这一篇博客我们介绍一下Netty服务端绑定端口的过程，我们通过跟踪代码一直到NIO原生绑定端口的操作。

绑定端口操作

ChannelFuture future = serverBootstrap.bind(8080).sync();

AbstractBootstrap中bind操作

public ChannelFuture bind(int inetPort) {
        return bind(new InetSocketAddress(inetPort));
}

 public ChannelFuture bind(SocketAddress localAddress) {
        validate();
        if (localAddress == null) {
            throw new NullPointerException("localAddress");
        }
        return doBind(localAddress);
    }

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) {
                        // 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.registered();

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

最终的操作是在doBind0函数中，在线程池中添加一个任务，任务的操作是通过Channel来绑定端口等，在线程池中添加一个任务时会开启Server端的服务监听线程。

private static void doBind0(
            final ChannelFuture regFuture, final Channel channel,
            final SocketAddress localAddress, final ChannelPromise promise) {

        // This method is invoked before channelRegistered() is triggered.  Give user handlers a chance to set up
        // the pipeline in its channelRegistered() implementation.
        channel.eventLoop().execute(new Runnable() {
            @Override
            public void run() {
                if (regFuture.isSuccess()) {
                    channel.bind(localAddress, promise).addListener(ChannelFutureListener.CLOSE_ON_FAILURE);
                } else {
                    promise.setFailure(regFuture.cause());
                }
            }
        });
    }

bind操作是在AbstractChannel

 @Override
    public ChannelFuture bind(SocketAddress localAddress, ChannelPromise promise) {
        return pipeline.bind(localAddress, promise);
    }

 @Override
    public final ChannelFuture bind(SocketAddress localAddress, ChannelPromise promise) {
        return tail.bind(localAddress, promise);
    }

private void invokeBind(SocketAddress localAddress, ChannelPromise promise) {
        if (invokeHandler()) {
            try {
                ((ChannelOutboundHandler) handler()).bind(this, localAddress, promise);
            } catch (Throwable t) {
                notifyOutboundHandlerException(t, promise);
            }
        } else {
            bind(localAddress, promise);
        }
    }

HeadContext中调用bind函数，具体实现在Unsafe中。

@Override
public void bind(
    ChannelHandlerContext ctx, SocketAddress localAddress, ChannelPromise promise)
                throws Exception {
    unsafe.bind(localAddress, promise);
}

AbstractUnsafe中调用bind函数

@Override
public final void bind(final SocketAddress localAddress, final ChannelPromise promise) {

	........

	doBind(localAddress);

	.......
}
		

最终在NioServerSocketChannle中调用doBind函数，javaChannel其实获得的对象就是ServerSocketChannel。

@Override
protected void doBind(SocketAddress localAddress) throws Exception {
    if (PlatformDependent.javaVersion() >= 7) {
        javaChannel().bind(localAddress, config.getBacklog());
    } else {
        javaChannel().socket().bind(localAddress, config.getBacklog());
    }
}

这样就完成了服务端的端口绑定过程，同时在绑定端口的过程会调用线程池中的任务，运行一个监听线程。