NettyServer源码分析

可以先看下NettyServer启动流程分析

以EchoServer为例，main方法如下：

    // Configure the server.
    EventLoopGroup bossGroup = new NioEventLoopGroup(1);
    EventLoopGroup workerGroup = new NioEventLoopGroup();
    try {
        ServerBootstrap b = new ServerBootstrap();
        b.group(bossGroup, workerGroup)
         .channel(NioServerSocketChannel.class)
         .option(ChannelOption.SO_BACKLOG, 100)
         .handler(new LoggingHandler(LogLevel.INFO))
         .childHandler(new ChannelInitializer() {
             @Override
             public void initChannel(SocketChannel ch) throws Exception {
                 ChannelPipeline p = ch.pipeline();
                 p.addLast(new EchoServerHandler());
             }
         });

        // Start the server.
        ChannelFuture f = b.bind(PORT).sync();

        // Wait until the server socket is closed.
        f.channel().closeFuture().sync();
    } finally {
        // Shut down all event loops to terminate all threads.
        bossGroup.shutdownGracefully();
        workerGroup.shutdownGracefully();
    }

ServerBootstrap通过group方法聚合了一个bossGroup和一个workerGroup，他们都是NioEventLoopGroup实例。
当执行ServerBootstrap的bind方法时，实际上执行的是其父类AbstractBootstrap的bind方法，其中调用了其doBind方法，如下：

  //AbstractBootstrap       
  private ChannelFuture doBind(final SocketAddress localAddress) {
    /**
     * 创建，初始化channel，将channel注册到selector
     */
    final ChannelFuture regFuture = initAndRegister();//(1)
    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);//(2)
        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) {
                    // Registration on the EventLoop failed so fail the ChannelPromise directly to not cause an
                    // IllegalStateException once we try to access the EventLoop of the Channel.
                    promise.setFailure(cause);
                } else {
                    // Registration was successful, so set the correct executor to use.
                    // See https://github.com/netty/netty/issues/2586
                    promise.executor = channel.eventLoop();

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

主要是(1)(2)两处的代码，先分析(1)处的initAndRegister方法，如下：

final ChannelFuture initAndRegister() {
    Channel channel = null;
    try {
        /**
         * 创建channel对象，对于server端为NioServerSocketChannel
         */
        channel = channelFactory().newChannel();(3)
        init(channel);(4)
    } 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);
    }

    /**
     * 在boss EventLoopGroup中注册该channel
     * 从boss EventLoopGroup中选出一个EventLoop注册该channel ->SingleThreadEventLoop.register -> channel.unsafe().register
     *
     */
    ChannelFuture regFuture = group().register(channel);(5)
    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;
}

(3)处会创建channel，因为在ServerBootstrap中设置了channel(NioServerSocketChannel.class)，因此这里创建的channel为NioServerSocketChannel类型。
(4)用于初始化该NioServerSocketChannel，调用了其子类ServerBootstrap.init(Channel channel)方法，如下：

/**
 * 该方法用于初始化创建的channel对象，channelfactory创建的channel
 */
@Override
void init(Channel channel) throws Exception {
    final Map, Object> options = options();
    synchronized (options) {
        channel.config().setOptions(options);
    }

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