netty源码解析（一）服务端channel创建及初始化

本系列源码是基于netty 4.1.6版本。

netty服务端启动代码如下：

public final class Server {

    public static void main(String[] args) throws Exception {
        EventLoopGroup bossGroup = new NioEventLoopGroup(1);
        EventLoopGroup workerGroup = new NioEventLoopGroup();

        try {
            ServerBootstrap b = new ServerBootstrap();
            b.group(bossGroup, workerGroup)
                    .channel(NioServerSocketChannel.class)
                    .childOption(ChannelOption.TCP_NODELAY, true)
                    .childAttr(AttributeKey.newInstance("childAttr"), "childAttrValue")
                    .handler(new ServerHandler())
                    .childHandler(new ChannelInitializer<SocketChannel>() {
                        @Override
                        public void initChannel(SocketChannel ch) {
                            ch.pipeline().addLast(new AuthHandler());
                            //..

                        }
                    });

            ChannelFuture f = b.bind(8000).sync();

            f.channel().closeFuture().sync();
        } finally {
            bossGroup.shutdownGracefully();
            workerGroup.shutdownGracefully();
        }
    }
}

进入b.bind(8000)，最终进入AbstractBootstrap的doBind方法中

该方法第一行final ChannelFuture regFuture = initAndRegister()就是本节讲解的内容。进入该方法：

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();
            }
        }
        return regFuture;
    }

由上述代码可知channelFactory.newChannel()创建了一个channel。

这个channelFactory的类型是ReflectiveChannelFactory，并且里面的属性clazz的值是NioServerSocketChannel.class。由此我们可以推测出上述的赋值是通过b.group(bossGroup, workerGroup).channel(NioServerSocketChannel.class)实现的。进入channel方法：

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

这个方法的作用就是给channelFactory赋值以及给ReflectiveChannelFactory中的clazz赋值。

进入ReflectiveChannelFactory的newChannel方法：

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

由此可知创建的channel是通过用户传入NioServerSocketChannel.class这个参数，然后通过反射机制构造出NioServerSocketChannel这个对象。

进入NioServerSocketChannel的构造函数：

public NioServerSocketChannel() {
        this(newSocket(DEFAULT_SELECTOR_PROVIDER));
    }

newSocket(DEFAULT_SELECTOR_PROVIDER)的作用是调用SelectorProvider.provider()..openServerSocketChannel()方法构造出java原生的serversocketchannel。进入下面方法：

public NioServerSocketChannel(ServerSocketChannel channel) {
        super(null, channel, SelectionKey.OP_ACCEPT);
        config = new NioServerSocketChannelConfig(this, javaChannel().socket());
    }

该方法的作用是调用父类构造函数，并且生成一个配置类对象。

至此，完成了服务端channel创建