Netty源码笔记（一）Netty服务端处理流程

最近开始了解Netty源码。此系列文章在于对Netty设计思想的个人理解做一些整理，以及关于网络通信延伸的一些思考。
备注：
Netty版本：4.1.6.Final
启动一个Netty服务端，代码示例：

        ServerBootstrap bootstrap = new ServerBootstrap();
                EventLoopGroup bossGroup = new NioEventLoopGroup(1);
        EventLoopGroup workerGroup = new NioEventLoopGroup(0);
        bootstrap.group(bossGroup , workerGroup );
        bootstrap.channel(NioServerSocketChannel.class);
                bootstrap.childHandler(new ChannelInitializer() {
            @Override
            protected void initChannel(SocketChannel ch) throws Exception {
                ChannelPipeline pipeline = ch.pipeline();
                                pipeline.addLast("decode", new MessageDecoder());
                pipeline.addLast("encode", new ClientMessageEncoder());
            }
        });
                
        bootstrap.childOption(ChannelOption.SO_REUSEADDR, true);
        bootstrap.childOption(ChannelOption.TCP_NODELAY, true);
        bootstrap.childOption(ChannelOption.SO_KEEPALIVE, true);
        try {
            bootstrap.bind(port).sync();
        } catch (Exception e) {
            logger.error("Started Netty Server Failed:" + port, e);
        }

EventLoopGroup 为Netty EventLoop线程组，服务端主要用来处理Accept事件，IO事件。默认线程数为CPU内核数*2。
NioServerSocketChannel是对java ServerSocketChannel进行了一层包装。
ChannelPipeline.addLast添加执行器链。
childOption设置TCP连接属性。

接下来从NioServerSocketChannel的初始化、端口的绑定、ChannelPipeline中处理器链的添加、Accept事件监听和初始化子连接四个方面来拆解Netty服务端处理流程。
本文不会分析每一个方法调用，只截取主干函数和主要代码便于迅速理解。

ServerBootstrap可以理解为一个引导类。

image.png

1》：代码入口AbstractBootstrap.doBind(SocketAddress localAddress)

    private ChannelFuture doBind(final SocketAddress localAddress) {
        //详细逻辑见代码片段2》
        final ChannelFuture regFuture = initAndRegister();
        final Channel channel = regFuture.channel();
        ////详细逻辑和说明代码片段5》
        doBind0(regFuture, channel, localAddress, promise);
    }

2》：initAndRegister()完成NioServerSocketChannel初始化。

image.png

final ChannelFuture initAndRegister() {
        //实例化NioServerSocketChannel，详细逻辑见代码片段3》
        Channel channel = channelFactory.newChannel();
        //详细逻辑见代码片段4》
        init(channel);
        ChannelFuture regFuture = config().group().register(channel);
        return regFuture;
}

3》：ReflectiveChannelFactory基于反射创建NioServerSocketChannel实例并进行一系列初始化

    //bootstrap.channel(NioServerSocketChannel.class);配置了ServerSocketChannel的包装类
    public B channel(Class channelClass) {
        if (channelClass == null) {
            throw new NullPointerException("channelClass");
        }
        return channelFactory(new ReflectiveChannelFactory(channelClass));
    }
    public NioServerSocketChannel(ServerSocketChannel channel) {
        //感兴趣的事件类型，后续会注册到selector上
        super(null, channel, SelectionKey.OP_ACCEPT);
        config = new NioServerSocketChannelConfig(this, javaChannel().socket());
    }
    protected AbstractNioMessageChannel(Channel parent, SelectableChannel ch, int readInterestOp) {
        super(parent, ch, readInterestOp);
    }
    protected AbstractNioChannel(Channel parent, SelectableChannel ch, int readInterestOp) {
        super(parent);
        this.ch = ch;
        this.readInterestOp = readInterestOp;
        ch.configureBlocking(false);
    }
    //创建unsafe和DefaultChannelPipeline
    protected AbstractChannel(Channel parent) {
        this.parent = parent;
        id = newId();
        unsafe = newUnsafe();
        pipeline = newChannelPipeline();
    }

4》：初始化NioServerSocketChannel的option, attr，并在上一步已经创建的pipeline对象中添加ServerBootstrapAcceptor处理器。关于ServerBootstrapAcceptor的实现逻辑在后文介绍，这里只需要注意在NioServerSocketChannel的pipeline中添加了一个这么一个处理器，处理的消息对象为ServerSocketChannel的Accept事件建立的子连接SocketChannel。

//初始化ServerSocketChannel的并在NioServerSocketChannel实添加一个handler
//
void init(Channel channel) throws Exception {
        final Map, Object> options = options0();
        synchronized (options) {
            channel.config().setOptions(options);
        }

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