Spring架构篇–2.7.3 远程通信基础–Netty原理–bind实现端口的绑定

前言：在对ServerBootstrap 进行属性赋值之后，通过bind 方法完成端口的绑定，并开始在NioEventLoop中进行轮询进行事件的处理；本文主要探究ServersocketChannel 在netty 中是如何完成注册，以及端口的绑定

1 Nio selector 多路复用模型：

为了更好的探究netty 的channel 的注册和端口的绑定，先来回顾下Nio selector 的事件注册以及端口绑定步骤以及netty 步骤的对应：

可以看到在 nio 中的步骤，在netty 也都是有的；然后对以上步骤的关键点在netty 中实现进行探究；

2 跟踪bind 方法： server.bind(this.port).sync()

2.1 AbstractBootstrap 类 server.bind(this.port)：
AbstractBootstrap bind 完成端口的绑定和监听

public ChannelFuture bind(int inetPort) {return this.bind(new InetSocketAddress(inetPort));}

使用new InetSocketAddress(inetPort) 构建要监听的端口，然后通过bind 方法进行绑定
AbstractBootstrap 的bind 方法：

public ChannelFuture bind(SocketAddress localAddress) {// 验证父类事件处理 的EventLoopGroup 和NioServerSocketChannel 工厂类是否赋值this.validate();// 端口的绑定和启动return this.doBind((SocketAddress)ObjectUtil.checkNotNull(localAddress, "localAddress"));}

关键点 ：2.2 进入 AbstractBootstrap doBind：在这里开始真正进行channel 管道的初始化，注册占位事件，完成端口的绑定

private ChannelFuture doBind(final SocketAddress localAddress) {// 初始化NioServerSocketChannel 对象并赋值 regFuture final ChannelFuture regFuture = this.initAndRegister();// 得到NioServerSocketChannelchannel 对象final Channel channel = regFuture.channel();if (regFuture.cause() != null) {return regFuture;} else if (regFuture.isDone()) {// 通道初始化完成ChannelPromise promise = channel.newPromise();// 地址绑定doBind0(regFuture, channel, localAddress, promise);return promise;} else {final PendingRegistrationPromise promise = new PendingRegistrationPromise(channel);regFuture.addListener(new ChannelFutureListener() {public void operationComplete(ChannelFuture future) throws Exception {Throwable cause = future.cause();if (cause != null) {promise.setFailure(cause);} else {promise.registered();AbstractBootstrap.doBind0(regFuture, channel, localAddress, promise);}}});return promise;}}

关键点 ：2.2.1 this.initAndRegister()：NioServerSocketChannel 对象的初始化，以及channel 管道的占位事件注册：
AbstractBootstrap 类中 initAndRegister 方法

final ChannelFuture initAndRegister() {Channel channel = null;try {//调用NioServerSocketChannel 无参构造方法 初始化 NioServerSocketChannel 对象channel = this.channelFactory.newChannel();// NioServerSocketChannel 对象初始化this.init(channel);} catch (Throwable var3) {if (channel != null) {channel.unsafe().closeForcibly();return (new DefaultChannelPromise(channel, GlobalEventExecutor.INSTANCE)).setFailure(var3);}return (new DefaultChannelPromise(new FailedChannel(), GlobalEventExecutor.INSTANCE)).setFailure(var3);}// 从父类group 中的EventLoopGroup 选出一个EventLoop 并注册到channel 管道上ChannelFuture regFuture = this.config().group().register(channel);if (regFuture.cause() != null) {// 注册失败 关闭ServerSocketChannel通道if (channel.isRegistered()) {channel.close();} else {channel.unsafe().closeForcibly();}}// 返回注册的结果， 由于是异步的方法 返回regFuture return regFuture;}

关键点 2.2.1.1 this.channelFactory.newChannel() 的方法：NioServerSocketChannel 以及jdk 原生的ServerSocketChannel 对象创建：

（1） this.channelFactory.newChannel() 通过反射机制调用NioServerSocketChannel 的鬼样子方法中，进行jdk 原生的ServerSocketChannel 对象创建：

// 获取 WindowsSelectorProvider 的实例对象private static final SelectorProvider DEFAULT_SELECTOR_PROVIDER = SelectorProvider.provider();public NioServerSocketChannel() {this(newSocket(DEFAULT_SELECTOR_PROVIDER));}private static java.nio.channels.ServerSocketChannel newSocket(SelectorProvider provider) {try {// 返回ServerSocketChannelImpl 的实例对象return provider.openServerSocketChannel();} catch (IOException var2) {throw new ChannelException("Failed to open a server socket.", var2);}}

this(newSocket(DEFAULT_SELECTOR_PROVIDER));
NioServerSocketChannel 的this 方法：

public NioServerSocketChannel(java.nio.channels.ServerSocketChannel channel) {// 父类方法对ServerSocketChannel对象设置属性super((Channel)null, channel, 16);// ServerSocketChannel 的config 对象进行初始化// private final ServerSocketChannelConfig config;this.config = new NioServerSocketChannelConfig(this, this.javaChannel().socket());}

super((Channel)null, channel, 16) 调用 AbstractNioMessageChannel：这里可以看到把感兴趣的事件accept 16 作为参数进行了传递

// parent 为null ，ch 为ServerSocketChannel channel 对象，// readInterestOp为int OP_ACCEPT = 1 << 4; 对应客户端连接服务器事件protected AbstractNioMessageChannel(Channel parent, SelectableChannel ch, int readInterestOp) {super(parent, ch, readInterestOp);}

关键点（2）继续进父类的调用:AbstractNioChannel: 对java 原生的ServerSocketChannel 的channel 管道的非阻塞属性进行了设置

private static final InternalLogger logger = InternalLoggerFactory.getInstance(AbstractNioChannel.class);private final SelectableChannel ch;protected final int readInterestOp;volatile SelectionKey selectionKey;boolean readPending;private final Runnable clearReadPendingRunnable = new Runnable() {public void run() {AbstractNioChannel.this.clearReadPending0();}};private ChannelPromise connectPromise;private ScheduledFuture<" />> connectTimeoutFuture;private SocketAddress requestedRemoteAddress;protected AbstractNioChannel(Channel parent, SelectableChannel ch, int readInterestOp) {super(parent);//将ServerSocketChannel 对象进行赋值this.ch = ch;// 感兴趣的事件 进行赋值 16 final int OP_ACCEPT = 1 << 4; 对应客户端连接服务器事件this.readInterestOp = readInterestOp;try {// 设置管道非阻塞ch.configureBlocking(false);} catch (IOException var7) {try {ch.close();} catch (IOException var6) {logger.warn("Failed to close a partially initialized socket.", var6);}throw new ChannelException("Failed to enter non-blocking mode.", var7);}}

关键点（3） super(parent); 调用父类：AbstractChannel:这里对java 原生的ServerSocketChannel 的pipeline 进行双向链表的初始化，以便后续加入对accept事件handler 的加入；

private static final InternalLogger logger = InternalLoggerFactory.getInstance(AbstractChannel.class);private final Channel parent;private final ChannelId id;private final Channel.Unsafe unsafe;private final DefaultChannelPipeline pipeline;private final VoidChannelPromise unsafeVoidPromise = new VoidChannelPromise(this, false);private final CloseFuture closeFuture = new CloseFuture(this);private volatile SocketAddress localAddress;private volatile SocketAddress remoteAddress;private volatile EventLoop eventLoop;private volatile boolean registered;private boolean closeInitiated;private Throwable initialCloseCause;private boolean strValActive;private String strVal;protected AbstractChannel(Channel parent) {// parent 为nullthis.parent = parent;// chanel 通道设置idthis.id = this.newId();// chanel 通过设置文件描述this.unsafe = this.newUnsafe();// 当前通道初始 pipeline this.pipeline = this.newChannelPipeline();}

this.newChannelPipeline()：

protected DefaultChannelPipeline newChannelPipeline() {return new DefaultChannelPipeline(this);}

DefaultChannelPipeline 类中对pipeline 完成初始化：

static final InternalLogger logger = InternalLoggerFactory.getInstance(DefaultChannelPipeline.class);private static final String HEAD_NAME = generateName0(HeadContext.class);private static final String TAIL_NAME = generateName0(TailContext.class);private static final FastThreadLocal<Map<Class<?>, String>> nameCaches = new FastThreadLocal<Map<Class<?>, String>>() {protected Map<Class<?>, String> initialValue() {return new WeakHashMap();}};private static final AtomicReferenceFieldUpdater<DefaultChannelPipeline, MessageSizeEstimator.Handle> ESTIMATOR = AtomicReferenceFieldUpdater.newUpdater(DefaultChannelPipeline.class, MessageSizeEstimator.Handle.class, "estimatorHandle");final AbstractChannelHandlerContext head;final AbstractChannelHandlerContext tail;private final Channel channel;private final ChannelFuture succeededFuture;private final VoidChannelPromise voidPromise;private final boolean touch = ResourceLeakDetector.isEnabled();private Map<EventExecutorGroup, EventExecutor> childExecutors;private volatile MessageSizeEstimator.Handle estimatorHandle;private boolean firstRegistration = true;private PendingHandlerCallback pendingHandlerCallbackHead;private boolean registered;// channel 为 NioServerSocketChannel对象protected DefaultChannelPipeline(Channel channel) {// chanel 赋值this.channel = (Channel)ObjectUtil.checkNotNull(channel, "channel");this.succeededFuture = new SucceededChannelFuture(channel, (EventExecutor)null);// 管道初始化异常 异常原因 promisethis.voidPromise = new VoidChannelPromise(channel, true);// pipeline 双向链表设置this.tail = new TailContext(this);this.head = new HeadContext(this);this.head.next = this.tail;this.tail.prev = this.head;}

（4）NioServerSocketChannel 的config 初始化：在完成了对NioServerSocketChannel 对象的创建并初始化pipeline和设置其为非阻塞流之后；回到 NioServerSocketChannel 类中：

this.config = new NioServerSocketChannelConfig(this, this.javaChannel().socket())private NioServerSocketChannelConfig(NioServerSocketChannel channel, ServerSocket javaSocket) {super(channel, javaSocket);}

super(channel, javaSocket) 方法调用到DefaultServerSocketChannelConfig 类：

protected final ServerSocket javaSocket;private volatile int backlog;public DefaultServerSocketChannelConfig(ServerSocketChannel channel, ServerSocket javaSocket) {super(channel);this.backlog = NetUtil.SOMAXCONN;// ServerSocket进行赋值this.javaSocket = (ServerSocket)ObjectUtil.checkNotNull(javaSocket, "javaSocket");}

继续super(channel);调用到 DefaultChannelConfig

//channel 为 ServerSocketChannel 对象public DefaultChannelConfig(Channel channel) {this(channel, new AdaptiveRecvByteBufAllocator());}protected DefaultChannelConfig(Channel channel, RecvByteBufAllocator allocator) {// channel NioServerSocketChannel allocator:通道所用的缓冲区this.allocator = ByteBufAllocator.DEFAULT;this.msgSizeEstimator = DEFAULT_MSG_SIZE_ESTIMATOR;// 通道属性的赋值this.connectTimeoutMillis = 30000;this.writeSpinCount = 16;this.autoRead = 1;this.autoClose = true;this.writeBufferWaterMark = WriteBufferWaterMark.DEFAULT;this.pinEventExecutor = true;this.setRecvByteBufAllocator(allocator, channel.metadata());this.channel = channel;}

2.2.1.2 继续看 initAndRegister() 中的this.init(channel) 方法：

// channel为 NioServerSocketChannel对象void init(Channel channel) {// 将父类AbstractBootstrap 的options 属性赋值到 NioServerSocketChannel对象setChannelOptions(channel, this.newOptionsArray(), logger);// 将父类AbstractBootstrap 的attrs属性赋值到 NioServerSocketChannel对象setAttributes(channel, (Map.Entry[])this.attrs0().entrySet().toArray(EMPTY_ATTRIBUTE_ARRAY));// 获取ServerSocketChannel初始化的pipeline DetaultChannelPipelineChannelPipeline p = channel.pipeline();// 获取 子事件轮询组final EventLoopGroup currentChildGroup = this.childGroup;// 获取子事件的 业务逻辑处理任务final ChannelHandler currentChildHandler = this.childHandler;// 获取的子事件中设置的配置项final Map.Entry[] currentChildOptions;synchronized(this.childOptions) {currentChildOptions = (Map.Entry[])this.childOptions.entrySet().toArray(EMPTY_OPTION_ARRAY);}// 子事件设置的属性final Map.Entry<AttributeKey<?>, Object>[] currentChildAttrs = (Map.Entry[])this.childAttrs.entrySet().toArray(EMPTY_ATTRIBUTE_ARRAY);p.addLast(new ChannelHandler[]{new ChannelInitializer<Channel>() {public void initChannel(final Channel ch) {// 获取 ServerSocketChannel 的pipelinefinal ChannelPipeline pipeline = ch.pipeline();// 添加 原有的handlerChannelHandler handler = ServerBootstrap.this.config.handler();if (handler != null) {pipeline.addLast(new ChannelHandler[]{handler});}// 添加 Acceptor hadler 到 ServerSocketChannel 的pipeline 双向链表中ch.eventLoop().execute(new Runnable() {public void run() {pipeline.addLast(new ChannelHandler[]{new ServerBootstrapAcceptor(ch, currentChildGroup, currentChildHandler, currentChildOptions, currentChildAttrs)});}});}}});}

关键点：（1）下面代码向ServerSocketChannel 中的pipeline 在双向链表的尾部增加了一个处理客户端连接事件的handler，并且可以看到这个添加handler 的操作是在ServerSocketChannel 初始化完成之后 由eventLoop 中的线程执行了任务：handler 在增加之后双向链表的构造：header->acceptor->tail

pipeline.addLast(new ChannelHandler[]{new ServerBootstrapAcceptor(ch, currentChildGroup, currentChildHandler, currentChildOptions, currentChildAttrs)})

到这里可以做下小结：

• this.channelFactory.newChannel() 完成了对NioServerSocketChannel 的创建，此处可以对应NIo 中的 SocketChannel socketChannel = SocketChannel.open()；
• this.init(channel); 对创建的SocketChannel 进行属性的初始化，对于事件处理的Pipeline进行双向链表的初始化，此处可以对应NIo 中的 socketChannel.configureBlocking(false);
• 并且在ServerSocketChannel 真正完成初始化之后会有nio 的Eventloop 线程将ServerBootstrapAcceptor的handler 增加到ServerSocketChannel pipeline 链表中；

（2）再回到 AbstractBootstrap 的 initAndRegister() 方法中：看下事件是如果被注册到ServerSocketChannel 上：

final ChannelFuture initAndRegister() {Channel channel = null;try {//调用NioServerSocketChannel 无参构造方法 初始化 NioServerSocketChannel 对象channel = this.channelFactory.newChannel();// NioServerSocketChannel 对象初始化this.init(channel);} catch (Throwable var3) {if (channel != null) {channel.unsafe().closeForcibly();return (new DefaultChannelPromise(channel, GlobalEventExecutor.INSTANCE)).setFailure(var3);}return (new DefaultChannelPromise(new FailedChannel(), GlobalEventExecutor.INSTANCE)).setFailure(var3);}// 从父类group 中的EventLoopGroup 选出一个EventLoop 并注册到channel 管道上ChannelFuture regFuture = this.config().group().register(channel);if (regFuture.cause() != null) {// 注册失败 关闭ServerSocketChannel通道if (channel.isRegistered()) {channel.close();} else {channel.unsafe().closeForcibly();}}// 返回注册的结果return regFuture;}

关键点（3）： ChannelFuture regFuture = this.config().group().register(channel)：事件轮询器注册到ServerSocketChannel 对象上：
调用 MultithreadEventLoopGroup 中的register 方法

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

next（） 方法： 使用在new NioEventLoopGroup 时设置的 选择器，从父类 EventExecutor[] children 数组中选择一个 EventExecutor ；然后继续看register 方法：
SingleThreadEventLoop 中的register：

public ChannelFuture register(Channel channel) {// 创建DefaultChannelPromise 的promise 对象 赋值 当前的ServerSocketChannel channel 和 子事件轮询器的EventExecutor ，并返回包装类的ChannelPromisereturn this.register((ChannelPromise)(new DefaultChannelPromise(channel, this)));}public ChannelFuture register(ChannelPromise promise) {ObjectUtil.checkNotNull(promise, "promise");//NioServerSocketChannel 的channel 进行registerpromise.channel().unsafe().register(this, promise);return promise;}

继续进入AbstractChannel 类中的register 方法：

public final void register(EventLoop eventLoop, final ChannelPromise promise) {ObjectUtil.checkNotNull(eventLoop, "eventLoop");if (AbstractChannel.this.isRegistered()) {promise.setFailure(new IllegalStateException("registered to an event loop already"));} else if (!AbstractChannel.this.isCompatible(eventLoop)) {promise.setFailure(new IllegalStateException("incompatible event loop type: " + eventLoop.getClass().getName()));} else {// 子事件轮训器进行赋值AbstractChannel.this.eventLoop = eventLoop;// eventLoop 是否有线程可以直接使用if (eventLoop.inEventLoop()) {this.register0(promise);} else {try {// new 一个线程来执行registereventLoop.execute(new Runnable() {public void run() {AbstractUnsafe.this.register0(promise);}});} catch (Throwable var4) {// 注册失败，设置异常原因AbstractChannel.logger.warn("Force-closing a channel whose registration task was not accepted by an event loop: {}", AbstractChannel.this, var4);this.closeForcibly();AbstractChannel.this.closeFuture.setClosed();this.safeSetFailure(promise, var4);}}}}

关键点（3.1 ）调用AbstractUnsafe register0 方法：完成占位事件的注册

private voidregister0(ChannelPromise promise) {try {if (!promise.setUncancellable() || !this.ensureOpen(promise)) {return;}// 第一次注册boolean firstRegistration = this.neverRegistered;// 事件注册AbstractChannel.this.doRegister();this.neverRegistered = false;// 注册成功AbstractChannel.this.registered = true;AbstractChannel.this.pipeline.invokeHandlerAddedIfNeeded();// promise 设置注册成功标识this.safeSetSuccess(promise);AbstractChannel.this.pipeline.fireChannelRegistered();if (AbstractChannel.this.isActive()) {if (firstRegistration) {AbstractChannel.this.pipeline.fireChannelActive();} else if (AbstractChannel.this.config().isAutoRead()) {this.beginRead();}}} catch (Throwable var3) {this.closeForcibly();AbstractChannel.this.closeFuture.setClosed();this.safeSetFailure(promise, var3);}}

关键点（3.2 ）AbstractChannel.this.doRegister()：占位事件0完成注册：
AbstractNioChannel 类中 doRegister 方法：

protected void doRegister() throws Exception {boolean selected = false;while(true) {try {//this.javaChannel() 为ServerSocketChannel对象//ServerSocketChannel对象 调用 register 方法this.selectionKey = this.javaChannel().register(this.eventLoop().unwrappedSelector(), 0, this);return;} catch (CancelledKeyException var3) {if (selected) {throw var3;}this.eventLoop().selectNow();selected = true;}}}

ServerSocketChannel对象 调用 register 方法
AbstractSelectableChannel 类中 register 方法

// Selector为 从父事件NioEventLoopGroup 中选择出来的某个NioEventLoop 对应的 Selector 选择器器// ops 感兴趣的事件 0 ，// att 属性NioServerSocketChannel 对象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;}}

调用AbstractSelector 的 register 方法完成将事件注册到selector多路复用器中：

protected final SelectionKey register(AbstractSelectableChannel var1, int var2, Object var3) {if (!(var1 instanceof SelChImpl)) {throw new IllegalSelectorException();} else {SelectionKeyImpl var4 = new SelectionKeyImpl((SelChImpl)var1, this);var4.attach(var3);synchronized(this.publicKeys) {this.implRegister(var4);}var4.interestOps(var2);return var4;}}

关键点（3.3） AbstractChannel.this.pipeline.invokeHandlerAddedIfNeeded() 在channel初始化完成之后回调到initAndRegister() 中的this.init(channel) 方法，完成accept handler 的增加：

可以看到具体进行accept 事件增加的处理是借由 EventLoop 中的execute 进行任务的提交；

关键点关键点（3.4） this.safeSetSuccess(promise) 为promise 设置成功结果后回调到AbstractBootstrap的doBind的operationComplete 方法进行端口号的绑定：

关键点 2.2.1.3 ServerSocketChannel 管道初始化完成之后 进行doBind0 端口的板绑定

private ChannelFuture doBind(final SocketAddress localAddress) {final ChannelFuture regFuture = this.initAndRegister();final Channel channel = regFuture.channel();if (regFuture.cause() != null) {return regFuture;} else if (regFuture.isDone()) {ChannelPromise promise = channel.newPromise();doBind0(regFuture, channel, localAddress, promise);return promise;} else {final PendingRegistrationPromise promise = new PendingRegistrationPromise(channel);// 监听注册完成的事件regFuture.addListener(new ChannelFutureListener() {public void operationComplete(ChannelFuture future) throws Exception {Throwable cause = future.cause();if (cause != null) {promise.setFailure(cause);} else {// 设置注册成功标识promise.registered();// 绑定端口AbstractBootstrap.doBind0(regFuture, channel, localAddress, promise);}}});return promise;}}

（1） AbstractBootstrap 的doBind0 方法：使用nio 线程进行channel 的绑定：

// ChannelFuture为 DefaultChannelPromise// channel 为NioServerSocketChannel// localAddress为 0.0.0.0/0.0.0.0:8080// promise为 AbstractBootstrap$PendingRegistrationPromiseprivate static void doBind0(final ChannelFuture regFuture, final Channel channel, final SocketAddress localAddress, final ChannelPromise promise) {channel.eventLoop().execute(new Runnable() {public void run() {if (regFuture.isSuccess()) {channel.bind(localAddress, promise).addListener(ChannelFutureListener.CLOSE_ON_FAILURE);} else {promise.setFailure(regFuture.cause());}}});}

AbstractChannel 的bind 方法：

public ChannelFuture bind(SocketAddress localAddress, ChannelPromise promise) {//this.pipeline 取的 NioServerSocketChannel 的pipeline return this.pipeline.bind(localAddress, promise);}

DefaultChannelPipeline 类中bind 方法：

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

（2） AbstractChannelHandlerContext 类中bind 方法， 找到第一个Outbound handler （head 节点）完成端口绑定：

public ChannelFuture bind(final SocketAddress localAddress, final ChannelPromise promise) {ObjectUtil.checkNotNull(localAddress, "localAddress");if (this.isNotValidPromise(promise, false)) {return promise;} else {final AbstractChannelHandlerContext next = this.findContextOutbound(512);EventExecutor executor = next.executor();if (executor.inEventLoop()) {// 端口绑定next.invokeBind(localAddress, promise);} else {safeExecute(executor, new Runnable() {public void run() {next.invokeBind(localAddress, promise);}}, promise, (Object)null, false);}return promise;}}

AbstractChannelHandlerContext 类 invokeBind 方法：

private void invokeBind(SocketAddress localAddress, ChannelPromise promise) {if (this.invokeHandler()) {try {((ChannelOutboundHandler)this.handler()).bind(this, localAddress, promise);} catch (Throwable var4) {notifyOutboundHandlerException(var4, promise);}} else {this.bind(localAddress, promise);}}

关键点（3） 最终进入NioServerSocketChannel 中的doBind 进行端口的绑定 使用原生的java ServerSocketChannel 进行端口的绑定：

关键点（4） 在dobind 之后：判断channel 管道是否已经就绪了，如果就绪 就依次调用 NioServerSocketChannel 的Pipeline 中的 每个handler 中的channelActive 方法：

目前Pipeline 中有3个handler ：header->acceptor->tail
（5） 最终进入head 头节点的 DefaultChannelPipeline 中的channelActive方法 ：

（6） 进入AbstractChannelHandlerContext read 方法，从tail 向前找到Outbound 的handler 然后进行操作：

关键点（7）：进入到AbstractNioChannel中doBeginRead() 然后关注accept 事件：

到这里我们看到了NioServerSocketChannel 对象的初始化，以及非阻塞属性的设置，已经最终对管道accept 事件 的注册；

3 对于bind 方法总结：

• initAndRegister 分为init 和register ；
• init 中可以理解为 在main 线程中创建了创 NioServerSocketChannel 对象并且， 在main 线程中 添加 NioServerSocketChannel 初始化 handler ；并随后在NioServerSocketChannel 管道初始化完成之后，使用nio 线程向NioServerSocketChannel 的Pipeline 增加accept 处理客户端连接的handler;
• register 中会切换到nio线程完成对NioServerSocketChannel 初始事件的绑定；
• 在dobind0 中 使用nio 线程完成NioServerSocketChannel 端口的绑定，并触发NioServerSocketChannel active 事件 进入到head 节点selectionKey.interestOps(SelectionKey.OP ACCEPT): 对于accept 事件进行绑定；
• 这里nio 线程指的是我们声明的boss NioEventLoopGroup；需要注意的是 NioEventLoopGroup 中存在多个NioEventLoop,每个NioEventLoop 都有自己的selector，任务队列；所以每个NioEventLoop 都可以处理io 事件，普通任务，和定时任务；值得注意的是每个NioEventLoop 的处理任务的线程只有一个（虽然有线程池，但是也只有一个线程，典型的多生产者，一个消费者）；