netty通过websocket主动向浏览器推送消息，代码示例

以下是一个使用Netty实现WebSocket主动向浏览器推送消息的示例代码：

public class WebSocketServer {

    public static void main(String[] args) throws Exception {

        EventLoopGroup bossGroup = new NioEventLoopGroup();
        EventLoopGroup workerGroup = new NioEventLoopGroup();

        try {
            ServerBootstrap bootstrap = new ServerBootstrap()
                    .group(bossGroup, workerGroup)
                    .channel(NioServerSocketChannel.class)
                    .childHandler(new ChannelInitializer<SocketChannel>() {
                        @Override
                        protected void initChannel(SocketChannel ch) throws Exception {
                            ChannelPipeline pipeline = ch.pipeline();
                            pipeline.addLast(new HttpServerCodec());
                            pipeline.addLast(new HttpObjectAggregator(65536));
                            pipeline.addLast(new WebSocketServerProtocolHandler("/ws"));
                            pipeline.addLast(new WebSocketServerHandler());
                        }
                    });

            ChannelFuture future = bootstrap.bind(8080).sync();
            future.channel().closeFuture().sync();
        } finally {
            bossGroup.shutdownGracefully();
            workerGroup.shutdownGracefully();
        }
    }

    private static class WebSocketServerHandler extends SimpleChannelInboundHandler<WebSocketFrame> {

        private static final Logger logger = LoggerFactory.getLogger(WebSocketServerHandler.class);

        @Override
        protected void channelRead0(ChannelHandlerContext ctx, WebSocketFrame frame) throws Exception {
            if (frame instanceof TextWebSocketFrame) {
                String request = ((TextWebSocketFrame) frame).text();
                logger.info("Received message: {}", request);
                // 向浏览器发送消息
                ctx.writeAndFlush(new TextWebSocketFrame("Hello, " + request + "!"));
            } else {
                String message = "Unsupported frame type: " + frame.getClass().getName();
                throw new UnsupportedOperationException(message);
            }
        }

        @Override
        public void exceptionCaught(ChannelHandlerContext ctx, Throwable cause) {
            logger.error("Exception caught: ", cause);
            ctx.close();
        }
    }
}

在这个示例中，我们首先创建了一个`ServerBootstrap`，并且添加了一个`WebSocketServerHandler`来处理WebSocket请求。在`WebSocketServerHandler`中，我们通过`channelRead0`方法获取到来自浏览器的消息，并且向浏览器发送一个回复。

需要注意的是，在这个示例中，我们使用了`HttpServerCodec`和`HttpObjectAggregator`来处理HTTP请求，使用了`WebSocketServerProtocolHandler`来处理WebSocket请求。同时，我们将WebSocket的路径设置为`/ws`，可以根据自己的需要进行修改。