Netty server 主动关闭

package com.braun.pumpd.netty;

import io.netty.bootstrap.ServerBootstrap;
import io.netty.channel.*;
import io.netty.channel.nio.NioEventLoopGroup;
import io.netty.channel.socket.SocketChannel;
import io.netty.channel.socket.nio.NioServerSocketChannel;

public class NettyServer {
    public static final int PORT = 5000;
    private Channel serverChannel;

    public void start() throws Exception {
        EventLoopGroup bossGroup = new NioEventLoopGroup();
        EventLoopGroup workerGroup = new NioEventLoopGroup();
        try {
            ServerBootstrap b = new ServerBootstrap();
            b.group(bossGroup, workerGroup)
                    .channel(NioServerSocketChannel.class)
                    .option(ChannelOption.SO_BACKLOG, 100)
                    .childHandler(new ChannelInitializer<SocketChannel>() {
                        @Override
                        public void initChannel(SocketChannel ch) throws Exception {

                        }
                    });

            ChannelFuture f = b.bind(PORT).sync();
            // 保存 serverChannel, 之后调用  close, closeFuture sync 的阻塞就会解除了
            serverChannel = f.channel();
            System.out.println("server started at port " + PORT);
            // 这里 sync 会一直阻塞
            f.channel().closeFuture().sync();

        } finally {
            bossGroup.shutdownGracefully();
            workerGroup.shutdownGracefully();
        }

        System.out.println("server closed");
    }

    public void stop() {
        if (serverChannel != null) {
            serverChannel.close();
            serverChannel = null;
        }
    }
}