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>() {
                 @Override
                 public 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);
        }
    }

    @Override
    public T newChannel() {
        try {
            return constructor.newInstance();
        } catch (Throwable t) {
            throw new ChannelException("Unable to create Channel from class " + constructor.getDeclaringClass(), t);
        }
    }

    @Override
    public 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() {
                @Override
                public 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.newChannel
            channel = channelFactory.newChannel();
            // 初始化channel
            init(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 GlobalEventExecutor
                return new DefaultChannelPromise(channel, GlobalEventExecutor.INSTANCE).setFailure(t);
            }
            // as the Channel is not registered yet we need to force the usage of the GlobalEventExecutor
            return 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 AbstractNioMessageChannel
                             implements 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 provider
                    continue;
                }
                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null
            ServerSocketChannel channel =
                    SelectorProviderUtil.newChannel(OPEN_SERVER_SOCKET_CHANNEL_WITH_FAMILY, provider, family);
            // 创建 JDK 底层的 ServerSocketChannel
            return 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AdaptiveRecvByteBufAllocator
        config = 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id
        unsafe = 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

    @Override
    void 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 Handler
        p.addLast(new ChannelInitializer<Channel>() {
            @Override
            public void initChannel(final Channel ch) {
                final ChannelPipeline pipeline = ch.pipeline();
                // 将handler(new LoggingHandler(LogLevel.INFO)) 中的handler加入pipeLine
                ChannelHandler 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() {
                    @Override
                    public 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。

    @Override
    public ChannelFuture register(Channel channel) {
        return next().register(channel);
    }


    @Override
    public EventExecutor next() {
        return chooser.next();
    }

    //获取绑定策略
    @Override
    public 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
    @Override
    public 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

    @Override
    public ChannelFuture register(Channel channel) {
        return register(new DefaultChannelPromise(channel, this));
    }

    @Override
    public ChannelFuture register(final ChannelPromise promise) {
        ObjectUtil.checkNotNull(promise, "promise");
        promise.channel().unsafe().register(this, promise);
        return promise;
    }

     @Override
        public 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Reactor
            AbstractChannel.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() {
                        @Override
                        public 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 eventLoop
                if (!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channelPipeline
                pipeline.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/4805
                        beginRead();
                    }
                }
            } catch (Throwable t) {
                // Close the channel directly to avoid FD leak.
                closeForcibly();
                closeFuture.setClosed();
                safeSetFailure(promise, t);
            }
        }

2.2.3.1 doRegister

    @Override
    protected 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 registration
                synchronized (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

    @Override
    public 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ChannelPipeline
    private 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ChannelInitializer
        if (ctx.isRemoved()) {
            initMap.remove(ctx);
        } else {
            ctx.executor().execute(new Runnable() {
                @Override
                public 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() {
                    @Override
                    public 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() {
                @Override
                public 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/2586
                        promise.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

    @Override
    public ChannelFuture bind(SocketAddress localAddress, ChannelPromise promise) {
        return pipeline.bind(localAddress, promise);
    }

    @Override
    public final ChannelFuture bind(SocketAddress localAddress, ChannelPromise promise) {
        return tail.bind(localAddress, promise);
    }

    @Override
    public ChannelFuture bind(final SocketAddress localAddress, final ChannelPromise promise) {
        ObjectUtil.checkNotNull(localAddress, "localAddress");
        if (isNotValidPromise(promise, false)) {
            // cancelled
            return promise;
        }

        final AbstractChannelHandlerContext next = findContextOutbound(MASK_BIND);
        EventExecutor executor = next.executor();
        if (executor.inEventLoop()) {
            next.invokeBind(localAddress, promise);
        } else {
            safeExecute(executor, new Runnable() {
                @Override
                public 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-8180450
                final 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_CAUGHT
          return (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

        @Override
        public void bind(
                ChannelHandlerContext ctx, SocketAddress localAddress, ChannelPromise promise) {
            unsafe.bind(localAddress, promise);
        }
   
         @Override
        public final void bind(final SocketAddress localAddress, final ChannelPromise promise) {
            assertEventLoop();

            if (!promise.setUncancellable() || !ensureOpen(promise)) {
                return;
            }

            // See: https://github.com/netty/netty/issues/576
            if (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() {
                    @Override
                    public void run() {
                        pipeline.fireChannelActive();
                    }
                });
            }

            safeSetSuccess(promise);
        }

  
    @SuppressJava6Requirement(reason = "Usage guarded by java version check")
    @Override
    protected 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

    @Override
    public 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。

    @Override
    public 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-8180450
                final 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();
        }
    }

        @Override
        public 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();
            }
        }

    @Override
    public Channel read() {
        pipeline.read();
        return this;
    }
    @Override
    public final ChannelPipeline read() {
        tail.read();
        return this;
    }


        @Override
        public void read(ChannelHandlerContext ctx) {
            unsafe.beginRead();
        }

        @Override
        public final void beginRead() {
            assertEventLoop();

            try {
                doBeginRead();
            } catch (final Exception e) {
                invokeLater(new Runnable() {
                    @Override
                    public void run() {
                        pipeline.fireExceptionCaught(e);
                    }
                });
                close(voidPromise());
            }
        }

    @Override
    protected void doBeginRead() throws Exception {
        // Channel.read() or ChannelHandlerContext.read() was called
        final 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。