Netty源码—1.服务端启动流程

大纲

1.服务端启动整体流程及关键方法

2.服务端启动的核心步骤

3.创建服务端Channel的源码

4.初始化服务端Channel的源码

5.注册服务端Channel的源码

6.绑定服务端端口的源码

7.服务端启动流程源码总结

 

1.服务端启动整体流程及关键方法

(1)关键方法

(2)整体流程

 

(1)关键方法

一.EventLoopGroup

服务端的线程模型外观类，Netty的线程模型是事件驱动的。也就是说，这个线程要做的事情就是不停地检测IO事件、处理IO事件、执行任务，并且不断重复这三个步骤。

 

二.ServerBootstrap

服务端的一个启动辅助类，通过给它设置一系列参数来绑定端口启动服务。

 

三.group(bossGroup, workerGroup)

设置服务端的线程模型，bossGroup的作用就是不断接收新的连接，并将新连接交给workerGroup来进行处理。

 

四.channel(NioServerSocketChannel.class)

设置服务端的IO类型为NIO，Netty是通过指定Channel的类型来指定IO类型的。Channel是Netty的一大组件，一个Channel就是一个连接或者一个服务端的bind动作。

 

五.handler()

表示在服务端的启动过程中，需要经过哪些流程。

 

六.childHandler()

设置ChannleHandler来处理每个连接上的数据。

 

七.ChannelFuture f = b.bind(8888).sync()

绑定端口并进行同步等待。绑定端口8888，等服务端启动完毕，才会进入下一行代码。

 

八.f.channel().closeFuture().sync()

等待服务端关闭端口绑定，这里的作用其实就是让程序不会退出。

 

九.bossGroup.shutdownGracefully()

关闭事件循环，关闭之后，main方法就结束了。

 

(2)整体流程

一.创建ServerBootstrap实例

ServerBootstrap是Netty服务端的启动辅助类，它提供了一系列方法用于设置服务端启动相关的参数。底层通过门面模式对各种能力进行抽象和封装，以让用户少和底层API交互，降低开发难度。ServerBootstrap只有一个无参的构造函数，它使用了Builder模式来处理参数过多的问题。

 

二.设置并绑定Reactor线程池

Netty的Reactor线程池是EventLoopGroup，而EventLoopGroup实际就是EventLoop的数组。EventLoop的职责是处理所有注册到本线程多路复用器Selector上的Channel。Selector的轮询操作是由其绑定的EventLoop线程run()方法驱动的，在一个循环体内循环执行。

 

三.设置并绑定服务端Channel

由于NIO服务端需要创建ServerSocketChannel，而Netty对NIO类库进行了封装，所以对应的就是NioServerSocketChannel。

 

Netty的ServerBootstrap方法提供了channel()方法用于指定服务端的Channel类型。Netty是通过工厂类(ServerBootstrap的父类AbstractBootstrap的ReflectiveChannelFactory实例)，利用反射创建NioServerSocketChannel对象的。由于启动时才调用，所以该反射对运行时的性能没有影响。

 

四.创建并初始化ChannelPipeline

ChannelPipeline不是NIO服务端必需的，它本质是一个负责处理网络事件的职责链。ChannelPipeline这个职责链会负责管理和执行ChannelHandler。网络事件以事件流的形式在ChannelPipeline中流转，由ChannelPipeline根据ChannelHandler的执行策略来调度执行。

 

五.添加并设置ChannelHandler

ChannelHandler是Netty提供给用户定制和扩展的关键接口。利用ChannelHandler用户可以完成大多数的功能定制，如消息编解码、心跳、安全认证、流量控制和流量整形。

 

六.绑定并启动监听端口

在绑定监听端口之前，系统会做一系列的初始化和检测工作。完成之后便会启动监听端口，并将ServerSocketChannel注册到Selector上，然后监听客户端连接。

 

七.Selector轮询

由Reactor线程NioEventLoop负责调度和执行Selector轮询操作，选择准备就绪的Channel集合。

 

八.执行ChannelPipeline和ChannelHandler

当轮询到准备就绪的Channel之后，就由Reactor线程NioEventLoop执行ChannelPipeline的相应方法。即ChannelPipeline会根据网络事件的类型调度并执行ChannelHandler，最终执行Netty自带的ChannelHandler或用户定制的ChannelHandler。

 

典型的网络事件有：

一.channelRegistered() 链路注册

二.channelActive() 链路激活

三.channelInActive() 链路断开

四.channelRead() 接收到请求消息

五.channelReadComplete() 处理完请求消息

六.exceptionCaugh() 链路发生异常

 

常用的ChannelHandler有：

一.ByteToMessageCodec 消息编解码Handler

二.LoggingHandler 码流日志打印Handler

三.SslHandler SSL安全认证Handler

四.IdleStateHandler 链路空闲检测Handler

五.LengthFieldBasedFrameDecoder 基于长度域的半包解码Handler

六.ChannelTrafficShapingHandler 进行流量整形的Handler

七.Base64Decoder和Base64Encoder Base64编解码Handler

 

2.服务端启动的核心步骤

(1)由启动辅助类的外观接口实现启动

(2)启动辅助类的bind()方法

(3)启动辅助类的initAndRegister()方法

(4)服务端启动的4个核心步骤

 

(1)由启动辅助类的外观接口实现启动

用户给启动辅助类ServerBootstrap设置好参数后，会通过它的外观接口来实现启动。

b.bind(8888).sync();

(2)启动辅助类的bind()方法

ServerBootstrap的bind()方法如下，来自其继承的抽象类AbstractBootstrap。

//Bootstrap sub-class which allows easy bootstrap of ServerChannel public class ServerBootstrap extends AbstractBootstrap<ServerBootstrap, ServerChannel> {     ...     ... }  //AbstractBootstrap is a helper class that makes it easy to bootstrap a Channel.  //It support method-chaining to provide an easy way to configure the AbstractBootstrap. //When not used in a ServerBootstrap context, the #bind() methods are useful for connectionless transports such as datagram (UDP). public abstract class AbstractBootstrap<B extends AbstractBootstrap<B, C>, C extends Channel> implements Cloneable {     ...     //Create a new Channel and bind it.     public ChannelFuture bind(int inetPort) {         //首先根据端口号创建一个InetSocketAddress对象，然后调用重载方法bind()         return bind(new InetSocketAddress(inetPort));     }          //Create a new Channel and bind it.     public ChannelFuture bind(SocketAddress localAddress) {         //验证服务启动需要的必要参数         validate();         if (localAddress == null) throw new NullPointerException("localAddress");         return doBind(ObjectUtil.checkNotNull(localAddress, "localAddress"));     }          private ChannelFuture doBind(final SocketAddress localAddress) {         final ChannelFuture regFuture = initAndRegister();//1.初始化和注册Channel         final Channel channel = regFuture.channel();         ...         doBind0(regFuture, channel, localAddress, promise);//2.绑定服务端端口         ...         return promise;     }     ... }

通过传入端口号调用AbstractBootstrap的bind()方法时，首先会根据端口号创建一个InetSocketAddress对象，然后继续调用重载方法bind()。重载方法bind()会先通过validate()方法验证服务启动需要的必要参数，然后调用doBind()方法。doBind()方法中的核心方法是：initAndRegister() + doBind0()。前者用于初始化和注册Channel，后者用于绑定服务端端口。

 

(3)启动辅助类的initAndRegister()方法

//AbstractBootstrap is a helper class that makes it easy to bootstrap a Channel.  //It support method-chaining to provide an easy way to configure the AbstractBootstrap. public abstract class AbstractBootstrap<B extends AbstractBootstrap<B, C>, C extends Channel> implements Cloneable {     ...     final ChannelFuture initAndRegister() {         Channel channel = null;         ...         //1.创建服务端Channel         channel = channelFactory.newChannel();         //2.初始化服务端Channel         init(channel);         ...         //3.注册服务端Channel，比如通过NioEventLoopGroup的register()方法进行注册         ChannelFuture regFuture = config().group().register(channel);         ...         return regFuture;     }     ... }

(4)服务端启动的4个核心步骤

步骤一：创建服务端Channel

步骤二：初始化服务端Channel

步骤三：注册服务端Channel到Selector

步骤四：绑定服务端端口

 

3.创建服务端Channel的源码

(1)Channel的概念

(2)Channel的创建

(3)ChannelFactory的创建

(4)通过反射创建Channel对象

(5)创建JDK底层NIO的Channel

(6)创建Channel配置类

(7)设置Channel类型为非阻塞

(8创建Channel的核心组件

(9)创建服务端Channel总结

 

(1)Channel的概念

Netty官方对Channel的描述是：Channel可以理解为一个网络连接或者一个具有"读、写、连接、绑定"等IO操作能力的组件。Netty的Channel由于是在服务启动的时候创建的，可以和BIO中的ServerSocket对应，也和NIO中的ServerSocketChannel对应，所以符合上述IO组件的概念。

 

(2)Channel的创建

Channel是通过ChannelFactory的newChannel()方法创建出来的。

//AbstractBootstrap is a helper class that makes it easy to bootstrap a Channel.  //It support method-chaining to provide an easy way to configure the AbstractBootstrap. public abstract class AbstractBootstrap<B extends AbstractBootstrap<B, C>, C extends Channel> implements Cloneable {     private volatile ChannelFactory<? extends C> channelFactory;     ...     final ChannelFuture initAndRegister() {         Channel channel = null;         ...         //1.创建服务端Channel         channel = channelFactory.newChannel();         //2.初始化服务端Channel         init(channel);         ...         //3.注册服务端Channel，比如通过NioEventLoopGroup的register()方法进行注册         ChannelFuture regFuture = config().group().register(channel);         ...         return regFuture;     }     ... }  public interface ChannelFactory<T extends Channel> {     //Creates a new channel.     T newChannel(); }

(3)ChannelFactory的创建

ChannelFactory是通过AbstractBootstrap的channel()方法创建出来的。

 

用户在调用启动辅助类的channel()方法时会将NioServerSocketChannel.class，作为ReflectiveChannelFactory的构造方法的参数，从而创建出一个ReflectiveChannelFactory对象，也就是ChannelFactory对象。

public class NettyServer {     ...     public void start() throws Exception {         ...         ServerBootstrap serverBootstrap = new ServerBootstrap();         serverBootstrap.group(bossGroup, workerGroup)         .channel(NioServerSocketChannel.class)//监听端口的ServerSocketChannel         .option(ChannelOption.SO_BACKLOG, 128)         .childOption(ChannelOption.SO_KEEPALIVE, true)         .childHandler(new ChannelInitializer<SocketChannel>() {//处理每个客户端连接的SocketChannel             @Override             protected void initChannel(SocketChannel socketChannel) throws Exception {                 ...             }         });         ChannelFuture channelFuture = serverBootstrap.bind(port).sync();//同步等待启动服务器监控端口         channelFuture.channel().closeFuture().sync();//同步等待关闭启动服务器的结果         ...     } }  //AbstractBootstrap is a helper class that makes it easy to bootstrap a Channel.  //It support method-chaining to provide an easy way to configure the AbstractBootstrap. public abstract class AbstractBootstrap<B extends AbstractBootstrap<B, C>, C extends Channel> implements Cloneable {     private volatile ChannelFactory<? extends C> channelFactory;     ...     //The Class which is used to create Channel instances from.     //You either use this or #channelFactory(io.netty.channel.ChannelFactory) if your Channel implementation has no no-args constructor.     public B channel(Class<? extends C> channelClass) {         if (channelClass == null) throw new NullPointerException("channelClass");         return channelFactory(new ReflectiveChannelFactory<C>(channelClass));     }          @SuppressWarnings({ "unchecked", "deprecation" })     public B channelFactory(io.netty.channel.ChannelFactory<? extends C> channelFactory) {         return channelFactory((ChannelFactory<C>) channelFactory);     }          public B channelFactory(ChannelFactory<? extends C> channelFactory) {         if (channelFactory == null) throw new NullPointerException("channelFactory");         if (this.channelFactory != null) throw new IllegalStateException("channelFactory set already");         this.channelFactory = channelFactory;         return (B) this;     }     ... }

(4)通过反射创建Channel对象

AbstractBootstrap.initAndRegister()方法中的channelFactory.newChannel()代码，最终调用的是ReflectiveChannelFactory.newChannel()方法，该方法会通过反射的方式创建出一个NioServerSocketChannel对象。

 

所以最终创建的服务端Channel相当于调用NioServerSocketChannel的默认构造函数来获得一个NioServerSocketChannel对象。

//A ChannelFactory that instantiates a new Channel by invoking its default constructor reflectively. public class ReflectiveChannelFactory<T extends Channel> implements ChannelFactory<T> {     private final Class<? extends T> clazz;     public ReflectiveChannelFactory(Class<? extends T> clazz) {         if (clazz == null) throw new NullPointerException("clazz");         this.clazz = clazz;     }      @Override     public T newChannel() {         ...         return clazz.newInstance();     } }

(5)创建JDK底层NIO的Channel

NioServerSocketChannel的默认构造方法会调用其newSocket()方法，而newSocket()方法中会通过SelectorProvider.openServerSocketChannel()方法创建一个ServerSocketChannel对象。这个对象也就是JDK底层的Channel，即NIO的Socket。

//A io.netty.channel.socket.ServerSocketChannel implementation which uses NIO selector based implementation to accept new connections. public class NioServerSocketChannel extends AbstractNioMessageChannel implements io.netty.channel.socket.ServerSocketChannel {     private static final SelectorProvider DEFAULT_SELECTOR_PROVIDER = SelectorProvider.provider();     private final ServerSocketChannelConfig config;     ...     //Create a new instance，默认的构造方法     public NioServerSocketChannel() {         //调用newSocket()创建出一个ServerSocketChannel对象后，再调用有参构造方法         this(newSocket(DEFAULT_SELECTOR_PROVIDER));     }          private static ServerSocketChannel newSocket(SelectorProvider provider) {         ...         //创建一个ServerSocketChannel对象         //这个对象也就是JDK底层的Channel，即NIO的Socket         return provider.openServerSocketChannel();     }          //Create a new instance using the given ServerSocketChannel，重载的构造方法     public NioServerSocketChannel(ServerSocketChannel channel) {         //传入要关心的ACCEPT事件         super(null, channel, SelectionKey.OP_ACCEPT);         this.config = new NioServerSocketChannelConfig(this, javaChannel().socket());     }     ... }

(6)创建Channel配置类

NioServerSocketChannel的默认构造方法还会调用其重载的构造方法；在重载的构造方法里，会创建一个NioServerSocketChannelConfig对象，其顶层接口为ChannelConfig。

 

(7)设置Channel类型为非阻塞

在NioServerSocketChannel的重载构造方法里，会逐层调用父类的构造方法，比如其中就会调用AbstractNioChannel的构造方法。

在AbstractNioChannel的构造方法中，会将前面provider.openServerSocketChannel()创建的ServerSocketChannel对象，保存到AbstractNioChannel的成员变量ch中，然后再将该Channel对象设置为非阻塞模式。以及将NioServerSocketChannel重载构造方法里传入的SelectionKey.OP_ACCEPT，设置到AbstractNioChannel的另一成员变量readInterestOp中，表示该Channel对象要关心ACCEPT事件。

注意：可以通过javaChannel()方法获取AbstractNioChannel的成员变量ch。

//A io.netty.channel.socket.ServerSocketChannel implementation which uses NIO selector based implementation to accept new connections. public class NioServerSocketChannel extends AbstractNioMessageChannel implements io.netty.channel.socket.ServerSocketChannel {     private static final SelectorProvider DEFAULT_SELECTOR_PROVIDER = SelectorProvider.provider();     private final ServerSocketChannelConfig config;     ...     //Create a new instance using the given ServerSocketChannel，重载的构造方法     public NioServerSocketChannel(ServerSocketChannel channel) {         //传入要关心的ACCEPT事件         super(null, channel, SelectionKey.OP_ACCEPT);         this.config = new NioServerSocketChannelConfig(this, javaChannel().socket());     }     ... }  //AbstractNioChannel} base class for Channels that operate on messages. public abstract class AbstractNioMessageChannel extends AbstractNioChannel {     ...     protected AbstractNioMessageChannel(Channel parent, SelectableChannel ch, int readInterestOp) {         super(parent, ch, readInterestOp);     }     ... }  //Abstract base class for Channel implementations which use a Selector based approach. public abstract class AbstractNioChannel extends AbstractChannel {     private final SelectableChannel ch;//这是NIO中的Channel     protected final int readInterestOp;     ...     //Create a new instance     //@param parent，the parent Channel by which this instance was created. May be null.     //@param ch，he underlying SelectableChannel on which it operates     //@param readInterestOp，the ops to set to receive data from the SelectableChannel     protected AbstractNioChannel(Channel parent, SelectableChannel ch, int readInterestOp) {         super(parent);         //NioServerSocketChannel.newSocket()方法通过JDK底层创建的Channel对象会被缓存在其父类AbstractNioChannel的变量ch中         //可以通过NioServerSocketChannel.javaChannel()方法获取其父类AbstractNioChannel的变量ch         this.ch = ch;         this.readInterestOp = readInterestOp;         ...         //设置Channel对象为非阻塞模式         ch.configureBlocking(false);         ...     }          protected SelectableChannel javaChannel() {         return ch;     }     ... }

(8创建Channel的核心组件

AbstractNioChannel的构造方法中还会调用其父类AbstractChannel的构造方法。在AbstractChannel的构造方法中，Netty创建了三大组件，分别赋值到其成员变量中。第一个组件是ChannelId，第二个组件是Unsafe，第三个组件是ChannelPipeline。

//A skeletal Channel implementation. public abstract class AbstractChannel extends DefaultAttributeMap implements Channel {     private final Channel parent;     private final ChannelId id;     private final Unsafe unsafe;     private final DefaultChannelPipeline pipeline;     ...     //Creates a new instance.     //@param parent，the parent of this channel. null if there's no parent.     protected AbstractChannel(Channel parent) {         this.parent = parent;         this.id = newId();         this.unsafe = newUnsafe();         this.pipeline = newChannelPipeline();     }     ... }

(9)创建服务端Channel总结

用户调用启动辅助类ServerBootstrap的bind()方法时，第一步通过反射创建服务端Channel会执行NioServerSocketChannel的默认构造方法，来创建一个NioServerSocketChannel对象，并且在创建过程中会创建Netty的一系列核心组件：如Channel、ChannelConfig、ChannelId、Unsafe、ChannelPipeline。

创建服务端Channel的关键脉络如下：

ServerBootstrap.bind() //用户代码入口   AbstractBootstrap.initAndRegister() //初始化并注册Channel     channelFactory.newChannel() //创建服务端Channel       NioServerSocketChannel.newSocket() //通过JDK来创建JDK底层NIO的Channel       new NioServerSocketChannelConfig() //对底层NIO的Channel进行TCP参数配置       new AbstractNioChannel() //调用AbstractNioChannel的构造方法         configureBlocking(false) //设置NIO的Channel为非阻塞模式         new AbstractChannel() //调用AbstractChannel的构造方法创建组件：id、unsafe、pipeline

4.初始化服务端Channel的源码

(1)初始化服务端Channel的时机

(2)初始化服务端Channel的三项工作

(3)初始化服务端Channel总结

(1)初始化服务端Channel的时机

AbstractBootstrap的initAndRegister()方法执行channelFactory.newChannel()创建服务端Channel后，便会继续执行init(channel)对服务端Channel进行初始化。

//AbstractBootstrap is a helper class that makes it easy to bootstrap a Channel.  //It support method-chaining to provide an easy way to configure the AbstractBootstrap. public abstract class AbstractBootstrap<B extends AbstractBootstrap<B, C>, C extends Channel> implements Cloneable {     ...     final ChannelFuture initAndRegister() {         Channel channel = null;         ...         //1.创建服务端Channel         channel = channelFactory.newChannel();         //2.初始化服务端Channel         init(channel);         ...         //3.注册服务端Channel，比如通过NioEventLoopGroup的register()方法进行注册         ChannelFuture regFuture = config().group().register(channel);         ...         return regFuture;     }          //init()方法的具体逻辑会由ServerBootstrap来实现     abstract void init(Channel channel) throws Exception;     ... }

(2)初始化服务端Channel的三项工作

AbstractBootstrap的init()方法只是一个抽象方法，具体的逻辑会在ServerBootstrap类中实现。

ServerBootstrap的init()方法初始化服务端Channel主要有三项工作：

一.设置服务端Channel的Option与Attr

二.设置客户端Channel的Option与Attr

三.配置服务端启动逻辑

其中，Netty把服务端启动过程中需要执行的启动逻辑分为两部分。一部分是添加用户自定义的处理逻辑到服务端启动流程，另一部分是添加一个特殊的逻辑处理ServerBootstrapAcceptor。

ServerBootstrapAcceptor是一个接入器，用来接收新请求以及把新请求传递给某个事件循环器。

//Bootstrap sub-class which allows easy bootstrap of ServerChannel public class ServerBootstrap extends AbstractBootstrap<ServerBootstrap, ServerChannel> {     ...     @Override     void init(Channel channel) throws Exception {         //1.设置服务端Channel的Option与Attr         final Map<ChannelOption<?>, Object> options = options0();         synchronized (options) {             channel.config().setOptions(options);         }         final Map<AttributeKey<?>, Object> attrs = attrs0();         synchronized (attrs) {             for (Entry<AttributeKey<?>, Object> e: attrs.entrySet()) {                 @SuppressWarnings("unchecked")                 AttributeKey<Object> key = (AttributeKey<Object>) e.getKey();                 channel.attr(key).set(e.getValue());             }         }                 //2.设置客户端Channel的Option与Attr         final EventLoopGroup currentChildGroup = childGroup;         final ChannelHandler currentChildHandler = childHandler;         final Entry<ChannelOption<?>, Object>[] currentChildOptions;         final Entry<AttributeKey<?>, Object>[] currentChildAttrs;         synchronized (childOptions) {             currentChildOptions = childOptions.entrySet().toArray(newOptionArray(childOptions.size()));         }         synchronized (childAttrs) {             currentChildAttrs = childAttrs.entrySet().toArray(newAttrArray(childAttrs.size()));         }                 //3.配置服务端启动逻辑         ChannelPipeline p = channel.pipeline();         //p.addLast()用于定义服务端启动过程中需要执行哪些逻辑         p.addLast(new ChannelInitializer<Channel>() {             @Override             public void initChannel(Channel ch) throws Exception {                 //一.添加用户自定义的Handler，注意这是handler，而不是childHandler                 final ChannelPipeline pipeline = ch.pipeline();                 ChannelHandler handler = config.handler();                 if (handler != null) pipeline.addLast(handler);                 //二.添加一个特殊的Handler用于接收新连接                 //自定义的childHandler会作为参数传入连接器ServerBootstrapAcceptor                 ch.eventLoop().execute(new Runnable() {                     @Override                     public void run() {                         pipeline.addLast(new ServerBootstrapAcceptor(                             currentChildGroup,                              currentChildHandler,                              currentChildOptions,                              currentChildAttrs)                         );                     }                 });             }         });     }     ... }

(3)初始化服务端Channel总结

AbstractBootstrap的initAndRegister()方法调用的ServerBootstrap实现的init()方法并没有启动服务，只是初始化一些基本配置和属性，以及在服务端启动逻辑中加入一个ServerBootstrapAcceptor接入器，用来专门接收新连接。

ServerBootstrap.bind() //用户代码入口   AbstractBootstrap.initAndRegister() //初始化并注册Channel     channelFactory.newChannel() //创建服务端Channel     ServerBootstrap.init() //初始化服务端Channel       setChannelOptions、setChannelAttrs //设置服务端的Option与Attr       setChildOptions、setChildAttrs //设置客户端的Option与Attr       config.handler() //设置服务端pipeline       addServerBootstrapAcceptor //添加接入器

5.注册服务端Channel的源码

(1)注册服务端Channel的入口

(2)注册Selector的主要步骤

(3)注册服务端Channel总结

(1)注册服务端Channel的入口

首先AbstractBootstrap的config()方法是一个抽象方法，会由ServerBootstrap来实现。

ServerBootstrap的config()方法会返回一个封装了ServerBootstrap对象的ServerBootstrapConfig对象。所以执行代码config().group()时会调用AbstractBootstrapConfig的group()方法，也就是执行ServerBootstrap的group()方法返回用户通过group()方法设置的一个NioEventLoopGroup对象。因此config().group().register(channel)最后会调用NioEventLoopGroup的register()方法。

//AbstractBootstrap is a helper class that makes it easy to bootstrap a Channel.  //It support method-chaining to provide an easy way to configure the AbstractBootstrap. public abstract class AbstractBootstrap<B extends AbstractBootstrap<B, C>, C extends Channel> implements Cloneable {     volatile EventLoopGroup group;     ...     final ChannelFuture initAndRegister() {         Channel channel = null;         ...         //1.创建服务端Channel         channel = channelFactory.newChannel();         //2.初始化服务端Channel         init(channel);         ...         //3.注册服务端Channel并启动一个NioEventLoop线程，通过NioEventLoopGroup的register()方法进行注册         ChannelFuture regFuture = config().group().register(channel);         ...         return regFuture;     }     //Returns the AbstractBootstrapConfig object that can be used to obtain the current config of the bootstrap.     public abstract AbstractBootstrapConfig<B, C> config();     //Returns the configured EventLoopGroup or null if non is configured yet.     public final EventLoopGroup group() {         return group;     }     ... }  //Bootstrap sub-class which allows easy bootstrap of ServerChannel public class ServerBootstrap extends AbstractBootstrap<ServerBootstrap, ServerChannel> {     private final ServerBootstrapConfig config = new ServerBootstrapConfig(this);     ...     @Override     public final ServerBootstrapConfig config() {         return config;     }     ... }  public abstract class AbstractBootstrapConfig<B extends AbstractBootstrap<B, C>, C extends Channel> {     protected final B bootstrap;     ...     protected AbstractBootstrapConfig(B bootstrap) {         this.bootstrap = ObjectUtil.checkNotNull(bootstrap, "bootstrap");     }     //Returns the configured EventLoopGroup or null if non is configured yet.     public final EventLoopGroup group() {         //比如返回一个NioEventLoopGroup对象         return bootstrap.group();     }     ... }

NioEventLoopGroup继承自抽象类MultithreadEventLoopGroup，调用NioEventLoopGroup的register()方法也就是调用MultithreadEventLoopGroup的register()方法。

调用NioEventLoopGroup的register()方法时，会先通过next()方法获取一个NioEventLoop对象，然后再调用NioEventLoop的register()方法。而调用NioEventLoop的register()方法，其实就是调用抽象类SingleThreadEventLoop的register()方法。

在SingleThreadEventLoop的register()方法中，promise.channel().unsafe()会返回一个Channel.Unsafe类型的对象。而AbstractChannel实现了Channel接口，AbstractChannel的内部类AbstractUnsafe也实现了Channel接口的内部接口Unsafe。

所以promise.channel().unsafe().register(this, promise)最后会调用AbstractUnsafe的register()方法。

//MultithreadEventLoopGroup implementations which is used for NIO Selector based Channels. public class NioEventLoopGroup extends MultithreadEventLoopGroup {     ...     ... }  //Abstract base class for EventLoopGroup implementations that handles their tasks with multiple threads at the same time. public abstract class MultithreadEventLoopGroup extends MultithreadEventExecutorGroup implements EventLoopGroup {     ...     @Override     public ChannelFuture register(Channel channel) {         //先通过next()方法获取一个NioEventLoop，然后通过NioEventLoop.register()方法注册服务端Channel         return next().register(channel);     }      @Override     public EventLoop next() {         return (EventLoop) super.next();     }     ... }  //SingleThreadEventLoop implementation which register the Channel's to a Selector and so does the multi-plexing of these in the event loop. public final class NioEventLoop extends SingleThreadEventLoop {     ...     ... }  //Abstract base class for EventLoops that execute all its submitted tasks in a single thread. public abstract class SingleThreadEventLoop extends SingleThreadEventExecutor implements EventLoop {     ...     @Override     public ChannelFuture register(Channel channel) {         return register(new DefaultChannelPromise(channel, this));     }      @Override     public ChannelFuture register(final ChannelPromise promise) {         ObjectUtil.checkNotNull(promise, "promise");         //调用AbstractUnsafe的register()方法         promise.channel().unsafe().register(this, promise);         return promise;     }     ... }

所以注册服务端Channel的关键逻辑其实就体现在AbstractUnsafe的register()方法上。该方法会先将EventLoop事件循环器绑定到服务端Channel即NioServerSocketChanel上，然后再调用AbstractUnsafe的register0()方法将服务端Channel注册到Selector上。

//A skeletal Channel implementation. public abstract class AbstractChannel extends DefaultAttributeMap implements Channel {     private volatile EventLoop eventLoop;     ...     //Unsafe implementation which sub-classes must extend and use.     protected abstract class AbstractUnsafe implements Unsafe {         ...         @Override         public final void register(EventLoop eventLoop, final ChannelPromise promise) {             ...             //绑定事件循环器，即绑定一个NioEventLoop到该Channel上             AbstractChannel.this.eventLoop = eventLoop;             //注册Selector，并启动一个NioEventLoop             if (eventLoop.inEventLoop()) {                 register0(promise);             } else {                 ...                 //通过启动这个NioEventLoop线程来调用register0()方法将这个服务端Channel注册到Selector上                 eventLoop.execute(new Runnable() {                     @Override                     public void run() {                         register0(promise);                     }                 });                 ...             }         }          private void register0(ChannelPromise promise) {             ...         }         ...     }     ... }

注意：AbstractUnsafe的register()方法会将前面获取到的一个NioEventLoop事件循环器绑定到服务端Channel上，之后便可以通过channel.eventLoop()来取出这个NioEventLoop事件循环器了。因此，一个服务端Channel对应一个NioEventLoop事件循环器。此外，会通过启动一个NioEventLoop线程来调用register0()方法将服务端Channel注册到Selector上。

总结：创建服务端Channel后，就会从NioEventLoopGroup中获取一个NioEventLoop出来进行绑定，并启动这个NioEventLoop线程将这个服务端Channel注册到Selector上以及执行线程的run()逻辑监听事件等。

(2)注册Selector的主要步骤

AbstractUnsafe.register0()方法主要有4个步骤。

步骤一：调用JDK底层注册服务端Channel到Selector上

doRegister()方法是由AbstractChannel的子类AbstractNioChannel来实现的。

在AbstractNioChannel的doRegister()方法中，首先获取前面创建的JDK底层NIO的Channel，然后调用JDK底层NIO的register()方法，将this也就是NioServerSocketChannel对象当作attachment绑定到JDK的Selector上。这样绑定是为了后续从Selector拿到对应的事件后，可以把Netty领域的Channel拿出来。而且注册的ops值是0，表示此时还不关注任何事件。

步骤二：回调handlerAdded事件

步骤三：传播channelRegisterd事件

步骤四：其他逻辑

//A skeletal Channel implementation. public abstract class AbstractChannel extends DefaultAttributeMap implements Channel {     private volatile EventLoop eventLoop;     ...     //Unsafe implementation which sub-classes must extend and use.     protected abstract class AbstractUnsafe implements Unsafe {         ...         @Override         public final void register(EventLoop eventLoop, final ChannelPromise promise) {             ...             //绑定事件循环器，即绑定一个NioEventLoop到该Channel上             AbstractChannel.this.eventLoop = eventLoop;             //注册Selector，并启动一个NioEventLoop             if (eventLoop.inEventLoop()) {                 register0(promise);             } else {                 ...                 //通过启动这个NioEventLoop线程来调用register0()方法将这个服务端Channel注册到Selector上                 eventLoop.execute(new Runnable() {                     @Override                     public void run() {                         register0(promise);                     }                 });                 ...             }         }          private void register0(ChannelPromise promise) {             ...             boolean firstRegistration = this.neverRegistered;             //1.调用JDK底层注册服务端Channel到Selector上             doRegister();             this.neverRegistered = false;             this.registered = true;             //2.回调handlerAdded事件             this.pipeline.invokeHandlerAddedIfNeeded();             safeSetSuccess(promise);             //3.传播channelRegisterd事件到用户代码里             this.pipeline.fireChannelRegistered();             //4.其他逻辑             if (isActive()) {                 if (firstRegistration) {                     this.pipeline.fireChannelActive();                 } else if (config().isAutoRead()) {                     beginRead();                 }             }             ...         }         ...     }          //Is called after the Channel is registered with its EventLoop as part of the register process.     //Sub-classes may override this method     protected void doRegister() throws Exception {         // NOOP     }     ... }  //Abstract base class for Channel implementations which use a Selector based approach. public abstract class AbstractNioChannel extends AbstractChannel {     private final SelectableChannel ch;//这是NIO中的Channel     protected final int readInterestOp;     volatile SelectionKey selectionKey;     ...     //Create a new instance     //@param parent，the parent Channel by which this instance was created. May be null.     //@param ch，he underlying SelectableChannel on which it operates     //@param readInterestOp，the ops to set to receive data from the SelectableChannel     protected AbstractNioChannel(Channel parent, SelectableChannel ch, int readInterestOp) {         super(parent);         //NioServerSocketChannel.newSocket()方法通过JDK底层创建的Channel对象会被缓存在其父类AbstractNioChannel的变量ch中         //可以通过NioServerSocketChannel.javaChannel()方法获取其父类AbstractNioChannel的变量ch         this.ch = ch;         this.readInterestOp = readInterestOp;         ...         //设置Channel对象为非阻塞模式         ch.configureBlocking(false);         ...     }          @Override     protected void doRegister() throws Exception {         boolean selected = false;         for (;;) {             ...             //首先获取前面创建的JDK底层NIO的Channel，然后调用JDK底层NIO的register()方法，             //将this也就是NioServerSocketChannel对象当作attachment绑定到JDK的Selector上；             //这样绑定是为了后续从Selector拿到对应的事件后，可以把Netty领域的Channel拿出来；             //而且注册的ops值是0，表示此时还不关注任何事件；             selectionKey = javaChannel().register(eventLoop().selector, 0, this);             return;             ...         }     }              protected SelectableChannel javaChannel() {         return ch;     }     ... }

(3)注册服务端Channel总结

注册服务端Channel的入口是AbstractChannel的内部类AbstractUnsafe的register()方法。

首先会把一个NioEventLoop线程和当前的Channel进行绑定，然后再调用AbstractUnsafe的register0()方法进行注册。而register0()方法会把前面创建的JDK底层NIO的Channel注册到Selector上，并且把Netty领域的Channel当作一个attachment绑定到Selector上去，最后回调handlerAdded事件以及传播channelRegistered事件到用户代码里。

ServerBootstrap.bind() //用户代码入口   AbstractBootstrap.initAndRegister() //初始化并注册Channel     channelFactory.newChannel() //创建服务端Channel     ServerBootstrap.init() //初始化服务端Channel     NioEventLoopGroup.register() //注册服务端Channel       NioEventLoop.register() //注册服务端Channel         AbstractChannel.AbstractUnsafe.register() //注册Channel入口           this.eventLoop = eventLoop //将Channel绑定NioEventLoop线程           AbstractChannel.AbstractUnsafe.register0() //实际注册             AbstractNioChannel.doRegister() //调用JDK底层注册Channel到Selector             invokeHandlerAddedIfNeeded() //回调handlerAdded事件             fireChannelRegistered() //传播channelRegistered事件

补充说明一：Java类是单继承的，Java接口却是多继承的。因为前者不能区分父类相同名字方法要用哪一个，后者则由于还没实现接口，即使父类有相同名字接口也不影响。

public interface EventLoop extends OrderedEventExecutor, EventLoopGroup {     ... }  public abstract class SingleThreadEventLoop extends SingleThreadEventExecutor implements EventLoop {     ... }

补充说明二：如果监听一个端口，就创建一个服务端Channel。如果监听多个端口，就创建多个服务端Channel。

每个Channel绑定于NioEventLoopGroup的next()方法返回的一个NioEventLoop。

6.绑定服务端端口的源码

(1)绑定服务端端口的时机

(2)AbstractUnsafe.bind()方法的主要工作

(3)调用JDK底层绑定端口

(4)传播ChannelActive事件

(5)注册ACCEPT事件到Selector

(6)绑定服务端端口总结

(1)绑定服务端端口的时机

ServerBootstrap的bind()方法，首先执行AbstractBootstrap的initAndRegister()方法完成了服务端Channel的初始化和注册后，就会调用AbstractBootstrap的doBind0()方法绑定端口。

//Bootstrap sub-class which allows easy bootstrap of ServerChannel public class ServerBootstrap extends AbstractBootstrap<ServerBootstrap, ServerChannel> {     ...     ... }  //AbstractBootstrap is a helper class that makes it easy to bootstrap a Channel.  //It support method-chaining to provide an easy way to configure the AbstractBootstrap. //When not used in a ServerBootstrap context, the #bind() methods are useful for connectionless transports such as datagram (UDP). public abstract class AbstractBootstrap<B extends AbstractBootstrap<B, C>, C extends Channel> implements Cloneable {     ...     //Create a new Channel and bind it.     public ChannelFuture bind(int inetPort) {         //首先根据端口号创建一个InetSocketAddress对象，然后调用重载方法bind()         return bind(new InetSocketAddress(inetPort));     }          //Create a new Channel and bind it.     public ChannelFuture bind(SocketAddress localAddress) {         //验证服务启动需要的必要参数         validate();         if (localAddress == null) throw new NullPointerException("localAddress");         return doBind(ObjectUtil.checkNotNull(localAddress, "localAddress"));     }          private ChannelFuture doBind(final SocketAddress localAddress) {         final ChannelFuture regFuture = initAndRegister();//1.初始化和注册Channel         final Channel channel = regFuture.channel();         ...         doBind0(regFuture, channel, localAddress, promise);//2.绑定服务端端口         ...         return promise;     }          private static void doBind0(final ChannelFuture regFuture, final Channel channel,         final SocketAddress localAddress, final ChannelPromise promise) {          //This method is invoked before channelRegistered() is triggered.         //Give user handlers a chance to set up the pipeline in its channelRegistered() implementation.         channel.eventLoop().execute(new Runnable() {             @Override             public void run() {                 if (regFuture.isSuccess()) {                     channel.bind(localAddress, promise).addListener(ChannelFutureListener.CLOSE_ON_FAILURE);                 } else {                     promise.setFailure(regFuture.cause());                 }             }         });     }     ... }

(2)AbstractUnsafe.bind()方法的主要工作

AbstractBootstrap的doBind0()方法会执行代码channel.bind()，这个channel其实就是通过channelFactory工厂反射生成的NioServerSocketChannel。

所以执行channel.bind()其实就是执行AbstractChannel的bind()方法。经过逐层调用，最后会落到调用AbstractChannel内部类AbstractUnsafe的bind()方法。

AbstractUnsafe的bind()方法主要做两件事：

一.调用JDK底层绑定端口

二.传播channelActive事件并注册ACCEPT事件

//A ServerSocketChannel implementation which uses NIO selector based implementation to accept new connections. public class NioServerSocketChannel extends AbstractNioMessageChannel implements ServerSocketChannel {     ... }  //AbstractNioChannel base class for Channels that operate on messages. public abstract class AbstractNioMessageChannel extends AbstractNioChannel {     ... }  //Abstract base class for Channel implementations which use a Selector based approach. public abstract class AbstractNioChannel extends AbstractChannel {     ... }  //A skeletal {@link Channel} implementation. public abstract class AbstractChannel extends DefaultAttributeMap implements Channel {     ...     private final DefaultChannelPipeline pipeline;     @Override     public ChannelFuture bind(SocketAddress localAddress, ChannelPromise promise) {         return pipeline.bind(localAddress, promise);     }     ... }  //The default ChannelPipeline implementation.  //It is usually created by a Channel implementation when the Channel is created. public class DefaultChannelPipeline implements ChannelPipeline {     final AbstractChannelHandlerContext head;     final AbstractChannelHandlerContext tail;     ...     @Override     public final ChannelFuture bind(SocketAddress localAddress, ChannelPromise promise) {         return tail.bind(localAddress, promise);     }     ... }  abstract class AbstractChannelHandlerContext extends DefaultAttributeMap implements ChannelHandlerContext, ResourceLeakHint {     ...     @Override     public ChannelFuture bind(final SocketAddress localAddress, final ChannelPromise promise) {         if (localAddress == null) throw new NullPointerException("localAddress");         if (!validatePromise(promise, false)) return promise;         final AbstractChannelHandlerContext next = findContextOutbound();         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);         }         return promise;     }          private void invokeBind(SocketAddress localAddress, ChannelPromise promise) {         if (invokeHandler()) {             try {                 //执行DefaultChannelPipeline.HeadContext的bind()方法                 ((ChannelOutboundHandler) handler()).bind(this, localAddress, promise);             } catch (Throwable t) {                 notifyOutboundHandlerException(t, promise);             }         } else {             bind(localAddress, promise);         }     }     ... }  //The default ChannelPipeline implementation.  //It is usually created by a Channel implementation when the Channel is created. public class DefaultChannelPipeline implements ChannelPipeline {     ...     final class HeadContext extends AbstractChannelHandlerContext implements ChannelOutboundHandler, ChannelInboundHandler {         private final Unsafe unsafe;         HeadContext(DefaultChannelPipeline pipeline) {             super(pipeline, null, HEAD_NAME, false, true);             unsafe = pipeline.channel().unsafe();             setAddComplete();         }         ...         @Override         public void bind(ChannelHandlerContext ctx, SocketAddress localAddress, ChannelPromise promise) throws Exception {             //执行AbstractChannel内部类AbstractUnsafe的bind()方法             unsafe.bind(localAddress, promise);         }         ...     }     ... }  //A skeletal {@link Channel} implementation. public abstract class AbstractChannel extends DefaultAttributeMap implements Channel {     private final DefaultChannelPipeline pipeline;     ...     //Unsafe implementation which sub-classes must extend and use.     protected abstract class AbstractUnsafe implements Unsafe {         ...         @Override         public final void bind(final SocketAddress localAddress, final ChannelPromise promise) {             ...             boolean wasActive = isActive();             try {                 //1.调用JDK底层绑定端口                 doBind(localAddress);             } catch (Throwable t) {                 safeSetFailure(promise, t);                 closeIfClosed();                 return;             }              if (!wasActive && isActive()) {                 invokeLater(new Runnable() {                     @Override                     public void run() {                         //2.传播channelActive事件并注册ACCEPT事件                         pipeline.fireChannelActive();                     }                 });             }             safeSetSuccess(promise);         }         ...     }     ...     //Bind the Channel to the SocketAddress     protected abstract void doBind(SocketAddress localAddress) throws Exception;     ... }

(3)调用JDK底层绑定端口

AbstractUnsafe的bind()方法中所调用的doBind()方法是属于AbstractChannel的抽象接口，会由NioServerSocketChannel来进行具体的实现，即调用JDK底层NIO的bind()方法来绑定端口。

//A skeletal {@link Channel} implementation. public abstract class AbstractChannel extends DefaultAttributeMap implements Channel {     ...     //Bind the Channel to the SocketAddress     protected abstract void doBind(SocketAddress localAddress) throws Exception;     ... }  //A ServerSocketChannel implementation which uses NIO selector based implementation to accept new connections. public class NioServerSocketChannel extends AbstractNioMessageChannel implements ServerSocketChannel {     ...     @Override     protected void doBind(SocketAddress localAddress) throws Exception {         if (PlatformDependent.javaVersion() >= 7) {             javaChannel().bind(localAddress, config.getBacklog());         } else {             javaChannel().socket().bind(localAddress, config.getBacklog());         }     }          @Override     protected ServerSocketChannel javaChannel() {         return (ServerSocketChannel) super.javaChannel();     }     ... }  //Abstract base class for Channel implementations which use a Selector based approach. public abstract class AbstractNioChannel extends AbstractChannel {     private final SelectableChannel ch;//这是NIO中的Channel     ...     protected SelectableChannel javaChannel() {         return ch;     }     ... }

(4)传播ChannelActive事件

绑定完端口后，就会执行代码pipeline.fireChannelActive()，也就是调用DefaultChannelPipeline.fireChannelActive()。

最后会调用DefaultChannelPipeline.HeadContext的channelActive()方法传播channelActive事件。

//The default ChannelPipeline implementation.   //It is usually created by a Channel implementation when the Channel is created. public class DefaultChannelPipeline implements ChannelPipeline {     final AbstractChannelHandlerContext head;     final AbstractChannelHandlerContext tail;     ...     @Override     public final ChannelPipeline fireChannelActive() {         AbstractChannelHandlerContext.invokeChannelActive(head);         return this;     }     ... }  abstract class AbstractChannelHandlerContext extends DefaultAttributeMap implements ChannelHandlerContext, ResourceLeakHint {     ...     static void invokeChannelActive(final AbstractChannelHandlerContext next) {         EventExecutor executor = next.executor();         if (executor.inEventLoop()) {             next.invokeChannelActive();         } else {             executor.execute(new Runnable() {                 @Override                 public void run() {                     next.invokeChannelActive();                 }             });         }     }          private void invokeChannelActive() {         if (invokeHandler()) {             try {                 //执行DefaultChannelPipeline.HeadContext的channelActive()方法                 ((ChannelInboundHandler) handler()).channelActive(this);             } catch (Throwable t) {                 notifyHandlerException(t);             }         } else {             fireChannelActive();         }     } }  //The default ChannelPipeline implementation.  //It is usually created by a Channel implementation when the Channel is created. public class DefaultChannelPipeline implements ChannelPipeline {     ...     final class HeadContext extends AbstractChannelHandlerContext implements ChannelOutboundHandler, ChannelInboundHandler {         ...         @Override         public void channelActive(ChannelHandlerContext ctx) throws Exception {             //1.传播channelActive事件             ctx.fireChannelActive();             //2.注册ACCEPT事件             readIfIsAutoRead();         }         ...     } }

(5)注册ACCEPT事件到Selector

传播完channelActive事件后，便会调用HeadContext.readIfIsAutoRead()方法。然后逐层调用到AbstractChannel内部类AbstractUnsafe的beginRead()方法，并最终调用到AbstractNioChannel的doBeginRead()方法来注册ACCEPT事件。

//The default ChannelPipeline implementation.  //It is usually created by a Channel implementation when the Channel is created. public class DefaultChannelPipeline implements ChannelPipeline {     private final Channel channel;     ...     final class HeadContext extends AbstractChannelHandlerContext implements ChannelOutboundHandler, ChannelInboundHandler {         ...         private void readIfIsAutoRead() {             //isAutoRead()方法默认会返回true             if (channel.config().isAutoRead()) {                 //调用AbstractChannel的read()方法                 channel.read();             }         }         ...     } }  //A skeletal {@link Channel} implementation. public abstract class AbstractChannel extends DefaultAttributeMap implements Channel {     private final DefaultChannelPipeline pipeline;     ...     @Override     public Channel read() {         pipeline.read();         return this;     }     ... }  //The default ChannelPipeline implementation.  //It is usually created by a Channel implementation when the Channel is created. public class DefaultChannelPipeline implements ChannelPipeline {     final AbstractChannelHandlerContext head;     final AbstractChannelHandlerContext tail;     ...     @Override     public final ChannelPipeline read() {         tail.read();         return this;     }     ... }  abstract class AbstractChannelHandlerContext extends DefaultAttributeMap implements ChannelHandlerContext, ResourceLeakHint {     ...     @Override     public ChannelHandlerContext read() {         final AbstractChannelHandlerContext next = findContextOutbound();         EventExecutor executor = next.executor();         if (executor.inEventLoop()) {             next.invokeRead();         } else {             Runnable task = next.invokeReadTask;             if (task == null) {                 next.invokeReadTask = task = new Runnable() {                     @Override                     public void run() {                         next.invokeRead();                     }                 };             }             executor.execute(task);         }         return this;     }      private void invokeRead() {         if (invokeHandler()) {             try {                 //执行DefaultChannelPipeline.HeadContext的read()方法                 ((ChannelOutboundHandler) handler()).read(this);             } catch (Throwable t) {                 notifyHandlerException(t);             }         } else {             read();         }     }     ... }  //The default ChannelPipeline implementation.  //It is usually created by a Channel implementation when the Channel is created. public class DefaultChannelPipeline implements ChannelPipeline {     ...     final class HeadContext extends AbstractChannelHandlerContext implements ChannelOutboundHandler, ChannelInboundHandler {         private final Unsafe unsafe;         HeadContext(DefaultChannelPipeline pipeline) {             super(pipeline, null, HEAD_NAME, false, true);             unsafe = pipeline.channel().unsafe();             setAddComplete();         }         ...         @Override         public void read(ChannelHandlerContext ctx) {             unsafe.beginRead();         }         ...     } }  //A skeletal {@link Channel} implementation. public abstract class AbstractChannel extends DefaultAttributeMap implements Channel {     private final DefaultChannelPipeline pipeline;     ...     //Unsafe implementation which sub-classes must extend and use.     protected abstract class AbstractUnsafe implements Unsafe {         ...         @Override         public final void beginRead() {             assertEventLoop();             if (!isActive()) return;             try {                 doBeginRead();             } catch (final Exception e) {                 invokeLater(new Runnable() {                     @Override                     public void run() {                         pipeline.fireExceptionCaught(e);                     }                 });                 close(voidPromise());             }         }         ...     }          //Schedule a read operation.     protected abstract void doBeginRead() throws Exception;     ... }  //Abstract base class for Channel implementations which use a Selector based approach. public abstract class AbstractNioChannel extends AbstractChannel {     protected final int readInterestOp;     volatile SelectionKey selectionKey;     boolean readPending;     ...     @Override     protected void doBeginRead() throws Exception {         //Channel.read() or ChannelHandlerContext.read() was called         //this.selectionKey就是前面注册服务端Channel时返回的对象         //注册服务端Channel时，注册ops的值是0，表示还不关注任何事件         final SelectionKey selectionKey = this.selectionKey;         if (!selectionKey.isValid()) return;          readPending = true;         final int interestOps = selectionKey.interestOps();         //这里的readInterestOp就是前面newChannel()时传入的SelectionKey.OP_ACCEPT         //所以这样要做的工作就是，告诉JDK的Selector一切工作准备就绪，只剩下把ACCEPT事件注册到Selector上         if ((interestOps & readInterestOp) == 0) {             //关注ACCEPT事件             selectionKey.interestOps(interestOps | readInterestOp);         }     }     ... }

(6)绑定服务端端口总结

绑定服务端端口，最终会调用JDK底层API去进行实际绑定。绑定端口成功后，会由DefaultChannelPipeline传播channelActive事件，以及把ACCEPT事件注册到Selector上，从而可以通过Selector监听新连接的接入。

ServerBootstrap.bind() //用户代码入口   AbstractBootstrap.initAndRegister() //初始化并注册Channel     channelFactory.newChannel() //创建服务端Channel     ServerBootstrap.init() //初始化服务端Channel     NioEventLoopGroup.register() //注册服务端Channel   AbstractBootstrap.doBind0() //绑定服务端端口     AbstractChannel.AbstractUnsafe.bind() //绑定服务端端口入口       NioServerSocketChannel.doBind() //NioServerSocketChannel实现         javaChannel().bind() //JDK底层API绑定端口       DefaultChannelPipeline.fireChannelActive() //传播channelActive事件         HeadContext.readIfIsAutoRead() //注册ACCEPT事件到Selector上

7.服务端启动流程源码总结

initAndRegister()里的newChannel()会通过反射创建JDK底层Channel，同时会创建该Channel对应的Config对象并设置该Channel为非阻塞模式。总之，创建服务端Channel时会完成Netty几大基本组件的创建。如Channel、ChannelConfig、ChannelId、Unsafe、ChannelPipeline。

初始化服务端Channel时，会设置服务端Channel和客户端Channel的Option和Attr，并且给服务端Channel添加连接接入器ServerBootstrapAcceptor用于接收新连接。

注册服务端Channel时，会调用JDK底层的API将Channel注册到Selector，同时将Netty领域的Channel当作attachment注册到Selector上，并且回调handlerAdded事件和传播channelRegistered事件到其他用户代码中。

绑定服务端端口时，会调用JDK底层API进行端口绑定并传播channelActive事件。当channelActive事件被传播后，才真正进行有效的服务端端口绑定，也就是把ACCEPT事件注册到Selector上。