Netty源码分析-Server端启动

本文主要对Netty中Server端启动进行分析，分析Server端是如何绑定端口，初始化Selector，启动NioEventLoop，并最终实现Reactor模式的。
首先看一段经典的Server启动代码。可以看到该过程中实例化了两个EventLoopGroup县城组：bossGroup、workerGroup。可以看到这个Server采用的是Reactor的多线程模式。创建的SimpleNettyServerHandler是通过childHandler()方法加入到workerGroup线程去执行的。

    public void bind(int port) throws Exception {
        EventLoopGroup bossGroup = new NioEventLoopGroup(1);
        EventLoopGroup workerGroup = new NioEventLoopGroup();
        try {
            ServerBootstrap serverBootstrap = new ServerBootstrap();
            serverBootstrap.group(bossGroup, workerGroup)
                .channel(NioServerSocketChannel.class)
                .option(ChannelOption.SO_BACKLOG, 1024)
                .childHandler(new ChannelInitializer() {
                    @Override
                    protected void initChannel(SocketChannel ch) throws Exception {
                        ch.pipeline().addLast(new SimpleNettyServerHandler());
                    }
                });
            ChannelFuture channelFuture = serverBootstrap.bind(port).sync();
            channelFuture.channel().closeFuture().sync();
        } finally {
            bossGroup.shutdownGracefully();
            workerGroup.shutdownGracefully();
        }
    }

1. bind()入口

下面就以serverBootstrap.bind(port)为入口，进一步分析实际Server的启动流程的，直接进入到doBind(final SocketAddress localAddress)方法:
1.会调用initAndRegister()初始化并注册一个Channel，此处的Channel是一个NioServerSocketChannel，该方法返回一个ChannelFuture。
2.判断ChannelFuture是否已经执行完成，如果执行完成会调用doBind0()方法；如果没有执行完成，也会通过添加监听器等待执行完成后再调用doBind0()方法。
因此，重点就落在initAndRegister()和doBind0()这两个方法上来了。

    private ChannelFuture doBind(final SocketAddress localAddress) {
        final ChannelFuture regFuture = initAndRegister();
        final Channel channel = regFuture.channel();
        if (regFuture.cause() != null) {
            return regFuture;
        }

        if (regFuture.isDone()) {
            // At this point we know that the registration was complete and successful.
            ChannelPromise promise = channel.newPromise();
            doBind0(regFuture, channel, localAddress, promise);
            return promise;
        } else {
            // Registration future is almost always fulfilled already, but just in case it's not.
            final PendingRegistrationPromise promise = new PendingRegistrationPromise(channel);
            regFuture.addListener(new ChannelFutureListener() {
                @Override
                public void operationComplete(ChannelFuture future) throws Exception {
                    Throwable cause = future.cause();
                    if (cause != null) {
                        promise.setFailure(cause);
                    } else {
                        promise.registered();

                        doBind0(regFuture, channel, localAddress, promise);
                    }
                }
            });
            return promise;
        }
    }

1.1 initAndRegister()分析

分析一下initAndRegister()方法。通过函数名也大致可以看出该方法工作分为两部分：init(Channel channel)和register。

    final ChannelFuture initAndRegister() {
        Channel channel = null;
        try {
            channel = channelFactory.newChannel();
            init(channel);
        } catch (Throwable t) {
            if (channel != null) {
                // channel can be null if newChannel crashed (eg SocketException("too many open files"))
                channel.unsafe().closeForcibly();
            }
            // as the Channel is not registered yet we need to force the usage of the GlobalEventExecutor
            return new DefaultChannelPromise(channel, GlobalEventExecutor.INSTANCE).setFailure(t);
        }

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

        // If we are here and the promise is not failed, it's one of the following cases:
        // 1) If we attempted registration from the event loop, the registration has been completed at this point.
        //    i.e. It's safe to attempt bind() or connect() now because the channel has been registered.
        // 2) If we attempted registration from the other thread, the registration request has been successfully
        //    added to the event loop's task queue for later execution.
        //    i.e. It's safe to attempt bind() or connect() now:
        //         because bind() or connect() will be executed *after* the scheduled registration task is executed
        //         because register(), bind(), and connect() are all bound to the same thread.

        return regFuture;
    }

1.1.1 init过程分析

看一下init(Channel channel)的实现。首先对Channel设置了ChannelOption和AttributeKey，然后会执行最重要的一步：在Channel的pipeline中添加ChannelHandler。具体ChannelPipeline的讲解会在单独的文章，可以看到该ChannelHandler的最终实现是一个ServerBootstrapAcceptor，通过名字就可以看出这是一个专门做Acceptor的handler，具体对它的分析可以放在文章的后边。值得注意的是给pipeline添加ServerBootstrapAcceptor的过程是通过ch.eventLoop().execute(new Runnable()执行的，ch.eventLoop()返回实际就是NioEventLoop（具体可见：），所以实际是通过NioEventLoop中执行非I/O任务执行的。

    @Override
    void init(Channel channel) throws Exception {
        final Map, Object> options = options0();
        synchronized (options) {
            setChannelOptions(channel, options, logger);
        }

        final Map, Object> attrs = attrs0();
        synchronized (attrs) {
            for (Entry, Object> e: attrs.entrySet()) {
                @SuppressWarnings("unchecked")
                AttributeKey