Netty服务端的启动源码分析

ServerBootstrap的构造：

public class ServerBootstrap extends AbstractBootstrap<ServerBootstrap, ServerChannel> {
    private static final InternalLogger logger = InternalLoggerFactory.getInstance(ServerBootstrap.class);
    private final Map<ChannelOption<?>, Object> childOptions = new LinkedHashMap();
    private final Map<AttributeKey<?>, Object> childAttrs = new LinkedHashMap();
    private final ServerBootstrapConfig config = new ServerBootstrapConfig(this);
    private volatile EventLoopGroup childGroup;
    private volatile ChannelHandler childHandler;

    public ServerBootstrap() {
    }
    ......
}

隐式地执行了父类的无参构造：

public abstract class AbstractBootstrap<B extends AbstractBootstrap<B, C>, C extends Channel> implements Cloneable {
    volatile EventLoopGroup group;
    private volatile ChannelFactory<? extends C> channelFactory;
    private volatile SocketAddress localAddress;
    private final Map<ChannelOption<?>, Object> options = new LinkedHashMap();
    private final Map<AttributeKey<?>, Object> attrs = new LinkedHashMap();
    private volatile ChannelHandler handler;

    AbstractBootstrap() {
    }
    ......
}

只是初始化了几个容器成员

在ServerBootstrap创建后，需要调用group方法，绑定EventLoopGroup，有关EventLoopGroup的创建在我之前博客中写过：Netty中NioEventLoopGroup的创建源码分析

ServerBootstrap的group方法：

public ServerBootstrap group(EventLoopGroup group) {
    return this.group(group, group);
}

public ServerBootstrap group(EventLoopGroup parentGroup, EventLoopGroup childGroup) {
    super.group(parentGroup);
    if (childGroup == null) {
        throw new NullPointerException("childGroup");
    } else if (this.childGroup != null) {
        throw new IllegalStateException("childGroup set already");
    } else {
        this.childGroup = childGroup;
        return this;
    }
}

首先调用父类的group方法绑定parentGroup：

public B group(EventLoopGroup group) {
    if (group == null) {
        throw new NullPointerException("group");
    } else if (this.group != null) {
        throw new IllegalStateException("group set already");
    } else {
        this.group = group;
        return this.self();
    }
}

private B self() {
    return this;
}

将传入的parentGroup绑定给AbstractBootstrap的group成员，将childGroup绑定给ServerBootstrap的childGroup成员。
group的绑定仅仅是交给了成员保存。

再来看看ServerBootstrap的channel方法,，是在AbstractBootstrap中实现的：

public B channel(Class<? extends C> channelClass) {
    if (channelClass == null) {
        throw new NullPointerException("channelClass");
    } else {
        return this.channelFactory((io.netty.channel.ChannelFactory)(new ReflectiveChannelFactory(channelClass)));
    }
}

使用channelClass构建了一个ReflectiveChannelFactory对象：

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");
        } else {
            this.clazz = clazz;
        }
    }

    public T newChannel() {
        try {
            return (Channel)this.clazz.getConstructor().newInstance();
        } catch (Throwable var2) {
            throw new ChannelException("Unable to create Channel from class " + this.clazz, var2);
        }
    }

    public String toString() {
        return StringUtil.simpleClassName(this.clazz) + ".class";
    }
}

可以看到ReflectiveChannelFactory的作用就是通过反射机制，产生clazz的实例（这里以NioServerSocketChannel为例）。

在创建完ReflectiveChannelFactory对象后， 调用channelFactory方法：

public B channelFactory(io.netty.channel.ChannelFactory<? extends C> channelFactory) {
    return this.channelFactory((ChannelFactory)channelFactory);
}

public B channelFactory(ChannelFactory<? extends C> channelFactory) {
    if (channelFactory == null) {
        throw new NullPointerException("channelFactory");
    } else if (this.channelFactory != null) {
        throw new IllegalStateException("channelFactory set already");
    } else {
        this.channelFactory = channelFactory;
        return this.self();
    }
}

将刚才创建的ReflectiveChannelFactory对象交给channelFactory成员，用于后续服务端NioServerSocketChannel的创建。

再来看ServerBootstrap的childHandler方法：

public ServerBootstrap childHandler(ChannelHandler childHandler) {
    if (childHandler == null) {
        throw new NullPointerException("childHandler");
    } else {
        this.childHandler = childHandler;
        return this;
    }
}

还是交给了childHandler成员保存，可以看到上述这一系列的操作，都是为了填充ServerBootstrap，而ServerBootstrap真正的启动是在bind时：
ServerBootstrap的bind方法，在AbstractBootstrap中实现：

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

public ChannelFuture bind(String inetHost, int inetPort) {
return this.bind(SocketUtils.socketAddress(inetHost, inetPort));
}

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

public ChannelFuture bind(SocketAddress localAddress) {
    this.validate();
    if (localAddress == null) {
        throw new NullPointerException("localAddress");
    } else {
        return this.doBind(localAddress);
    }
}

可以看到首先调用了ServerBootstrap的validate方法，：

public ServerBootstrap validate() {
    super.validate();
    if (this.childHandler == null) {
        throw new IllegalStateException("childHandler not set");
    } else {
        if (this.childGroup == null) {
            logger.warn("childGroup is not set. Using parentGroup instead.");
            this.childGroup = this.config.group();
        }
    
        return this;
    }
}

先调用了AbstractBootstrap的validate方法：

public B validate() {
    if (this.group == null) {
        throw new IllegalStateException("group not set");
    } else if (this.channelFactory == null) {
        throw new IllegalStateException("channel or channelFactory not set");
    } else {
        return this.self();
    }
}

这个方法就是用来检查是否绑定了group和channel以及childHandler，所以在执行bind方法前，无论如何都要执行group、channel和childHandler方法。

实际的bind交给了doBind来完成：

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 AbstractBootstrap.PendingRegistrationPromise promise = new AbstractBootstrap.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;
    }
}

首先调用initAndRegister，完成ServerSocketChannel的创建以及注册：

final ChannelFuture initAndRegister() {
    Channel channel = null;

    try {
        channel = this.channelFactory.newChannel();
        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);
    }

    ChannelFuture regFuture = this.config().group().register(channel);
    if (regFuture.cause() != null) {
        if (channel.isRegistered()) {
            channel.close();
        } else {
            channel.unsafe().closeForcibly();
        }
    }

    return regFuture;
}

首先调用channelFactory的newChannel通过反射机制构建Channel实例，也就是NioServerSocketChannel，

NioServerSocketChannel的无参构造：

public class NioServerSocketChannel extends AbstractNioMessageChannel implements ServerSocketChannel {
    private static final SelectorProvider DEFAULT_SELECTOR_PROVIDER = SelectorProvider.provider();
    
    public NioServerSocketChannel() {
        this(newSocket(DEFAULT_SELECTOR_PROVIDER));
    }
    ......
}

SelectorProvider 是JDK的，关于SelectorProvider在我之前的博客中有介绍：【Java】NIO中Selector的创建源码分析

在Windows系统下默认产生WindowsSelectorProvider，即DEFAULT_SELECTOR_PROVIDER，再来看看newSocket方法：

private static java.nio.channels.ServerSocketChannel newSocket(SelectorProvider provider) {
    try {
        return provider.openServerSocketChannel();
    } catch (IOException var2) {
        throw new ChannelException("Failed to open a server socket.", var2);
    }
}

使用WindowsSelectorProvider创建了一个ServerSocketChannelImpl，其实看到这里就明白了，NioServerSocketChannel是为了封装JDK的ServerSocketChannel

接着调用另一个重载的构造：

public NioServerSocketChannel(java.nio.channels.ServerSocketChannel channel) {
    super((Channel)null, channel, 16);
    this.config = new NioServerSocketChannel.NioServerSocketChannelConfig(this, this.javaChannel().socket());
}

首先调用父类的三参构造，其中16对应的是JDK中SelectionKey的ACCEPT状态：

public static final int OP_ACCEPT = 1 << 4;

其父类的构造处于一条继承链上：

AbstractNioMessageChannel：

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

AbstractNioChannel：

protected AbstractNioChannel(Channel parent, SelectableChannel ch, int readInterestOp) {
    super(parent);
    this.ch = ch;
    this.readInterestOp = readInterestOp;

    try {
        ch.configureBlocking(false);
    } catch (IOException var7) {
        try {
            ch.close();
        } catch (IOException var6) {
            if (logger.isWarnEnabled()) {
                logger.warn("Failed to close a partially initialized socket.", var6);
            }
        }

        throw new ChannelException("Failed to enter non-blocking mode.", var7);
    }
}

AbstractChannel：

private final ChannelId id;
private final Channel parent;
private final Unsafe unsafe;
private final DefaultChannelPipeline pipeline;

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

在AbstractChannel中使用newUnsafe和newChannelPipeline分别创建了一个Unsafe和一个DefaultChannelPipeline对象，
在前面的博客介绍NioEventLoopGroup时候，在NioEventLoop的run方法中，每次轮询完调用processSelectedKeys方法时，都是通过这个unsafe根据SelectedKey来完成数据的读或写，unsafe是处理基础的数据读写
（unsafe在NioServerSocketChannel创建时，产生NioMessageUnsafe实例，在NioSocketChannel创建时产生NioSocketChannelUnsafe实例）

而pipeline的实现是一条双向责任链，负责处理unsafe提供的数据，进而进行用户的业务逻辑 （Netty中的ChannelPipeline源码分析）

在AbstractNioChannel中调用configureBlocking方法给JDK的ServerSocketChannel设置为非阻塞模式，且让readInterestOp成员赋值为16用于未来注册ACCEPT事件。

在调用完继承链后回到NioServerSocketChannel构造，调用了javaChannel方法：

protected java.nio.channels.ServerSocketChannel javaChannel() {
    return (java.nio.channels.ServerSocketChannel)super.javaChannel();
}

其实这个javaChannel就是刚才出传入到AbstractNioChannel中的ch成员：

protected SelectableChannel javaChannel() {
    return this.ch;
}

也就是刚才创建的JDK的ServerSocketChannelImpl，用其socket方法，得到一个ServerSocket对象，然后产生了一个NioServerSocketChannelConfig对象，用于封装相关信息。

NioServerSocketChannel构建完毕，回到initAndRegister方法，使用刚创建的NioServerSocketChannel调用init方法，这个方法是在ServerBootstrap中实现的：

void init(Channel channel) throws Exception {
    Map<ChannelOption<?>, Object> options = this.options0();
    synchronized(options) {
        setChannelOptions(channel, options, logger);
    }

    Map<AttributeKey<?>, Object> attrs = this.attrs0();
    synchronized(attrs) {
        Iterator var5 = attrs.entrySet().iterator();

        while(true) {
            if (!var5.hasNext()) {
                break;
            }

            Entry<AttributeKey<?>, Object> e = (Entry)var5.next();
            AttributeKey<Object> key = (AttributeKey)e.getKey();
            channel.attr(key).set(e.getValue());
        }
    }

    ChannelPipeline p = channel.pipeline();
    final EventLoopGroup currentChildGroup = this.childGroup;
    final ChannelHandler currentChildHandler = this.childHandler;
    Map var9 = this.childOptions;
    final Entry[] currentChildOptions;
    synchronized(this.childOptions) {
        currentChildOptions = (Entry[])this.childOptions.entrySet().toArray(newOptionArray(0));
    }

    var9 = this.childAttrs;
    final Entry[] currentChildAttrs;
    synchronized(this.childAttrs) {
        currentChildAttrs = (Entry[])this.childAttrs.entrySet().toArray(newAttrArray(0));
    }

    p.addLast(new ChannelHandler[]{new ChannelInitializer<Channel>() {
        public void initChannel(final Channel ch) throws Exception {
            final ChannelPipeline pipeline = ch.pipeline();
            ChannelHandler handler = ServerBootstrap.this.config.handler();
            if (handler != null) {
                pipeline.addLast(new ChannelHandler[]{handler});
            }

            ch.eventLoop().execute(new Runnable() {
                public void run() {
                    pipeline.addLast(new ChannelHandler[]{new ServerBootstrap.ServerBootstrapAcceptor(ch, currentChildGroup, currentChildHandler, currentChildOptions, currentChildAttrs)});
                }
            });
        }
    }});
}

首先对attrs和options这两个成员进行了填充属性配置，这不是重点，然后获取刚才创建的NioServerSocketChannel的责任链pipeline，通过addLast将ChannelInitializer加入责任链，在ChannelInitializer中重写了initChannel方法，首先根据handler是否是null（这个handler是ServerBootstrap调用handler方法添加的，和childHandler方法不一样），若是handler不是null，将handler加入责任链，无论如何，都会异步将一个ServerBootstrapAcceptor对象加入责任链（后面会说为什么是异步）

这个ChannelInitializer的initChannel方法的执行需要等到后面注册时才会被调用，在后面pipeline处理channelRegistered请求时，此initChannel方法才会被执行 （Netty中的ChannelPipeline源码分析）

ChannelInitializer的channelRegistered方法：

public final void channelRegistered(ChannelHandlerContext ctx) throws Exception {
    if (initChannel(ctx)) {
        ctx.pipeline().fireChannelRegistered();
    } else {
        ctx.fireChannelRegistered();
    }
}

首先调用initChannel方法（和上面的initChannel不是一个）：

private boolean initChannel(ChannelHandlerContext ctx) throws Exception {
    if (initMap.putIfAbsent(ctx, Boolean.TRUE) == null) { 
        try {
            initChannel((C) ctx.channel());
        } catch (Throwable cause) {
            exceptionCaught(ctx, cause);
        } finally {
            remove(ctx);
        }
        return true;
    }
    return false;
}

可以看到，这个ChannelInitializer只会在pipeline中初始化一次，仅用于Channel的注册，在完成注册后，会调用remove方法将其从pipeline中移除：
remove方法：

private void remove(ChannelHandlerContext ctx) {
    try {
        ChannelPipeline pipeline = ctx.pipeline();
        if (pipeline.context(this) != null) {
            pipeline.remove(this);
        }
    } finally {
        initMap.remove(ctx);
    }
}

在移除前，就会回调用刚才覆盖的initChannel方法，异步向pipeline添加了ServerBootstrapAcceptor，用于后续的NioServerSocketChannel侦听到客户端连接后，完成在服务端的NioSocketChannel的注册。

回到initAndRegister，在对NioServerSocketChannel初始化完毕，接下来就是注册逻辑：

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

首先调用config().group()，这个就得到了一开始在ServerBootstrap的group方法传入的parentGroup，调用parentGroup的register方法，parentGroup是NioEventLoopGroup，这个方法是在子类MultithreadEventLoopGroup中实现的：

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

首先调用next方法：

public EventLoop next() {
    return (EventLoop)super.next();
}

实际上调用父类MultithreadEventExecutorGroup的next方法：

public EventExecutor next() {
    return this.chooser.next();
}

关于chooser在我之前博客：Netty中NioEventLoopGroup的创建源码分析 介绍过，在NioEventLoopGroup创建时，默认会根据cpu个数创建二倍个NioEventLoop，而chooser就负责通过取模，每次选择一个NioEventLoop使用

所以在MultithreadEventLoopGroup的register方法实际调用了NioEventLoop的register方法：

NioEventLoop的register方法在子类SingleThreadEventLoop中实现：

public ChannelFuture register(Channel channel) {
    return this.register((ChannelPromise)(new DefaultChannelPromise(channel, this)));
}

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

先把channel包装成ChannelPromise，默认是DefaultChannelPromise （Netty中的ChannelFuture和ChannelPromise），用于处理异步操作

调用重载方法，而在重载方法里，可以看到，实际上的register操作交给了channel的unsafe来实现：

unsafe的register方法在AbstractUnsafe中实现：

public final void register(EventLoop eventLoop, final ChannelPromise promise) {
    if (eventLoop == null) {
        throw new NullPointerException("eventLoop");
    } else 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 {
        AbstractChannel.this.eventLoop = eventLoop;
        if (eventLoop.inEventLoop()) {
            this.register0(promise);
        } else {
            try {
                eventLoop.execute(new Runnable() {
                    public void run() {
                        AbstractUnsafe.this.register0(promise);
                    }
                });
            } catch (Throwable var4) {
                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);
            }
        }

    }
}

前面的判断做了一些检查就不细说了，直接看到else块
首先给当前Channel绑定了eventLoop，即通过刚才chooser选择的eventLoop，该Channel也就是NioServerSocketChannel
由于Unsafe的操作是在轮询线程中异步执行的，所里，这里需要判断inEventLoop是否处于轮询中
在之前介绍NioEventLoopGroup的时候说过，NioEventLoop在没有调用doStartThread方法时并没有启动轮询的，所以inEventLoop判断不成立

那么就调用eventLoop的execute方法，实际上的注册方法可以看到调用了AbstractUnsafe的register0方法，而将这个方法封装为Runnable交给eventLoop作为一个task去异步执行
先来看eventLoop的execute方法实现，是在NioEventLoop的子类SingleThreadEventExecutor中实现的：

public void execute(Runnable task) {
    if (task == null) {
        throw new NullPointerException("task");
    } else {
        boolean inEventLoop = this.inEventLoop();
        this.addTask(task);
        if (!inEventLoop) {
            this.startThread();
            if (this.isShutdown() && this.removeTask(task)) {
                reject();
            }
        }

        if (!this.addTaskWakesUp && this.wakesUpForTask(task)) {
            this.wakeup(inEventLoop);
        }

    }
}

这里首先将task，即刚才的注册事件放入阻塞任务队列中，然后调用startThread方法：

private void startThread() {
    if (this.state == 1 && STATE_UPDATER.compareAndSet(this, 1, 2)) {
        try {
            this.doStartThread();
        } catch (Throwable var2) {
            STATE_UPDATER.set(this, 1);
            PlatformDependent.throwException(var2);
        }
    }

}

NioEventLoop此时还没有轮询，所以状态是1，对应ST_NOT_STARTED，此时利用CAS操作，将状态修改为2，即ST_STARTED ，标志着NioEventLoop要启动轮询了，果然，接下来就调用了doStartThread开启轮询线程：

private void doStartThread() {
    assert this.thread == null;

    this.executor.execute(new Runnable() {
        public void run() {
            SingleThreadEventExecutor.this.thread = Thread.currentThread();
            if (SingleThreadEventExecutor.this.interrupted) {
                SingleThreadEventExecutor.this.thread.interrupt();
            }

            boolean success = false;
            SingleThreadEventExecutor.this.updateLastExecutionTime();
            boolean var112 = false;

            int oldState;
            label1907: {
                try {
                    var112 = true;
                    SingleThreadEventExecutor.this.run();
                    success = true;
                    var112 = false;
                    break label1907;
                } catch (Throwable var119) {
                    SingleThreadEventExecutor.logger.warn("Unexpected exception from an event executor: ", var119);
                    var112 = false;
                } finally {
                    if (var112) {
                        int oldStatex;
                        do {
                            oldStatex = SingleThreadEventExecutor.this.state;
                        } while(oldStatex < 3 && !SingleThreadEventExecutor.STATE_UPDATER.compareAndSet(SingleThreadEventExecutor.this, oldStatex, 3));

                        if (success && SingleThreadEventExecutor.this.gracefulShutdownStartTime == 0L && SingleThreadEventExecutor.logger.isErrorEnabled()) {
                            SingleThreadEventExecutor.logger.error("Buggy " + EventExecutor.class.getSimpleName() + " implementation; " + SingleThreadEventExecutor.class.getSimpleName() + ".confirmShutdown() must be called before run() implementation terminates.");
                        }

                        try {
                            while(!SingleThreadEventExecutor.this.confirmShutdown()) {
                                ;
                            }
                        } finally {
                            try {
                                SingleThreadEventExecutor.this.cleanup();
                            } finally {
                                SingleThreadEventExecutor.STATE_UPDATER.set(SingleThreadEventExecutor.this, 5);
                                SingleThreadEventExecutor.this.threadLock.release();
                                if (!SingleThreadEventExecutor.this.taskQueue.isEmpty() && SingleThreadEventExecutor.logger.isWarnEnabled()) {
                                    SingleThreadEventExecutor.logger.warn("An event executor terminated with non-empty task queue (" + SingleThreadEventExecutor.this.taskQueue.size() + ')');
                                }

                                SingleThreadEventExecutor.this.terminationFuture.setSuccess((Object)null);
                            }
                        }

                    }
                }

                do {
                    oldState = SingleThreadEventExecutor.this.state;
                } while(oldState < 3 && !SingleThreadEventExecutor.STATE_UPDATER.compareAndSet(SingleThreadEventExecutor.this, oldState, 3));

                if (success && SingleThreadEventExecutor.this.gracefulShutdownStartTime == 0L && SingleThreadEventExecutor.logger.isErrorEnabled()) {
                    SingleThreadEventExecutor.logger.error("Buggy " + EventExecutor.class.getSimpleName() + " implementation; " + SingleThreadEventExecutor.class.getSimpleName() + ".confirmShutdown() must be called before run() implementation terminates.");
                }

                try {
                    while(!SingleThreadEventExecutor.this.confirmShutdown()) {
                        ;
                    }

                    return;
                } finally {
                    try {
                        SingleThreadEventExecutor.this.cleanup();
                    } finally {
                        SingleThreadEventExecutor.STATE_UPDATER.set(SingleThreadEventExecutor.this, 5);
                        SingleThreadEventExecutor.this.threadLock.release();
                        if (!SingleThreadEventExecutor.this.taskQueue.isEmpty() && SingleThreadEventExecutor.logger.isWarnEnabled()) {
                            SingleThreadEventExecutor.logger.warn("An event executor terminated with non-empty task queue (" + SingleThreadEventExecutor.this.taskQueue.size() + ')');
                        }

                        SingleThreadEventExecutor.this.terminationFuture.setSuccess((Object)null);
                    }
                }
            }

            do {
                oldState = SingleThreadEventExecutor.this.state;
            } while(oldState < 3 && !SingleThreadEventExecutor.STATE_UPDATER.compareAndSet(SingleThreadEventExecutor.this, oldState, 3));

            if (success && SingleThreadEventExecutor.this.gracefulShutdownStartTime == 0L && SingleThreadEventExecutor.logger.isErrorEnabled()) {
                SingleThreadEventExecutor.logger.error("Buggy " + EventExecutor.class.getSimpleName() + " implementation; " + SingleThreadEventExecutor.class.getSimpleName() + ".confirmShutdown() must be called before run() implementation terminates.");
            }

            try {
                while(!SingleThreadEventExecutor.this.confirmShutdown()) {
                    ;
                }
            } finally {
                try {
                    SingleThreadEventExecutor.this.cleanup();
                } finally {
                    SingleThreadEventExecutor.STATE_UPDATER.set(SingleThreadEventExecutor.this, 5);
                    SingleThreadEventExecutor.this.threadLock.release();
                    if (!SingleThreadEventExecutor.this.taskQueue.isEmpty() && SingleThreadEventExecutor.logger.isWarnEnabled()) {
                        SingleThreadEventExecutor.logger.warn("An event executor terminated with non-empty task queue (" + SingleThreadEventExecutor.this.taskQueue.size() + ')');
                    }

                    SingleThreadEventExecutor.this.terminationFuture.setSuccess((Object)null);
                }
            }

        }
    });
}

关于doStartThread方法，我在 Netty中NioEventLoopGroup的创建源码分析 中已经说的很细了，这里就不再一步一步分析了

因为此时还没真正意义上的启动轮询，所以thread等于null成立的，然后调用executor的execute方法，这里的executor是一个线程池，在之前说过的，所以里面的run方法是处于一个线程里面的，然后给thread成员赋值为当前线程，表明正式进入了轮询。
而SingleThreadEventExecutor.this.run()才是真正的轮询逻辑，这在之前也说过，这个run的实现是在父类NioEventLoop中：

protected void run() {
    while(true) {
        while(true) {
            try {
                switch(this.selectStrategy.calculateStrategy(this.selectNowSupplier, this.hasTasks())) {
                case -2:
                    continue;
                case -1:
                    this.select(this.wakenUp.getAndSet(false));
                    if (this.wakenUp.get()) {
                        this.selector.wakeup();
                    }
                default:
                    this.cancelledKeys = 0;
                    this.needsToSelectAgain = false;
                    int ioRatio = this.ioRatio;
                    if (ioRatio == 100) {
                        try {
                            this.processSelectedKeys();
                        } finally {
                            this.runAllTasks();
                        }
                    } else {
                        long ioStartTime = System.nanoTime();
                        boolean var13 = false;

                        try {
                            var13 = true;
                            this.processSelectedKeys();
                            var13 = false;
                        } finally {
                            if (var13) {
                                long ioTime = System.nanoTime() - ioStartTime;
                                this.runAllTasks(ioTime * (long)(100 - ioRatio) / (long)ioRatio);
                            }
                        }

                        long ioTime = System.nanoTime() - ioStartTime;
                        this.runAllTasks(ioTime * (long)(100 - ioRatio) / (long)ioRatio);
                    }
                }
            } catch (Throwable var21) {
                handleLoopException(var21);
            }

            try {
                if (this.isShuttingDown()) {
                    this.closeAll();
                    if (this.confirmShutdown()) {
                        return;
                    }
                }
            } catch (Throwable var18) {
                handleLoopException(var18);
            }
        }
    }
}

首先由于task已经有一个了，就是刚才的注册事件，所以选择策略calculateStrategy最终调用selectNow（也是之前说过的）：

private final IntSupplier selectNowSupplier = new IntSupplier() {
    public int get() throws Exception {
        return NioEventLoop.this.selectNow();
    }
};

int selectNow() throws IOException {
    int var1;
    try {
        var1 = this.selector.selectNow();
    } finally {
        if (this.wakenUp.get()) {
            this.selector.wakeup();
        }

    }

    return var1;
}

使用JDK原生Selector进行selectNow，由于此时没有任何Channel的注册，所以selectNow会立刻返回0，此时就进入default逻辑，由于没有任何注册，processSelectedKeys方法也做不了什么，所以在这一次的轮询实质上只进行了runAllTasks方法，此方法会执行阻塞队列中的task的run方法（还是在之前博客中介绍过），由于轮询是在线程池中的一个线程中运行的，所以task的执行是一个异步操作。（在执行完task，将task移除阻塞对立，线程继续轮询）

这时就可以回到AbstractChannel的register方法中了，由上面可以知道task实际上异步执行了：

AbstractUnsafe.this.register0(promise);

register0方法：

private void register0(ChannelPromise promise) {
    try {
        if (!promise.setUncancellable() || !this.ensureOpen(promise)) {
            return;
        }

        boolean firstRegistration = this.neverRegistered;
        AbstractChannel.this.doRegister();
        this.neverRegistered = false;
        AbstractChannel.this.registered = true;
        AbstractChannel.this.pipeline.invokeHandlerAddedIfNeeded();
        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);
    }

}

可以看到实际上的注册逻辑又交给了AbstractChannel的doRegister，而这个方法在AbstractNioChannel中实现：

protected void doRegister() throws Exception {
    boolean selected = false;

    while(true) {
        try {
            this.selectionKey = this.javaChannel().register(this.eventLoop().unwrappedSelector(), 0, this);
            return;
        } catch (CancelledKeyException var3) {
            if (selected) {
                throw var3;
            }

            this.eventLoop().selectNow();
            selected = true;
        }
    }
}

javaChannel就是之前产生的JDK的ServerSocketChannel，unwrappedSelector在之前说过，就是未经修改的JDK原生Selector，这个Selector和eventLoop是一对一绑定的，可以看到调用JDK原生的注册方法，完成了对ServerSocketChannel的注册，但是注册的是一个0状态（缺省值），而传入的this，即AbstractNioChannel对象作为了一个附件，用于以后processSelectedKeys方法从SelectionKey中得到对应的Netty的Channel（还是之前博客说过）
关于缺省值，是由于AbstractNioChannel不仅用于NioServerSocketChannel的注册，还用于NioSocketChannel的注册，只有都使用缺省值注册才不会产生异常  【Java】NIO中Channel的注册源码分析 ，并且，在以后processSelectedKeys方法会对0状态判断，再使用unsafe进行相应的逻辑处理。

在完成JDK的注册后，调用pipeline的invokeHandlerAddedIfNeeded方法（Netty中的ChannelPipeline源码分析），处理ChannelHandler的handlerAdded的回调，即调用用户添加的ChannelHandler的handlerAdded方法。
调用safeSetSuccess，标志异步操作完成：

protected final void safeSetSuccess(ChannelPromise promise) {
    if (!(promise instanceof VoidChannelPromise) && !promise.trySuccess()) {
        logger.warn("Failed to mark a promise as success because it is done already: {}", promise);
    }
}

关于异步操作我在之前的博客中说的很清楚了：Netty中的ChannelFuture和ChannelPromise

接着调用pipeline的fireChannelRegistered方法，也就是在责任链上调用channelRegistered方法，这时，就会调用之在ServerBootstrap中向pipeline添加的ChannelInitializer的channelRegistered，进而回调initChannel方法，完成对ServerBootstrapAcceptor的添加。

回到register0方法，在处理完pipeline的责任链后，根据当前AbstractChannel即NioServerSocketChannel的isActive：

public boolean isActive() {
    return this.javaChannel().socket().isBound();
}

获得NioServerSocketChannel绑定的JDK的ServerSocketChannel，进而获取ServerSocket，判断isBound：

public boolean isBound() {
   // Before 1.3 ServerSockets were always bound during creation
    return bound || oldImpl;
}

这里实际上就是判断ServerSocket是否调用了bind方法，前面说过register0方法是一个异步操作，在多线程环境下不能保证执行顺序，若是此时已经完成了ServerSocket的bind，根据firstRegistration判断是否需要pipeline传递channelActive请求，首先会执行pipeline的head即HeadContext的channelActive方法：

@Override
public void channelActive(ChannelHandlerContext ctx) throws Exception {
    ctx.fireChannelActive();

    readIfIsAutoRead();
}

在HeadContext通过ChannelHandlerContext 传递完channelActive请求后，会调用readIfIsAutoRead方法：

private void readIfIsAutoRead() {
    if (channel.config().isAutoRead()) {
        channel.read();
    }
}

此时调用AbstractChannel的read方法：

public Channel read() {
    pipeline.read();
    return this;
}

最终在请求链由HeadContext执行read方法：

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

终于可以看到此时调用unsafe的beginRead方法：

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());
    }
}

最终执行了doBeginRead方法，由AbstractNioChannel实现：

protected void doBeginRead() throws Exception {
    final SelectionKey selectionKey = this.selectionKey;
    if (!selectionKey.isValid()) {
        return;
    }
    
    readPending = true;
    
    final int interestOps = selectionKey.interestOps();
    if ((interestOps & readInterestOp) == 0) {
        selectionKey.interestOps(interestOps | readInterestOp);
    }
}

这里，就完成了向Selector注册readInterestOp事件，从前面来看就是ACCEPT事件

回到AbstractBootstrap的doBind方法，在initAndRegister逻辑结束后，由上面可以知道，实际上的register注册逻辑是一个异步操作，在register0中完成
根据ChannelFuture来判断异步操作是否完成，如果isDone，则表明异步操作先完成，即完成了safeSetSuccess方法，
然后调用newPromise方法：

public ChannelPromise newPromise() {
    return pipeline.newPromise();
}

给channel的pipeline绑定异步操作ChannelPromise
然后调用doBind0方法完成ServerSocket的绑定，若是register0这个异步操作还没完成，就需要给ChannelFuture产生一个异步操作的侦听ChannelFutureListener对象，等到register0方法调用safeSetSuccess时，在promise的trySuccess中会回调ChannelFutureListener的operationComplete方法，进而调用doBind0方法

doBind0方法：

private static void doBind0(
        final ChannelFuture regFuture, final Channel channel,
        final SocketAddress localAddress, final ChannelPromise promise) {
    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());
            }
        }
    });
}

向轮询线程提交了一个任务，异步处理bind，可以看到，只有在regFuture异步操作成功结束后，调用channel的bind方法：

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

实际上的bind又交给pipeline，去完成，pipeline中就会交给责任链去完成，最终会交给HeadContext完成：

public void bind(
                ChannelHandlerContext ctx, SocketAddress localAddress, ChannelPromise promise)
                throws Exception {
    unsafe.bind(localAddress, promise);
}

可以看到，绕了一大圈，交给了unsafe完成：

public final void bind(final SocketAddress localAddress, final ChannelPromise promise) {
    assertEventLoop();

    if (!promise.setUncancellable() || !ensureOpen(promise)) {
        return;
    }
    
    if (Boolean.TRUE.equals(config().getOption(ChannelOption.SO_BROADCAST)) &&
        localAddress instanceof InetSocketAddress &&
        !((InetSocketAddress) localAddress).getAddress().isAnyLocalAddress() &&
        !PlatformDependent.isWindows() && !PlatformDependent.maybeSuperUser()) {
        logger.warn(
                "A non-root user can't receive a broadcast packet if the socket " +
                "is not bound to a wildcard address; binding to a non-wildcard " +
                "address (" + localAddress + ") anyway as requested.");
    }

    boolean wasActive = isActive();
    try {
        doBind(localAddress);
    } catch (Throwable t) {
        safeSetFailure(promise, t);
        closeIfClosed();
        return;
    }

    if (!wasActive && isActive()) {
        invokeLater(new Runnable() {
            @Override
            public void run() {
                pipeline.fireChannelActive();
            }
        });
    }

    safeSetSuccess(promise);
}

然而，真正的bind还是回调了doBind方法，最终是由NioServerSocketChannel来实现：

@Override
protected void doBind(SocketAddress localAddress) throws Exception {
    if (PlatformDependent.javaVersion() >= 7) {
        javaChannel().bind(localAddress, config.getBacklog());
    } else {
        javaChannel().socket().bind(localAddress, config.getBacklog());
    }
}

在这里终于完成了对JDK的ServerSocketChannel的bind操作

在上面的

if (!wasActive && isActive()) {
    invokeLater(new Runnable() {
        @Override
        public void run() {
            pipeline.fireChannelActive();
        }
    });
}

这个判断，就是确保在register0中isActive时，还没完成绑定，也就没有beginRead操作来向Selector注册ACCEPT事件，那么就在这里进行注册，进而让ServerSocket去侦听客户端的连接

在服务端ACCEPT到客户端的连接后，在NioEventLoop轮询中，就会调用processSelectedKeys处理ACCEPT的事件就绪，然后交给unsafe的read去处理  Netty中NioEventLoopGroup的创建源码分析

在服务端，由NioMessageUnsafe实现：

public void read() {
        assert eventLoop().inEventLoop();
        final ChannelConfig config = config();
        final ChannelPipeline pipeline = pipeline();
        final RecvByteBufAllocator.Handle allocHandle = unsafe().recvBufAllocHandle();
        allocHandle.reset(config);

        boolean closed = false;
        Throwable exception = null;
        try {
            try {
                do {
                    int localRead = doReadMessages(readBuf);
                    if (localRead == 0) {
                        break;
                    }
                    if (localRead < 0) {
                        closed = true;
                        break;
                    }

                    allocHandle.incMessagesRead(localRead);
                } while (allocHandle.continueReading());
            } catch (Throwable t) {
                exception = t;
            }

            int size = readBuf.size();
            for (int i = 0; i < size; i ++) {
                readPending = false;
                pipeline.fireChannelRead(readBuf.get(i));
            }
            readBuf.clear();
            allocHandle.readComplete();
            pipeline.fireChannelReadComplete();

            if (exception != null) {
                closed = closeOnReadError(exception);

                pipeline.fireExceptionCaught(exception);
            }

            if (closed) {
                inputShutdown = true;
                if (isOpen()) {
                    close(voidPromise());
                }
            }
        } finally {
            if (!readPending && !config.isAutoRead()) {
                removeReadOp();
            }
        }
    }
}

核心在doReadMessages方法，由NioServerSocketChannel实现：

protected int doReadMessages(List<Object> buf) throws Exception {
    SocketChannel ch = SocketUtils.accept(javaChannel());

    try {
        if (ch != null) {
            buf.add(new NioSocketChannel(this, ch));
            return 1;
        }
    } catch (Throwable t) {
        logger.warn("Failed to create a new channel from an accepted socket.", t);

        try {
            ch.close();
        } catch (Throwable t2) {
            logger.warn("Failed to close a socket.", t2);
        }
    }

    return 0;
}

SocketUtils的accept方法其实就是用来调用JDK中ServerSocketChannel原生的accept方法，来得到一个JDK的SocketChannel对象，然后通过这个SocketChannel对象，将其包装成NioSocketChannel对象添加在buf这个List中

由此可以看到doReadMessages用来侦听所有就绪的连接，包装成NioSocketChannel将其放在List中
然后遍历这个List，调用 NioServerSocketChannel的pipeline的fireChannelRead方法，传递channelRead请求，、
在前面向pipeline中添加了ServerBootstrapAcceptor这个ChannelHandler，此时，它也会响应这个请求，回调channelRead方法：

public void channelRead(ChannelHandlerContext ctx, Object msg) {
    final Channel child = (Channel) msg;

    child.pipeline().addLast(childHandler);

    setChannelOptions(child, childOptions, logger);

    for (Entry<AttributeKey<?>, Object> e: childAttrs) {
        child.attr((AttributeKey<Object>) e.getKey()).set(e.getValue());
    }

    try {
        childGroup.register(child).addListener(new ChannelFutureListener() {
            @Override
            public void operationComplete(ChannelFuture future) throws Exception {
                if (!future.isSuccess()) {
                    forceClose(child, future.cause());
                }
            }
        });
    } catch (Throwable t) {
        forceClose(child, t);
    }
}

msg就是侦听到的NioSocketChannel对象，给该对象的pipeline添加childHandler，也就是我们在ServerBootstrap中通过childHandler方法添加的
然后通过register方法完成对NioSocketChannel的注册（和NioServerSocketChannel注册逻辑一样）

至此Netty服务端的启动结束。