Building a Netty + WebSocket Message Push Server in Java

The article introduces Netty, a popular NIO framework, and demonstrates how to build a simple Netty + WebSocket message‑push example.

Netty Server : Shows a @Component NettyServer class with fields for the port, EventLoopGroups, and lifecycle methods start() and destroy() . The start() method creates boss and worker groups, configures ServerBootstrap , binds to a port, and waits for the channel to close.

@Component
public class NettyServer {

    static final Logger log = LoggerFactory.getLogger(NettyServer.class);

    /**
     * 端口号
     */
    @Value("${webSocket.netty.port:8888}")
    int port;

    EventLoopGroup bossGroup;
    EventLoopGroup workGroup;

    @Autowired
    ProjectInitializer nettyInitializer;

    @PostConstruct
    public void start() throws InterruptedException {
        new Thread(() -> {
            bossGroup = new NioEventLoopGroup();
            workGroup = new NioEventLoopGroup();
            ServerBootstrap bootstrap = new ServerBootstrap();
            // bossGroup辅助客户端的tcp连接请求, workGroup负责与客户端之前的读写操作
            bootstrap.group(bossGroup, workGroup);
            // 设置NIO类型的channel
            bootstrap.channel(NioServerSocketChannel.class);
            // 设置监听端口
            bootstrap.localAddress(new InetSocketAddress(port));
            // 设置管道
            bootstrap.childHandler(nettyInitializer);

            // 配置完成，开始绑定server，通过调用sync同步方法阻塞直到绑定成功
            ChannelFuture channelFuture = null;
            try {
                channelFuture = bootstrap.bind().sync();
                log.info("Server started and listen on:{}", channelFuture.channel().localAddress());
                // 对关闭通道进行监听
                channelFuture.channel().closeFuture().sync();
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }).start();
    }

    /**
     * 释放资源
     */
    @PreDestroy
    public void destroy() throws InterruptedException {
        if (bossGroup != null) {
            bossGroup.shutdownGracefully().sync();
        }
        if (workGroup != null) {
            workGroup.shutdownGracefully().sync();
        }
    }
}

Netty Configuration : Provides a NettyConfig class that maintains a global ChannelGroup for all channels, a concurrent map linking user IDs to channels, and two lock objects for thread‑safe lazy initialization.

public class NettyConfig {

    /**
     * 定义全局单利channel组 管理所有channel
     */
    private static volatile ChannelGroup channelGroup = null;

    /**
     * 存放请求ID与channel的对应关系
     */
    private static volatile ConcurrentHashMap
channelMap = null;

    /**
     * 定义两把锁
     */
    private static final Object lock1 = new Object();
    private static final Object lock2 = new Object();

    public static ChannelGroup getChannelGroup() {
        if (null == channelGroup) {
            synchronized (lock1) {
                if (null == channelGroup) {
                    channelGroup = new DefaultChannelGroup(GlobalEventExecutor.INSTANCE);
                }
            }
        }
        return channelGroup;
    }

    public static ConcurrentHashMap
getChannelMap() {
        if (null == channelMap) {
            synchronized (lock2) {
                if (null == channelMap) {
                    channelMap = new ConcurrentHashMap<>();
                }
            }
        }
        return channelMap;
    }

    public static Channel getChannel(String userId) {
        if (null == channelMap) {
            return getChannelMap().get(userId);
        }
        return channelMap.get(userId);
    }
}

Pipeline Configuration : Defines ProjectInitializer extending ChannelInitializer<SocketChannel> to set up the channel pipeline with HTTP codec, object encoder, chunked writer, HTTP aggregator, WebSocket protocol handler, and a custom WebSocketHandler .

@Component
public class ProjectInitializer extends ChannelInitializer
{

    /**
     * webSocket协议名
     */
    static final String WEBSOCKET_PROTOCOL = "WebSocket";

    /**
     * webSocket路径
     */
    @Value("${webSocket.netty.path:/webSocket}")
    String webSocketPath;
    @Autowired
    WebSocketHandler webSocketHandler;

    @Override
    protected void initChannel(SocketChannel socketChannel) throws Exception {
        // 设置管道
        ChannelPipeline pipeline = socketChannel.pipeline();
        // webSocket协议本身是基于http协议的，所以这边也要使用http编解码器
        pipeline.addLast(new HttpServerCodec());
        pipeline.addLast(new ObjectEncoder());
        // 以块的方式来写的处理器
        pipeline.addLast(new ChunkedWriteHandler());
        pipeline.addLast(new HttpObjectAggregator(8192));
        pipeline.addLast(new WebSocketServerProtocolHandler(webSocketPath, WEBSOCKET_PROTOCOL, true, 65536 * 10));
        // 自定义的handler，处理业务逻辑
        pipeline.addLast(webSocketHandler);
    }
}

Custom Handler : Implements WebSocketHandler extending SimpleChannelInboundHandler<TextWebSocketFrame> . It logs new connections, adds channels to the global group, parses incoming JSON messages to associate user IDs with channels, stores the user ID as a channel attribute, and sends acknowledgment frames. It also handles channel removal and exceptions, cleaning up mappings.

@Component
@ChannelHandler.Sharable
public class WebSocketHandler extends SimpleChannelInboundHandler
{
    private static final Logger log = LoggerFactory.getLogger(NettyServer.class);

    /**
     * 一旦连接，第一个被执行
     */
    @Override
    public void handlerAdded(ChannelHandlerContext ctx) throws Exception {
        log.info("有新的客户端链接：[{}]", ctx.channel().id().asLongText());
        // 添加到channelGroup 通道组
        NettyConfig.getChannelGroup().add(ctx.channel());
    }

    /**
     * 读取数据
     */
    @Override
    protected void channelRead0(ChannelHandlerContext ctx, TextWebSocketFrame msg) throws Exception {
        log.info("服务器收到消息：{}", msg.text());

        // 获取用户ID,关联channel
        JSONObject jsonObject = JSONUtil.parseObj(msg.text());
        String uid = jsonObject.getStr("uid");
        NettyConfig.getChannelMap().put(uid, ctx.channel());

        // 将用户ID作为自定义属性加入到channel中，方便随时channel中获取用户ID
        AttributeKey
key = AttributeKey.valueOf("userId");
        ctx.channel().attr(key).setIfAbsent(uid);

        // 回复消息
        ctx.channel().writeAndFlush(new TextWebSocketFrame("服务器收到消息啦"));
    }

    @Override
    public void handlerRemoved(ChannelHandlerContext ctx) throws Exception {
        log.info("用户下线了:{}", ctx.channel().id().asLongText());
        // 删除通道
        NettyConfig.getChannelGroup().remove(ctx.channel());
        removeUserId(ctx);
    }

    @Override
    public void exceptionCaught(ChannelHandlerContext ctx, Throwable cause) throws Exception {
        log.info("异常：{}", cause.getMessage());
        // 删除通道
        NettyConfig.getChannelGroup().remove(ctx.channel());
        removeUserId(ctx);
        ctx.close();
    }

    /**
     * 删除用户与channel的对应关系
     */
    private void removeUserId(ChannelHandlerContext ctx) {
        AttributeKey
key = AttributeKey.valueOf("userId");
        String userId = ctx.channel().attr(key).get();
        NettyConfig.getChannelMap().remove(userId);
    }
}

Push Message Service : Declares a PushMsgService interface with methods to push a message to a specific user or to all users. The implementation PushMsgServiceImpl retrieves the target channel(s) from NettyConfig and sends a TextWebSocketFrame .

public interface PushMsgService {

    /**
     * 推送给指定用户
     */
    void pushMsgToOne(String userId, String msg);

    /**
     * 推送给所有用户
     */
    void pushMsgToAll(String msg);
}

@Service
public class PushMsgServiceImpl implements PushMsgService {

    @Override
    public void pushMsgToOne(String userId, String msg) {
        Channel channel = NettyConfig.getChannel(userId);
        if (Objects.isNull(channel)) {
            throw new RuntimeException("未连接socket服务器");
        }
        channel.writeAndFlush(new TextWebSocketFrame(msg));
    }

    @Override
    public void pushMsgToAll(String msg) {
        NettyConfig.getChannelGroup().writeAndFlush(new TextWebSocketFrame(msg));
    }
}

Testing : The article includes screenshots (omitted here) that show how to connect to the server, send a JSON message containing a user ID, and see the server push messages back to the client, confirming that the Netty WebSocket push service works.

In summary, the tutorial walks through a complete, minimal Netty + WebSocket implementation for real‑time message pushing in a Java backend.