史上最不详细的Netty源码分析-Netty启动流程

首先要说明的是标题。没错！当然是为了博眼球，改成史上最详细的Netty源码分析是自取其辱。改成史上最不详细的Netty源码分析，万一说的不对也没人喷。话分两头，这个是我分析的初篇，我还是很认真和用心的写的。班门弄斧，如果有错误请指出。

为什么要写Netty源码分析? 用《Netty权威指南》话说
随着大数据，互联网和云计算的发展，传统的垂直架构将逐渐被分布式，弹性伸缩的新架构替代。
系统只要分布式部署，就存在多个节点之间的通讯问题，强调高可扩展和高性能，因此往往会选择高性能的通讯分式。使用
Netty+二进制编解码这些内部私有协议，已经逐渐成为业界主流。

简单的例子

启动下面程序后，命令行里输入 telnet 127.0.0.1 9988后，随便敲个字符，就会有welcome!!!打印出来，并且在console里会打印出敲入的字符

代码如下

public class WelcomeServer {
    private int port;
    public WelcomeServer(int port) {
        this.port = port;
    }
    public void run() throws Exception {
        EventLoopGroup bossGroup = new NioEventLoopGroup();
        EventLoopGroup workerGroup = new NioEventLoopGroup();
        try {
            ServerBootstrap b = new ServerBootstrap();
            b.group(bossGroup, workerGroup)
                    .channel(NioServerSocketChannel.class)
                    .childHandler(new ChannelInitializer<SocketChannel>() {
                        @Override
                        public void initChannel(SocketChannel ch) throws Exception {
                            ch.pipeline().addLast(new WelcomeServerHandler());
                        }
                    })
                    .option(ChannelOption.SO_BACKLOG, 128)
                    .childOption(ChannelOption.SO_KEEPALIVE, true);
            ChannelFuture f = b.bind(port).sync();
            f.channel().closeFuture().sync();
        } finally {
            workerGroup.shutdownGracefully();
            bossGroup.shutdownGracefully();
        }
    }
    public static void main(String[] args) throws Exception {
        int port = 9988;
        if (args.length > 0) {
            port = Integer.parseInt(args[0]);
        }
        new WelcomeServer(port).run();
    }
}

public class WelcomeServerHandler extends ChannelInboundHandlerAdapter {
    public void channelRead(ChannelHandlerContext ctx, Object msg) {
        ByteBuf bb=(ByteBuf)msg;
        byte[] b= new byte[bb.readableBytes()];
        bb.readBytes(b);
        System.out.println("read message:"+new String(b));
        byte[] welcomebyte="welcome!!!".getBytes();
        bb.writeBytes(welcomebyte);
        ctx.writeAndFlush(bb);
    }
    public void exceptionCaught(ChannelHandlerContext ctx, Throwable cause) { 
        cause.printStackTrace();
        ctx.close();
    }
}

开始分析

从ChannelFuture f = b.bind(port).sync();入手，在一系列的调用后会到达AbstractBootstrap#doBind

private ChannelFuture doBind(final SocketAddress localAddress) {
 final ChannelFuture regFuture = initAndRegister();
 ...   
}

在上面的类里最重要的就是initAndRegister()，顾名思义就是初始并注册。
下面的类去掉了没相关的代码，只保留必要的用于分析的代码

final ChannelFuture initAndRegister() {
        //初始
        Channel channel = null;
        try {
            channel = channelFactory.newChannel();//(1)
            init(channel);//(2)
        } catch (Throwable t) {
         ...
        }
        //注册
        ChannelFuture regFuture = config().group().register(channel);//(3)
        if (regFuture.cause() != null) {
            if (channel.isRegistered()) {
                channel.close();
            } else {
                channel.unsafe().closeForcibly();
            }
        }

        return regFuture;
    }

接下来就分初始和注册2部分进行分析

初始

(1)channelFactory 哪里初始化？

在例子WelcomeServer中有下面代码

ServerBootstrap b = new ServerBootstrap();
b.group(bossGroup, workerGroup).channel(NioServerSocketChannel.class);

channel(NioServerSocketChannel.class)，它的代码在下面

 public B channel(Class<? extends C> channelClass) {
        if (channelClass == null) {
            throw new NullPointerException("channelClass");
        }
        return channelFactory(new ReflectiveChannelFactory<C>(channelClass));//(1)
    }

public B channelFactory(ChannelFactory<? extends C> channelFactory) {
        ...
        //初始化出 channelFactory
        this.channelFactory = channelFactory;
        return (B) this;
    }

下面是ReflectiveChannelFactory的newChannel()

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() {
        try {
            return clazz.newInstance();
        } catch (Throwable t) {
            throw new ChannelException("Unable to create Channel from class " + clazz, t);
        }
    }
   
}

new ReflectiveChannelFactory<C>(channelClass)，
我们传入的class是
NioServerSocketChannel.class.
看到ReflectiveChannelFactoryd.newChannel()就是反射创建出一个NioServerSocketChannel对象。
创建完NioServerSocketChanne后，传入下面的init方法

(2) init(channel)都干了什么?

void init(Channel channel) throws Exception {
  ChannelPipeline p = channel.pipeline();
 
  p.addLast(new ChannelInitializer<Channel>() { //(3)
            @Override
            public void initChannel(final Channel ch) throws Exception {
            
              final Map<ChannelOption<?>, Object> options = options0();
              synchronized (options) {    
                 channel.config().setOptions(options);
               }
                // 拿到ChannelPipeline
                final ChannelPipeline pipeline = ch.pipeline();
                // 拿到config里的handler,这些handler就是WelcomeServer里构造出ServerBootStrap里的handler()方法放入的,注意不是childHandler()
                //WelcomeServer里目前没有写handler()
                ChannelHandler handler = config.handler();
                if (handler != null) {
                    pipeline.addLast(handler);
                }

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

channel.pipeline()是什么？这得从
NioServerSocketChannel构造过程入手，我们知道NioServerSocketChannel是clazz.newInstance()构造出来的。构造函数如下

public class NioServerSocketChannel extends AbstractNioMessageChannel
                             implements io.netty.channel.socket.ServerSocketChannel {
    
    //JDK里的NIO里的SelectorProvider                        
    private static final SelectorProvider DEFAULT_SELECTOR_PROVIDER = SelectorProvider.provider();
    
    public NioServerSocketChannel() {
        this(newSocket(DEFAULT_SELECTOR_PROVIDER));
    }
    
    private static ServerSocketChannel newSocket(SelectorProvider provider) {
        try {
            return provider.openServerSocketChannel();
        } catch (IOException e) {
            throw new ChannelException(
                    "Failed to open a server socket.", e);
        }
    }
    
     public NioServerSocketChannel(ServerSocketChannel channel) {
        super(null, channel, SelectionKey.OP_ACCEPT);
        config = new NioServerSocketChannelConfig(this, javaChannel().socket());
    }
}

super(null, channel, SelectionKey.OP_ACCEPT);的调用栈如下

class AbstractNioMessageChannel{
    protected AbstractNioMessageChannel(Channel parent, SelectableChannel ch, int readInterestOp) {
           super(parent, ch, readInterestOp);
    }
}

class AbstractNioChannel{
 protected AbstractNioChannel(Channel parent, SelectableChannel ch, int readInterestOp) {
        super(parent);
        this.ch = ch;
        this.readInterestOp = readInterestOp;
        try {
            ch.configureBlocking(false);
        } catch (IOException e) {
           ...
        }
    }
}

class AbstractChannel{
    protected AbstractChannel(Channel parent) {
        this.parent = parent;
        id = newId();
        unsafe = newUnsafe();
        pipeline = newChannelPipeline();
    }
}

channel.pipeline()返回的ChannelPipeline 就是上面的newChannelPipeline()得出的.
下面是newChannelPipeline()的代码

class AbstractChannel{
    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;
    }
}

从上面的代码里分析得出pipeline其实是双向链表的东西，如下

HeadContext.next-->TailContext
TailContext.prev-->HeadContext

有tail和head的全局引用分别指向TailContext和HeadContext
TailContext和HeadContext的区别

TailContext

final class TailContext extends AbstractChannelHandlerContext implements ChannelInboundHandler {}

HeadContext

final class HeadContext extends AbstractChannelHandlerContext
            implements ChannelOutboundHandler, ChannelInboundHandler {}

可以看出HeadContext比TailContext多了一个ChannelOutboundHandler对象。ChannelOutboundHandler和ChannelInboundHandler的作用后续再介绍

回到代码，看addLast

ChannelPipeline p = channel.pipeline(); 
  p.addLast(new ChannelInitializer<Channel>() { //(3)
            @Override
            public void initChannel(final Channel ch) throws Exception {
            
              final Map<ChannelOption<?>, Object> options = options0();
              synchronized (options) {     channel.config().setOptions(options);
               }
                // 拿到ChannelPipeline
                final ChannelPipeline pipeline = ch.pipeline();
                // 拿到config里的handler,这些handler就是构造出ServerBootStrap里的childHandler()方法放入的
                ChannelHandler handler = config.handler();
                if (handler != null) {
                    pipeline.addLast(handler);
                }

                ch.eventLoop().execute(new Runnable() {
                    @Override
                    public void run() {
                      // 这个处理client连接请求的关键Context的handler,这个在后面会分析，这里暂时不关心
                        pipeline.addLast(new ServerBootstrapAcceptor(
                                ch, currentChildGroup, currentChildHandler, currentChildOptions, currentChildAttrs));
                    }
                });
            }
        });

addLast(...)，最后会调用到下面代码，关注addLast0

 public final ChannelPipeline addLast(EventExecutorGroup group, String name, ChannelHandler handler) {
        final AbstractChannelHandlerContext newCtx;
        synchronized (this) {
            checkMultiplicity(handler);
            //将handler分装成为一个Context对象
            newCtx = newContext(group, filterName(name, handler), handler);
            //将这个Context对象加入到双向链表的tail节点之前
            addLast0(newCtx);          
            // If the registered is false it means that the channel was not registered on an eventloop yet.
            // In this case we add the context to the pipeline and add a task that will call
            // ChannelHandler.handlerAdded(...) once the channel is registered.           
            //registered=false 表示该channel 还没注册到selector
            //会最后调用到ChannelHandler.handlerAdded(...)方法里，这个方法里，最终会initChannel(ctx)
            if (!registered) {
                newCtx.setAddPending();
                //将这个任务加入到pipeline的pendingHandlerCallbackHead链表上
                callHandlerCallbackLater(newCtx, true);
                return this;
            }

            EventExecutor executor = newCtx.executor();
            if (!executor.inEventLoop()) {
                newCtx.setAddPending();
                executor.execute(new Runnable() {
                    @Override
                    public void run() {
                        callHandlerAdded0(newCtx);
                    }
                });
                return this;
            }
        }
        callHandlerAdded0(newCtx);
        return this;
    }

addLast0,回顾上面的pipleline里的双向链表，就容易了解到将这个AbstractChannelHandlerContext加入到双向链表的tail的上个节点

private void addLast0(AbstractChannelHandlerContext newCtx) {
        AbstractChannelHandlerContext prev = tail.prev;
        newCtx.prev = prev;
        newCtx.next = tail;
        prev.next = newCtx;
        tail.prev = newCtx;
}

上面的流程就是初始的过程了，总结下，简单说就是下面2步

new出一个Channel, 这个channel是NioServerSocketChannel,通过反射调用无参数够造函数构造出来。
够造过程里创建了一个pipeline对象，这个pipeline对象是个双向链表的结构
初始化这个channel,在channel的pipeline里加入从handler(…)里拿到的handler封装成的Context对象
如下

 b.group(bossGroup, workerGroup)
                    .channel(NioServerSocketChannel.class)
                    .handler(...)

4.最后会往pipeline加入ServerBootstrapAcceptor这个处理client连接的handler的context
5.如上分析server端pipeline应该是
在这里插入图片描述

注册

(3)ChannelFuture regFuture = config().group().register(channel);

config(),调用到最后会发现这是ServerBootstrap的一个全局变量ServerBootstrapConfig，ServerBootstrapConfig的有个传入参数this指向自己，如下代码

public class ServerBootstrap extends AbstractBootstrap<ServerBootstrap, ServerChannel> {

    private final ServerBootstrapConfig config = new ServerBootstrapConfig(this);
}

config().group()其实就是调用到
ServerBootstrap.group().ServerBootstrap.group()是什么呢？得看我们创建 WelcomeServer的时候，有new了2个EventLoopGroup，一个是bossGroup,另个是workerGroup。代码如下

EventLoopGroup bossGroup = new NioEventLoopGroup();
EventLoopGroup workerGroup = new NioEventLoopGroup();
ServerBootstrap b = new ServerBootstrap();
b.group(bossGroup, workerGroup).channel(NioServerSocketChannel.class)

ServerBootstrap中的group(EventLoopGroup parentGroup, EventLoopGroup childGroup)方法如下

public class ServerBootstrap extends AbstractBootstrap<ServerBootstrap, ServerChannel>{
    public ServerBootstrap group(EventLoopGroup parentGroup, EventLoopGroup childGroup) {
        super.group(parentGroup);
        ...
        this.childGroup = childGroup;
        return this;
    }
}

super.group(parentGroup)代码如下

public abstract class AbstractBootstrap<B extends AbstractBootstrap<B, C>, C extends Channel> implements Cloneable {
   public B group(EventLoopGroup group) {
        this.group = group;
        return (B) this;
    }
}

做的事就是

bossGroup付给了ServerBootstrap里全局变量group
workerGroup付给了ServerBootstrap里全局变量childGroup.

config().group() 指向的就是bossGroup这个NioEventLoopGroup

因此下面的代码

ChannelFuture regFuture = config().group().register(channel);

就是把channel注册到EventLoopGroup里，确切的说
把NIOServerChannel注册到bossGroup这个NioEventLoopGroup里.

NioEventLoopGroup里的register(channel)方法是在它的父类MultithreadEventLoopGroup定义的。

public class MultithreadEventLoopGroup extends MultithreadEventExecutorGroup{
    public EventLoop next() {
        return (EventLoop) super.next();
    }
    public ChannelFuture register(Channel channel) {
        return next().register(channel);
    }
}

public class MultithreadEventExecutorGroup{
  public EventExecutor next() {
        return chooser.next();
    }
}

这个chooser又是个什么对象，以及怎么初始化出来?
这得从下面的代码入手

 EventLoopGroup bossGroup = new NioEventLoopGroup();

NioEventLoopGroup无参构造函数最后调用super(…)

public class NioEventLoopGroup extends MultithreadEventLoopGroup {
   public NioEventLoopGroup() {
        this(0);
    }
    
    public NioEventLoopGroup(int nThreads) {
        this(nThreads, (Executor) null);
    }
    
    public NioEventLoopGroup(int nThreads, Executor executor) {
        this(nThreads, executor, SelectorProvider.provider());
    }
    
    public NioEventLoopGroup(
            int nThreads, Executor executor, final SelectorProvider selectorProvider) {
        this(nThreads, executor, selectorProvider, DefaultSelectStrategyFactory.INSTANCE);
    }
    
    public NioEventLoopGroup(int nThreads, Executor executor, final SelectorProvider selectorProvider,
                             final SelectStrategyFactory selectStrategyFactory) {
        super(nThreads, executor, selectorProvider, selectStrategyFactory, RejectedExecutionHandlers.reject());
    }
}

public abstract class MultithreadEventLoopGroup extends MultithreadEventExecutorGroup implements EventLoopGroup {

  private static final int DEFAULT_EVENT_LOOP_THREADS;

  static {
        DEFAULT_EVENT_LOOP_THREADS = Math.max(1, SystemPropertyUtil.getInt(
                "io.netty.eventLoopThreads", Runtime.getRuntime().availableProcessors() * 2));
  }
  //nThreads==0，所以取DEFAULT_EVENT_LOOP_THREADS，取DEFAULT_EVENT_LOOP_THREAD默认就是cpu核心数*2
  protected MultithreadEventLoopGroup(int nThreads, Executor executor, Object... args) {
        super(nThreads == 0 ? DEFAULT_EVENT_LOOP_THREADS : nThreads, executor, args);
  }
}

public abastract class MultithreadEventExecutorGroup extends AbstractEventExecutorGroup{
   
    private final EventExecutor[] children;

    protected MultithreadEventExecutorGroup(int nThreads, Executor executor, Object... args) {
        this(nThreads, executor, DefaultEventExecutorChooserFactory.INSTANCE, args);
    }
    
     protected MultithreadEventExecutorGroup(int nThreads, Executor executor,EventExecutorChooserFactory chooserFactory, Object... args) {
        ...
        if (executor == null) {
            executor = new ThreadPerTaskExecutor(newDefaultThreadFactory());
        }

        children = new EventExecutor[nThreads];

        for (int i = 0; i < nThreads; i ++) {
            boolean success = false;
            try {
               //EventExecutor 构造出来的是NioEventLoop
                children[i] = newChild(executor, args);
                success = true;
            } catch (Exception e) {
               ...
            } finally {
               ...
            }
        }

       //!!!!注意这里-chooser 在此构造出来，它是传入的chooserFactory，
       //即DefaultEventExecutorChooserFactory.INSTANCE
        chooser = chooserFactory.newChooser(children);

        final FutureListener<Object> terminationListener = new FutureListener<Object>() {
            @Override
            public void operationComplete(Future<Object> future) throws Exception {
                if (terminatedChildren.incrementAndGet() == children.length) {
                    terminationFuture.setSuccess(null);
                }
            }
        };

        for (EventExecutor e: children) {
            e.terminationFuture().addListener(terminationListener);
        }

        Set<EventExecutor> childrenSet = new LinkedHashSet<EventExecutor>(children.length);
        Collections.addAll(childrenSet, children);
        readonlyChildren = Collections.unmodifiableSet(childrenSet);                             
     }
}

DefaultEventExecutorChooserFactory 是个单例

public final class DefaultEventExecutorChooserFactory implements EventExecutorChooserFactory {

    public static final DefaultEventExecutorChooserFactory INSTANCE = new DefaultEventExecutorChooserFactory();

    private DefaultEventExecutorChooserFactory() { }

    public EventExecutorChooser newChooser(EventExecutor[] executors) {
       //如果传入的executors是2的n次方
       //在这里我们传入的executors的个数是cpu核心数*2，一定是2的n次方，所以返回的是PowerOfTowEventExecutorChooser
        if (isPowerOfTwo(executors.length)) {
            return new PowerOfTowEventExecutorChooser(executors);
        } else {
            return new GenericEventExecutorChooser(executors);
        }
    }
    }

我们要关注的就是PowerOfTowEventExecutorChooser.next()，executors[idx.getAndIncrement() & executors.length - 1]说白了就是 EventExecutor[]里挨个被选中去执行任务。

private static final class PowerOfTowEventExecutorChooser implements EventExecutorChooser {
        private final AtomicInteger idx = new AtomicInteger();
        private final EventExecutor[] executors;

        PowerOfTowEventExecutorChooser(EventExecutor[] executors) {
            this.executors = executors;
        }

        @Override
        public EventExecutor next() {
            return executors[idx.getAndIncrement() & executors.length - 1];
        }
    }

回到

public class MultithreadEventLoopGroup extends MultithreadEventExecutorGroup{
    public EventLoop next() {
        return (EventLoop) super.next();
    }
    public ChannelFuture register(Channel channel){
    //next()就是获得executors数组里的某个executor
    //(io.netty.channel.nio.NioEventLoop)
    //然后执行注册channel
        return next().register(channel);
    }
}

next()得到是的executor 是EventExecutor[] children里的一个，也就是io.netty.channel.nio.NioEventLoop

接下来需要明确NioEventLoop和NioEventLoopGroup关系

NioEventLoopGroup 类

public class NioEventLoopGroup extends MultithreadEventLoopGroup

public abstract class MultithreadEventLoopGroup extends MultithreadEventExecutorGroup implements EventLoopGroup

public abstract class MultithreadEventExecutorGroup extends AbstractEventExecutorGroup {
    private final EventExecutor[] children;
}

NioEventLoop类

public final class NioEventLoop extends SingleThreadEventLoop

public abstract class SingleThreadEventLoop extends SingleThreadEventExecutor implements EventLoop

public abstract class AbstractScheduledEventExecutor extends AbstractEventExecutor

public abstract class AbstractEventExecutor extends AbstractExecutorService implements EventExecutor

NioEventLoop的远祖就是EventExecutor。然后可以回到它两关系，NioEventLoopGroup里有个EventExecutor[] children的数组，里面包含的真实对象就是NioEventLoop

next().register(channel)就是从EventExecutor[] children的数组里拿到一个NioEventLoop,NioEventLoop.register(channel)

接下来就看NioEventLoop的register(Channel channel)方法，这个方法其实是它的父类SingleThreadEventLoop里的方法

public class SingleThreadEventLoop{
    public ChannelFuture register(Channel channel) {
        return register(new DefaultChannelPromise(channel, this));
    }
    public ChannelFuture register(final ChannelPromise promise) {
        ObjectUtil.checkNotNull(promise, "promise");
        promise.channel().unsafe().register(this, promise);
        return promise;
    }
}

promise.channel()我们知道是NioServerSocketChannel,它的unsafe()方法返回了一个Unsafe接口对象,Unsafe是个什么鬼？

还记得NioServerSocketChannel构造到最后调用如下代码，其中有newUnsafe()

 protected AbstractChannel(Channel parent) {
        this.parent = parent;
        id = newId();
        unsafe = newUnsafe();
        pipeline = newChannelPipeline();
    }

让我们进入newUnsafe()方法，构造出一个Unsafe的接口对象。看下Unsafe接口的说明。正常使用是不会碰到用到这些类

Unsafe operations that should never be called from user-code.

初略的看了下这个接口提供的方法，例如

void register(EventLoop eventLoop, ChannelPromise promise);
void bind(SocketAddress localAddress, ChannelPromise promise);
可以断定这个类是用来做连接相关的

debug会得到最后的对象是NioMessageUnsafe

private final class NioMessageUnsafe extends AbstractNioUnsafe {
}

我们关注的是register(EventLoop eventLoop, ChannelPromise promise)的实现，这个实现是在AbstractNioUnsafe的父类AbstractUnsafe里实现的

  protected abstract class AbstractUnsafe implements Unsafe {
       public final void register(EventLoop eventLoop, final ChannelPromise promise) {
            ...
         //eventLoop 是传入NioEventLoop AbstractChannel.this.eventLoop = eventLoop;
         //第一次进来eventLoop.inEventLoop()是false
            if (eventLoop.inEventLoop()) {
                register0(promise);
            } else {
                try {
                   //关注点-eventLoop.execute
                   
                   eventLoop.execute(new Runnable() {
                        @Override
                        public void run() {
                            register0(promise);
                        }
                    });
                } catch (Throwable t) {
                 ...
                }
            }
        }
  }

上面的代码我们关注eventLoop.execute(new Runnable() {...}); ,这个execute是NioEventLoop的父类SingleThreadEventExecutor里实现的，如下代码

public abstract class SingleThreadEventExecutor extends AbstractScheduledEventExecutor implements OrderedEventExecutor {
 private final Executor executor;
 public void execute(Runnable task) {
        if (task == null) {
            throw new NullPointerException("task");
        }

        boolean inEventLoop = inEventLoop();
        if (inEventLoop) {
            addTask(task);
        } else {
           // 第一次肯定是到这里
           //关注点-启动线程!!!
            startThread();
            //关注点-加入任务!!!
            addTask(task);
            if (isShutdown() && removeTask(task)) {
                reject();
            }
        }

        if (!addTaskWakesUp && wakesUpForTask(task)) {
            wakeup(inEventLoop);
        }
    }
    
  private void startThread() {
        if (STATE_UPDATER.get(this) == ST_NOT_STARTED) {
            if (STATE_UPDATER.compareAndSet(this, ST_NOT_STARTED, ST_STARTED)) {
                doStartThread();
            }
        }
    }
    
    private void doStartThread() {
        assert thread == null;
        //这个executor又是什么?
        executor.execute(new Runnable() {
            @Override
            public void run() {
                thread = Thread.currentThread();
                if (interrupted) {
                    thread.interrupt();
                }

                boolean success = false;
                updateLastExecutionTime();
                try {
                    SingleThreadEventExecutor.this.run();
                    success = true;
                } catch (Throwable t) {
                    ...
                } finally {
                    ...
                }
            }
        });
    }
}

上面的代码里这个executor是什么?
我们在create NioEventLoopGroup的时候，会调用到它的父类构造函数中，如下面传入的executor.但是传入的是null,为空则构造出ThreadPerTaskExecutor(newDefaultThreadFactory())赋给executor。看下面代码

class MultithreadEventExecutorGroup{
   
     private final EventExecutor[] children;

    protected MultithreadEventExecutorGroup(int nThreads, Executor executor, Object... args) {
        this(nThreads, executor, DefaultEventExecutorChooserFactory.INSTANCE, args);
    }
    
     protected MultithreadEventExecutorGroup(int nThreads, Executor executor,EventExecutorChooserFactory chooserFactory, Object... args) {
        ...
        if (executor == null) {
            executor = new ThreadPerTaskExecutor(newDefaultThreadFactory());
        }
        ...
   }
   
}

因此这个executor真实身份是
io.netty.util.concurrent.ThreadPerTaskExecutor

ThreadPerTaskExecutor的类如下

public final class ThreadPerTaskExecutor implements Executor {
    private final ThreadFactory threadFactory;

    public ThreadPerTaskExecutor(ThreadFactory threadFactory) {
        if (threadFactory == null) {
            throw new NullPointerException("threadFactory");
        }
        this.threadFactory = threadFactory;
    }

    @Override
    public void execute(Runnable command) {
        threadFactory.newThread(command).start();
    }
}

到此startThread()就很明确了，就是启动传入的command线程。最后就是调用SingleThreadEventExecutor.this.run(); 这个run()在SingleThreadEventExecutor里是
protected abstract void run(),它的实现在NIOEventLoop里，如下,这是个死循环

public class NIOEventLoop{
protected void run() {
        for (;;) {
            try {
              //死循环从一个 private final Queue<Runnable> taskQueue里去不断的获取任务,taskQueue是哪里建立出来的？找下它在哪里
                switch (selectStrategy.calculateStrategy(selectNowSupplier, hasTasks())) {
                    case SelectStrategy.CONTINUE:
                        continue;
                    case SelectStrategy.SELECT:
                        select(wakenUp.getAndSet(false));
                       
                        // (OK - no wake-up required).

                        if (wakenUp.get()) {
                            selector.wakeup();
                        }
                    default:
                        // fallthrough
                }

                cancelledKeys = 0;
                needsToSelectAgain = false;
                final int ioRatio = this.ioRatio;
                if (ioRatio == 100) {
                    try {
                        processSelectedKeys();
                    } finally {
                        // Ensure we always run tasks.
                        runAllTasks();
                    }
                } else {
                    final long ioStartTime = System.nanoTime();
                    try {
                        processSelectedKeys();
                    } finally {
                        // Ensure we always run tasks.
                        final long ioTime = System.nanoTime() - ioStartTime;
                        runAllTasks(ioTime * (100 - ioRatio) / ioRatio);
                    }
                }
            } catch (Throwable t) {
                handleLoopException(t);
            }
            // Always handle shutdown even if the loop processing threw an exception.
            try {
                if (isShuttingDown()) {
                    closeAll();
                    if (confirmShutdown()) {
                        return;
                    }
                }
            } catch (Throwable t) {
                handleLoopException(t);
            }
        }
    }
}

死循环从一个 private final Queue taskQueue里去不断的获取任务,taskQueue是哪里建立出来的？找下它在哪里建立就知道了，它是在下面的方法里

protected SingleThreadEventExecutor(EventExecutorGroup parent, Executor executor,
                                        boolean addTaskWakesUp, int maxPendingTasks,
                                        RejectedExecutionHandler rejectedHandler) {
        super(parent);
        this.addTaskWakesUp = addTaskWakesUp;
        this.maxPendingTasks = Math.max(16, maxPendingTasks);
        this.executor = ObjectUtil.checkNotNull(executor, "executor");
        //看这里构造出taskQueue
        taskQueue = newTaskQueue(this.maxPendingTasks);
        rejectedExecutionHandler = ObjectUtil.checkNotNull(rejectedHandler, "rejectedHandler");
    }

启动完线程后，就是加入任务了，代码比较简单，
往taskQueue里放任务

protected void addTask(Runnable task) {
        if (task == null) {
            throw new NullPointerException("task");
        }
        if (!offerTask(task)) {
            reject(task);
        }
    }

到此register的代码终于可以告一段落，简单的总结下initandregister的过程

创建了一个NioServerSocketChannel
创建了一个NioEventLoopGroup
NioEventLoopGroup里有个child[]的数组来存放了NioEventLoop
将NioServerSocketChannel注册到NioEventLoopGroup
NioServerChannel注册到NioEventLoopGroup，不是直接注册,而是从child[]的数组中挑一个NioEventLoop，挑哪个由choser来决定。然后将NioServerChannel注册到NioEventLoop
将NioServerChannel注册到NioEventLoop，也不是直接注册，而是先启动一个线程，执行的是NioEventLoop的run()
run()方法是个死循环，会不断从taskqueue这个队列里去获取到是否有runable的任务
addTask(task) 这个方法才真正的把注册channel的任务加入到taskqueue里
run()方法里检测到taskqueue队列里有任务了就进行相应的处理

run()里的分析待后续分解