() {@Overridepublic void run() {pipeline.addLast(new ServerBootstrapAcceptor(ch, currentChildGroup, currentChildHandler, currentChildOptions, currentChildAttrs));}});}});}

• ServerBootstrapAcceptor：也就是对应MainReactor中的acceptor,本质上也是一种ChannelHandler，主要负责在客户端连接建立好后，初始化客户端NioSocketChannel，在从Reactor线程组中选取一个SubReactor，将客户端NioSocketChannel注册到SubReactor中的selector上。

• 初始化NioServerSocketChannel中pipeline的时机是：当NioServerSocketChannel注册到Main Reactor之后，绑定端口地址之前，同时为了保证线程安全地初始化pipeline，初始化的动作netty统一交给了Reactor线程进行

• ServerBootstrapAcceptor 的注册过程为什么又需要封装成异步 task 呢？因为本文案例是

  handler(new LoggingHandler(LogLevel.INFO))但是考虑到过程中可能为new ChannelInitializer<SocketChannel>() ，那么在后续Main Reactor处理register0任务invokeHandlerAddedIfNeeded方法时会调用具体的ChannelInitializer的initChannel方法进行实例会进行添加到最后一个处理节点，如果这里不是异步task那么就会导致该Acceptor为pipeline的一个中间Handler，因此为了保证ServerBootstrapAcceptor是最后一个处理节点，所以本文就封装了一个异步任务。 等到新连接接入时，就可以调用pipeline.fireChannelRead();从head节点依次往下进行传播，直到传播到ServerBootstrapAcceptor

2.2 注册channel到mainReactor中

2.2.1 轮询选取MainReactor

 ChannelFuture regFuture = config().group().register(channel);
从ServerBootstrap获取主Reactor线程组NioEventLoopGroup，将NioServerSocketChannel注册到NioEventLoopGroup中。@Overridepublic ChannelFuture register(Channel channel) {return next().register(channel);}@Overridepublic EventExecutor next() {return chooser.next();}//获取绑定策略@Overridepublic EventExecutorChooser newChooser(EventExecutor[] executors) {// 判断2的次幂if (isPowerOfTwo(executors.length)) {return new PowerOfTwoEventExecutorChooser(executors);} else {return new GenericEventExecutorChooser(executors);}}//采用轮询round-robin的方式选择Reactor@Overridepublic EventExecutor next() {return executors[(int) Math.abs(idx.getAndIncrement() % executors.length)];}private static boolean isPowerOfTwo(int val) {return (val & -val) == val;}正数的补码，反码，原码都是一样的。
负数的补码为反码加1，负数的反码为除符号位原码按位取反。

2.2.2 register

    @Overridepublic ChannelFuture register(Channel channel) {return register(new DefaultChannelPromise(channel, this));}@Overridepublic ChannelFuture register(final ChannelPromise promise) {ObjectUtil.checkNotNull(promise, "promise");promise.channel().unsafe().register(this, promise);return promise;}@Overridepublic final void register(EventLoop eventLoop, final ChannelPromise promise) {ObjectUtil.checkNotNull(eventLoop, "eventLoop");if (isRegistered()) {promise.setFailure(new IllegalStateException("registered to an event loop already"));return;}if (!isCompatible(eventLoop)) {promise.setFailure(new IllegalStateException("incompatible event loop type: " + eventLoop.getClass().getName()));return;}//在channel上设置绑定的ReactorAbstractChannel.this.eventLoop = eventLoop;/*** 执行channel注册的操作必须是Reactor线程来完成** 1: 如果当前执行线程是Reactor线程，则直接执行register0进行注册* 2：如果当前执行线程是外部线程，则需要将register0注册操作 封装程异步Task 由Reactor线程执行* */if (eventLoop.inEventLoop()) {register0(promise);} else {//外部线程调用try {eventLoop.execute(new Runnable() {@Overridepublic void run() {register0(promise);}});} catch (Throwable t) {logger.warn("Force-closing a channel whose registration task was not accepted by an event loop: {}",AbstractChannel.this, t);closeForcibly();closeFuture.setClosed();safeSetFailure(promise, t);}}}

当前执行线程并不是Reactor线程，而是用户程序的启动线程Main线程,所以提交异步task并进行了启动Reactor线程

//Reactor线程的启动是在向Reactor提交第一个异步任务的时候启动的。private void execute(Runnable task, boolean immediate) {boolean inEventLoop = inEventLoop();addTask(task);if (!inEventLoop) {startThread();if (isShutdown()) {boolean reject = false;try {if (removeTask(task)) {reject = true;}} catch (UnsupportedOperationException e) {// The task queue does not support removal so the best thing we can do is to just move on and// hope we will be able to pick-up the task before its completely terminated.// In worst case we will log on termination.}if (reject) {reject();}}}if (!addTaskWakesUp && immediate) {wakeup(inEventLoop);}}private void startThread() {if (state == ST_NOT_STARTED) {if (STATE_UPDATER.compareAndSet(this, ST_NOT_STARTED, ST_STARTED)) {boolean success = false;try {doStartThread();success = true;} finally {if (!success) {STATE_UPDATER.compareAndSet(this, ST_STARTED, ST_NOT_STARTED);}}}}}

2.2.3 register0()-MainReactor异步任务执行

//一开始Reactor中的任务队列中只有一个任务register0，Reactor线程启动后，会从任务队列中取出任务执行。
private void register0(ChannelPromise promise) {try {// check if the channel is still open as it could be closed in the mean time when the register// call was outside of the eventLoopif (!promise.setUncancellable() || !ensureOpen(promise)) {return;}boolean firstRegistration = neverRegistered;// 调用 JDK 底层的 register() 进行注册doRegister();neverRegistered = false;registered = true;// Ensure we call handlerAdded(...) before we actually notify the promise. This is needed as the// user may already fire events through the pipeline in the ChannelFutureListener.//回调pipeline中添加的ChannelInitializer的handlerAdded方法，在这里初始化channelPipelinepipeline.invokeHandlerAddedIfNeeded(); // 触发 handlerAdded 事件safeSetSuccess(promise);// channelRegistered 事件是由 fireChannelRegistered() 方法触发，沿着 Pipeline 的 Head 节点传播到 Tail 节点pipeline.fireChannelRegistered();// Only fire a channelActive if the channel has never been registered. This prevents firing// multiple channel actives if the channel is deregistered and re-registered.//对于服务端ServerSocketChannel来说 只有绑定端口地址成功后 channel的状态才是active的。//此时绑定操作作为异步任务在Reactor的任务队列中，绑定操作还没开始，所以这里的isActive()是false                if (isActive()) {if (firstRegistration) {pipeline.fireChannelActive();} else if (config().isAutoRead()) {// This channel was registered before and autoRead() is set. This means we need to begin read// again so that we process inbound data.//// See https://github.com/netty/netty/issues/4805beginRead();}}} catch (Throwable t) {// Close the channel directly to avoid FD leak.closeForcibly();closeFuture.setClosed();safeSetFailure(promise, t);}}

2.2.3.1 doRegister

    @Overrideprotected void doRegister() throws Exception {boolean selected = false;for (;;) {try {selectionKey = javaChannel().register(eventLoop().unwrappedSelector(), 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.eventLoop().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;}}}}public final SelectionKey register(Selector sel, int ops,Object att)throws ClosedChannelException{synchronized (regLock) {if (!isOpen())throw new ClosedChannelException();if ((ops & ~validOps()) != 0)throw new IllegalArgumentException();if (blocking)throw new IllegalBlockingModeException();SelectionKey k = findKey(sel);if (k != null) {k.interestOps(ops);k.attach(att);}if (k == null) {// New registrationsynchronized (keyLock) {if (!isOpen())throw new ClosedChannelException();k = ((AbstractSelector)sel).register(this, ops, att);addKey(k);}}return k;}}

       javaChannel().register() 负责调用 JDK 底层，将 Channel 注册到 Selector 上，register() 的第三个入参传入的是 Netty 自己实现的 Channel 对象，调用 register() 方法会将它绑定在 JDK 底层 Channel 的attachment上。这样在每次 Selector 对象进行事件循环时，Netty 都可以从返回的 JDK 底层 Channel 中获得自己的 Channel 对象。

2.2.3.2 handlerAdded

初始化ChannelPipeline的时机是当Channel向对应的Reactor注册成功后，在handlerAdded事件回调中利用ChannelInitializer进行初始化。

io.netty.channel.ChannelInitializer#handlerAdded

    @Overridepublic void handlerAdded(ChannelHandlerContext ctx) throws Exception {// 必须是注册以后if (ctx.channel().isRegistered()) {// This should always be true with our current DefaultChannelPipeline implementation.// The good thing about calling initChannel(...) in handlerAdded(...) is that there will be no ordering// surprises if a ChannelInitializer will add another ChannelInitializer. This is as all handlers// will be added in the expected order.if (initChannel(ctx)) {// We are done with init the Channel, removing the initializer now.removeState(ctx);}}}//ChannelInitializer实例是被所有的Channel共享的，用于初始化ChannelPipeline//通过Set集合保存已经初始化的ChannelPipeline，避免重复初始化同一ChannelPipelineprivate final Set<ChannelHandlerContext> initMap = Collections.newSetFromMap(new ConcurrentHashMap<ChannelHandlerContext, Boolean>());private boolean initChannel(ChannelHandlerContext ctx) throws Exception {if (initMap.add(ctx)) { // Guard against re-entrance.try {initChannel((C) ctx.channel());} catch (Throwable cause) {exceptionCaught(ctx, cause);} finally {ChannelPipeline pipeline = ctx.pipeline();if (pipeline.context(this) != null) {//初始化完毕后，从pipeline中移除自身pipeline.remove(this);}}return true;}return false;}//匿名类实现，这里指定具体的初始化逻辑protected abstract void initChannel(C ch) throws Exception;private void removeState(final ChannelHandlerContext ctx) {//从initMap防重Set集合中删除ChannelInitializerif (ctx.isRemoved()) {initMap.remove(ctx);} else {ctx.executor().execute(new Runnable() {@Overridepublic void run() {initMap.remove(ctx);}});}}

执行完成后pipeline如下：

执行完整个 register0() 的注册流程之后，EventLoop 线程会将 ServerBootstrapAcceptor 添加到 Pipeline 当中(提交的任务执行)

ch.eventLoop().execute(new Runnable() {@Overridepublic void run() {pipeline.addLast(new ServerBootstrapAcceptor(ch, currentChildGroup, currentChildHandler, currentChildOptions, currentChildAttrs));}}

也就是统一在EventLoop 线程中初始化pipeLine，保证线程安全

2.2.4 Reactor线程触发注册成功safeSetSuccess(promise);

Reactor设置注册成功后，启动线程监听到完成任务，那么就进行接下来的绑定端口操作

        // 若执行完毕进行端口绑定if (regFuture.isDone()) {// At this point we know that the registration was complete and successful.ChannelPromise promise = channel.newPromise();doBind0(regFuture, channel, localAddress, promise);return promise;} else {// Registration future is almost always fulfilled already, but just in case it's not.final PendingRegistrationPromise promise = new PendingRegistrationPromise(channel);regFuture.addListener(new ChannelFutureListener() {@Overridepublic void operationComplete(ChannelFuture future) throws Exception {Throwable cause = future.cause();if (cause != null) {// Registration on the EventLoop failed so fail the ChannelPromise directly to not cause an// IllegalStateException once we try to access the EventLoop of the Channel.promise.setFailure(cause);} else {// Registration was successful, so set the correct executor to use.// See https://github.com/netty/netty/issues/2586promise.registered();doBind0(regFuture, channel, localAddress, promise);}}});return promise;}

2.3 端口绑定

    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
sss if (regFuture.isSuccess()) {channel.bind(localAddress, promise).addListener(ChannelFutureListener.CLOSE_ON_FAILURE);} else {promise.setFailure(regFuture.cause());}}});}

提交到异步任务到Reactor，绑定逻辑需要注册逻辑处理完之后运行，如上面的ServerBootstrapAcceptor异步任务执行完

    @Overridepublic ChannelFuture bind(SocketAddress localAddress, ChannelPromise promise) {return pipeline.bind(localAddress, promise);}@Overridepublic final ChannelFuture bind(SocketAddress localAddress, ChannelPromise promise) {return tail.bind(localAddress, promise);}@Overridepublic ChannelFuture bind(final SocketAddress localAddress, final ChannelPromise promise) {ObjectUtil.checkNotNull(localAddress, "localAddress");if (isNotValidPromise(promise, false)) {// cancelledreturn promise;}final AbstractChannelHandlerContext next = findContextOutbound(MASK_BIND);EventExecutor executor = next.executor();if (executor.inEventLoop()) {next.invokeBind(localAddress, promise);} else {safeExecute(executor, new Runnable() {@Overridepublic void run() {next.invokeBind(localAddress, promise);}}, promise, null, false);}return promise;}private void invokeBind(SocketAddress localAddress, ChannelPromise promise) {if (invokeHandler()) {try {// DON'T CHANGE// Duplex handlers implements both out/in interfaces causing a scalability issue// see https://bugs.openjdk.org/browse/JDK-8180450final ChannelHandler handler = handler();final DefaultChannelPipeline.HeadContext headContext = pipeline.head;if (handler == headContext) {headContext.bind(this, localAddress, promise);} else if (handler instanceof ChannelDuplexHandler) {((ChannelDuplexHandler) handler).bind(this, localAddress, promise);} else {((ChannelOutboundHandler) handler).bind(this, localAddress, promise);}} catch (Throwable t) {notifyOutboundHandlerException(t, promise);}} else {bind(localAddress, promise);}}

调用pipeline.bind(localAddress, promise)在pipeline中传播bind事件，触发回调pipeline中所有ChannelHandler的bind方法。

事件在pipeline中的传播具有方向性：

• inbound事件从HeadContext开始逐个向后传播直到TailContext。

• outbound事件则是反向传播，从TailContext开始反向向前传播直到HeadContext。

•     private AbstractChannelHandlerContext findContextOutbound(int mask) {AbstractChannelHandlerContext ctx = this;EventExecutor currentExecutor = executor();do {ctx = ctx.prev;
  // 跳过了 private static class ServerBootstrapAcceptor extends ChannelInboundHandlerAdapter} while (skipContext(ctx, currentExecutor, mask, MASK_ONLY_OUTBOUND));return ctx;}private static boolean skipContext(AbstractChannelHandlerContext ctx, EventExecutor currentExecutor, int mask, int onlyMask) {// Ensure we correctly handle MASK_EXCEPTION_CAUGHT which is not included in the MASK_EXCEPTION_CAUGHTreturn (ctx.executionMask & (onlyMask | mask)) == 0 ||// We can only skip if the EventExecutor is the same as otherwise we need to ensure we offload// everything to preserve ordering.//// See https://github.com/netty/netty/issues/10067(ctx.executor() == currentExecutor && (ctx.executionMask & mask) == 0);}

通过上面代码可以知道bind事件在Netty中被定义为outbound事件，所以它在pipeline中是反向传播。先从TailContext开始反向传播直到HeadContext。

因此bind的核心逻辑也正是实现在HeadContext中。

headContext.bind(this, localAddress, promise);

底层实际就是

        @Overridepublic void bind(ChannelHandlerContext ctx, SocketAddress localAddress, ChannelPromise promise) {unsafe.bind(localAddress, promise);}@Overridepublic final void bind(final SocketAddress localAddress, final ChannelPromise promise) {assertEventLoop();if (!promise.setUncancellable() || !ensureOpen(promise)) {return;}// See: https://github.com/netty/netty/issues/576if (Boolean.TRUE.equals(config().getOption(ChannelOption.SO_BROADCAST)) &&localAddress instanceof InetSocketAddress &&!((InetSocketAddress) localAddress).getAddress().isAnyLocalAddress() &&!PlatformDependent.isWindows() && !PlatformDependent.maybeSuperUser()) {// Warn a user about the fact that a non-root user can't receive a// broadcast packet on *nix if the socket is bound on non-wildcard address.logger.warn("A non-root user can't receive a broadcast packet if the socket " +"is not bound to a wildcard address; binding to a non-wildcard " +"address (" + localAddress + ") anyway as requested.");}boolean wasActive = isActive();try {doBind(localAddress);} catch (Throwable t) {safeSetFailure(promise, t);closeIfClosed();return;}if (!wasActive && isActive()) {invokeLater(new Runnable() {@Overridepublic void run() {pipeline.fireChannelActive();}});}safeSetSuccess(promise);}@SuppressJava6Requirement(reason = "Usage guarded by java version check")@Overrideprotected void doBind(SocketAddress localAddress) throws Exception {if (PlatformDependent.javaVersion() >= 7) {javaChannel().bind(localAddress, config.getBacklog());} else {javaChannel().socket().bind(localAddress, config.getBacklog());}}

Netty 会根据 JDK 版本的不同，分别调用 JDK 底层不同的 bind() 方法。我使用的是 JDK8，所以会调用 JDK 原生 Channel 的 bind() 方法。执行完 doBind() 之后，服务端 JDK 原生的 Channel 真正已经完成端口绑定了。

2.3.1 判断是否为激活

    @Overridepublic boolean isActive() {// As java.nio.ServerSocketChannel.isBound() will continue to return true even after the channel was closed// we will also need to check if it is open.return isOpen() && javaChannel().socket().isBound();}

2.3.2 channelActive事件

   完成端口绑定之后，Channel 处于活跃 Active 状态，然后会调用 pipeline.fireChannelActive() 方法触发 channelActive 事件。

    channelActive事件在Netty中定义为inbound事件，所以它在pipeline中的传播为正向传播，从HeadContext一直到TailContext为止。

在channelActive事件回调中需要触发向Selector指定需要监听的IO事件~~OP_ACCEPT事件。

    @Overridepublic final ChannelPipeline fireChannelActive() {AbstractChannelHandlerContext.invokeChannelActive(head);return this;}private void invokeChannelActive() {if (invokeHandler()) {try {// DON'T CHANGE// Duplex handlers implements both out/in interfaces causing a scalability issue// see https://bugs.openjdk.org/browse/JDK-8180450final ChannelHandler handler = handler();final DefaultChannelPipeline.HeadContext headContext = pipeline.head;if (handler == headContext) {headContext.channelActive(this);} else if (handler instanceof ChannelDuplexHandler) {((ChannelDuplexHandler) handler).channelActive(this);} else {((ChannelInboundHandler) handler).channelActive(this);}} catch (Throwable t) {invokeExceptionCaught(t);}} else {fireChannelActive();}}@Overridepublic void channelActive(ChannelHandlerContext ctx) {//pipeline中继续向后传播channelActive事件ctx.fireChannelActive();//如果是autoRead 则自动触发read事件传播//在read回调函数中 触发OP_ACCEPT注册readIfIsAutoRead();}private void readIfIsAutoRead() {if (channel.config().isAutoRead()) {channel.read();}}

    @Overridepublic Channel read() {pipeline.read();return this;}@Overridepublic final ChannelPipeline read() {tail.read();return this;}@Overridepublic void read(ChannelHandlerContext ctx) {unsafe.beginRead();}@Overridepublic final void beginRead() {assertEventLoop();try {doBeginRead();} catch (final Exception e) {invokeLater(new Runnable() {@Overridepublic void run() {pipeline.fireExceptionCaught(e);}});close(voidPromise());}}@Overrideprotected void doBeginRead() throws Exception {// Channel.read() or ChannelHandlerContext.read() was calledfinal SelectionKey selectionKey = this.selectionKey;if (!selectionKey.isValid()) {return;}readPending = true;final int interestOps = selectionKey.interestOps();if ((interestOps & readInterestOp) == 0) {selectionKey.interestOps(interestOps | readInterestOp);}}

在执行完 channelActive 事件传播之后，会调用 readIfIsAutoRead() 方法触发 Channel 的 read 事件，而它最终调用到 AbstractNioChannel 中的 doBeginRead() 方法，其中 readInterestOp 参数就是在前面初始化 Channel 所传入的 SelectionKey.OP_ACCEPT 事件，所以 OP_ACCEPT 事件会被注册到 Channel 的事件集合中。

2.4 服务启动总结

• 创建服务端 Channel：本质是创建 JDK 底层原生的 Channel，并初始化几个重要的属性，包括 id、unsafe、pipeline 等。
• 初始化服务端 Channel：设置 Socket 参数以及用户自定义属性，并添加两个特殊的处理器 ChannelInitializer 和 ServerBootstrapAcceptor。
• 注册服务端 Channel：调用 JDK 底层将 Channel 注册到 Selector 上。
• 端口绑定：调用 JDK 底层进行端口绑定，并触发 channelActive 事件，把 OP_ACCEPT 事件注册到 Channel 的事件集合中。