Netty doesn't complain if port is already used

Question

Following problem: I expect an exception if a port is already used.

So I tried to start my server twice in a thread

    public void start() {
    new Thread(new Runnable() {
        @Override
        public void run() {
            EventLoopGroup bossGroup = new EpollEventLoopGroup();
            EventLoopGroup workerGroup = new EpollEventLoopGroup();

            ServerBootstrap serverBootstrap = new ServerBootstrap();
            serverBootstrap.group(bossGroup, workerGroup)
                    .channel(NioServerSocketChannel.class)
                    .childHandler(new ChannelInitializer<SocketChannel>() {
                        @Override
                        protected void initChannel(final SocketChannel socketChannel) throws Exception {
                            socketChannel.pipeline().addLast("decoder", new Decoder(Server.this));
                            socketChannel.pipeline().addLast("encoder", new Encoder(Server.this));
                        }
                    });

            serverBootstrap.bind(port).channel().closeFuture().syncUninterruptibly();
        }
    }).start();
}

But got no exception. I also tried to add an channel handler and catch a exception there, no luck :/

Surrounding the line with bind(port) to catch an exception has also no effect. But I noticed when I added a ChannelListener to the ChannelFuture, that one of the started instances never reached that point.

Anyone an idea?