nio server启动的第一步，都是要创建一个serverSocketChannel，我截取一段启动代码，一步步分析：

public void afterPropertiesSet() throws Exception {
    // 创建rpc工厂
    ThreadFactory threadRpcFactory = new NamedThreadFactory("NettyRPC ThreadFactory");
    //执行worker线程池数量
    int parallel = Runtime.getRuntime().availableProcessors() * 2;
    // boss
    EventLoopGroup boss = new NioEventLoopGroup(1);
    // worker
    EventLoopGroup worker = new NioEventLoopGroup(parallel,threadRpcFactory, SelectorProvider.provider());
    try {
        ServerBootstrap bootstrap = new ServerBootstrap();
        bootstrap.group(boss, worker).channel(NioServerSocketChannel.class)
                .childHandler(new MessageRecvChannelInitializer(handlerMap))
                .option(ChannelOption.SO_BACKLOG, 128)
                .childOption(ChannelOption.SO_KEEPALIVE, true);

        ChannelFuture future = bootstrap.bind(serverAddress, Integer.valueOf(port)).sync();
        System.out.printf("[author chenjianye] Netty RPC Server start success ip:%s port:%s\n", serverAddress, port);

        // 注册zookeeper服务
        serviceRegistry.register(serverAddress + ":" + port);
        // wait
        future.channel().closeFuture().sync();
    } finally {
        worker.shutdownGracefully();
        boss.shutdownGracefully();
    }
}
入口就在

ChannelFuture future = bootstrap.bind(serverAddress, Integer.valueOf(port)).sync();

顺序往下执行，直到AbstractBootstrap类的doBind方法：

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 promise1 = channel.newPromise();
        doBind0(regFuture, channel, localAddress, promise1);
        return promise1;
    } else {
        final AbstractBootstrap.PendingRegistrationPromise promise = new AbstractBootstrap.PendingRegistrationPromise(channel, null);
        regFuture.addListener(new ChannelFutureListener() {
            public void operationComplete(ChannelFuture future) throws Exception {
                Throwable cause = future.cause();
                if(cause != null) {
                    promise.setFailure(cause);
                } else {
                    promise.executor = channel.eventLoop();
                }

                AbstractBootstrap.doBind0(regFuture, channel, localAddress, promise);
            }
        });
        return promise;
    }
}
=============================================
这个方法需要重点分析，我做个标记，doBind方法分析如下：

1.final ChannelFuture regFuture = this.initAndRegister();

final ChannelFuture initAndRegister() {
    // 通过channel工厂反射创建ServerSocketChannel，并创建了作用于serverSocketChannel的channelPipeline管道
    // 这个管道维护了ctx为元素的双向链表，到目前为止，pipeline的顺序为：head(outBound) ----> tail(inbound)
    Channel channel = this.channelFactory().newChannel();

    try {
　　　　 // 这个init方法第一个主要是设置channel的属性，我不细说了
        // 第二个作用是增加了inbound处理器，channelInitializer，里面有一个initChannel（）方法会在特定时刻被触发，什么时候被触发，后面我会说到。
        this.init(channel);
    } catch (Throwable var3) {
        channel.unsafe().closeForcibly();
        return (new DefaultChannelPromise(channel, GlobalEventExecutor.INSTANCE)).setFailure(var3);
    }

　　 // 到了这里，就开始执行register逻辑，这个是关键，我把register的代码贴出来，跟着后面的代码继续看，跳转到2。
    ChannelFuture regFuture = this.group().register(channel);
    if(regFuture.cause() != null) {
        if(channel.isRegistered()) {
            channel.close();
        } else {
            channel.unsafe().closeForcibly();
        }
    }

    return regFuture;
}

2.AbstractChannel里的register方法：

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;
        // 到这里还是主线程启动，所以会执行else，启动了boss线程,register0方法跳转到3
        if(eventLoop.inEventLoop()) {
            this.register0(promise);
        } else {
            try {
                eventLoop.execute(new OneTimeTask() {
                    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);
            }
        }

    }
}

3.register0方法

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

        boolean t = this.neverRegistered;
        // 这个方法主要是 this.selectionKey = this.javaChannel().register(this.eventLoop().selector, 0, this);

        AbstractChannel.this.doRegister();
        this.neverRegistered = false;
        AbstractChannel.this.registered = true;
        
        // 回调之前doBind方法的listener，boss线程添加了新的任务：bind任务
        this.safeSetSuccess(promise);
        
        // 这个方法东西内容很多
        // 第一步：this.head.fireChannelRegistered()没有什么实质内容，最终执行到inbound的next.invokeChannelRegistered()方法
        // 第二步：根据上文，目前的pipeline顺序为head---->initializer---->tail
        // 第三步：执行initializer
        // 第四步：添加一个新的inbound处理器，ServerBootstrapAcceptor，此时的顺序为：head---->initizlizer---->ServerBootstrapAcceptor---->tail
        // 第五步：移除initizlizer，此时pipeline顺序为：head---->ServerBootstrapAcceptor---->tail
        // 第六步：么有什么实质内容，这个方法就算执行结束了
        AbstractChannel.this.pipeline.fireChannelRegistered();
        
        // 这里channel还没有绑定，所以isActive()方法返回false，不会继续执行，目前boss线程还剩下bind任务
        if(t && AbstractChannel.this.isActive()) {
            AbstractChannel.this.pipeline.fireChannelActive();
        }
    } catch (Throwable var3) {
        this.closeForcibly();
        AbstractChannel.this.closeFuture.setClosed();
        this.safeSetFailure(promise, var3);
    }

}

4.bind任务
bind任务是一个outbound，所以会按照tail---->head的顺序执行，目前只有head是outbound。
headHandler最终会执行AbstractUnsafe的bind方法：

AbstractChannel.this.doBind(localAddress);

public final void bind(SocketAddress localAddress, ChannelPromise promise) {
    if(promise.setUncancellable() && this.ensureOpen(promise)) {
        if(Boolean.TRUE.equals(AbstractChannel.this.config().getOption(ChannelOption.SO_BROADCAST)) && localAddress instanceof InetSocketAddress && !((InetSocketAddress)localAddress).getAddress().isAnyLocalAddress() && !PlatformDependent.isWindows() && !PlatformDependent.isRoot()) {
            AbstractChannel.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 = AbstractChannel.this.isActive();

        try {
　　　　　　  //由于我们是nioServerSocketChannel，所以：this.javaChannel().socket().bind(localAddress, this.config.getBacklog());

            AbstractChannel.this.doBind(localAddress);

        } catch (Throwable var5) {

            this.safeSetFailure(promise, var5);
            this.closeIfClosed();
            return;
        }

　　　　 // 此时已经绑定了，所以isActive()返回true，执行
        if(!wasActive && AbstractChannel.this.isActive()) {
            // 重新往boss线程加入了任务
            this.invokeLater(new OneTimeTask() {
                public void run() {
                    AbstractChannel.this.pipeline.fireChannelActive();
                }
            });
        }

        this.safeSetSuccess(promise);
    }
}

public ChannelPipeline fireChannelActive() {
    // 来回调用，貌似这里没有什么实质内容
    this.head.fireChannelActive();
    if(this.channel.config().isAutoRead()) {
        // 这个是outBound，最终会触发head
　　　　 // head会执行 this.unsafe.beginRead()，最终会执行abstractNioChannel里的doBeginRead（）方法，最终会执行到5

        this.channel.read();
    }

    return this;
}

5.

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

        }
    }
}

终于看到我们熟悉的东西了，最终把selectionKey的interestOps设置为SelectionKey.OP_ACCEPT。

netty源码分析之一：server的启动