How to Use Spring Boot, Netty, and WebSocket for Server‑to‑Client Push

Introduction

Netty provides a powerful abstraction over Java NIO with a simple API and a large open‑source community. This guide demonstrates a basic Netty + WebSocket example that enables the backend to push messages to the frontend.

Netty Server

@Component
public class NettyServer {
    static final Logger log = LoggerFactory.getLogger(NettyServer.class);
    @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 handles TCP connection requests, workGroup handles read/write with clients
            bootstrap.group(bossGroup, workGroup);
            // Use NIO channel type
            bootstrap.channel(NioServerSocketChannel.class);
            // Set listening port
            bootstrap.localAddress(new InetSocketAddress(port));
            // Set pipeline
            bootstrap.childHandler(nettyInitializer);
            // Bind server and block until successful
            ChannelFuture channelFuture = null;
            try {
                channelFuture = bootstrap.bind().sync();
                log.info("Server started and listen on:{}", channelFuture.channel().localAddress());
                // Listen for channel close
                channelFuture.channel().closeFuture().sync();
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }).start();
    }
    @PreDestroy
    public void destroy() throws InterruptedException {
        if (bossGroup != null) {
            bossGroup.shutdownGracefully().sync();
        }
        if (workGroup != null) {
            workGroup.shutdownGracefully().sync();
        }
    }
}

Netty Configuration

public class NettyConfig {
    // Global singleton channel group to manage all channels
    private static volatile ChannelGroup channelGroup = null;
    // Map request ID to channel
    private static volatile ConcurrentHashMap<String, Channel> channelMap = null;
    // Two lock objects
    private static final Object lock1 = new Object();
    private static final Object lock2 = new Object();
    public static ChannelGroup getChannelGroup() {
        if (channelGroup == null) {
            synchronized (lock1) {
                if (channelGroup == null) {
                    channelGroup = new DefaultChannelGroup(GlobalEventExecutor.INSTANCE);
                }
            }
        }
        return channelGroup;
    }
    public static ConcurrentHashMap<String, Channel> getChannelMap() {
        if (channelMap == null) {
            synchronized (lock2) {
                if (channelMap == null) {
                    channelMap = new ConcurrentHashMap<>();
                }
            }
        }
        return channelMap;
    }
    public static Channel getChannel(String userId) {
        if (channelMap == null) {
            return getChannelMap().get(userId);
        }
        return channelMap.get(userId);
    }
}

Pipeline Configuration

@Component
public class ProjectInitializer extends ChannelInitializer<SocketChannel> {
    static final String WEBSOCKET_PROTOCOL = "WebSocket";
    @Value("${webSocket.netty.path:/webSocket}")
    String webSocketPath;
    @Autowired
    WebSocketHandler webSocketHandler;
    @Override
    protected void initChannel(SocketChannel socketChannel) throws Exception {
        // Set up the pipeline
        ChannelPipeline pipeline = socketChannel.pipeline();
        // HTTP codec (WebSocket is based on HTTP)
        pipeline.addLast(new HttpServerCodec());
        pipeline.addLast(new ObjectEncoder());
        // Chunked write handler
        pipeline.addLast(new ChunkedWriteHandler());
        pipeline.addLast(new HttpObjectAggregator(8192));
        pipeline.addLast(new WebSocketServerProtocolHandler(webSocketPath, WEBSOCKET_PROTOCOL, true, 65536 * 10));
        // Custom business logic handler
        pipeline.addLast(webSocketHandler);
    }
}

Custom Handler

@Component
@ChannelHandler.Sharable
public class WebSocketHandler extends SimpleChannelInboundHandler<TextWebSocketFrame> {
    private static final Logger log = LoggerFactory.getLogger(NettyServer.class);
    /** Executed when a new connection is added */
    @Override
    public void handlerAdded(ChannelHandlerContext ctx) throws Exception {
        log.info("New client connected: [{}]", ctx.channel().id().asLongText());
        // Add to global channel group
        NettyConfig.getChannelGroup().add(ctx.channel());
    }
    /** Read incoming data */
    @Override
    protected void channelRead0(ChannelHandlerContext ctx, TextWebSocketFrame msg) throws Exception {
        log.info("Server received message: {}", msg.text());
        // Parse user ID and associate with channel
        JSONObject jsonObject = JSONUtil.parseObj(msg.text());
        String uid = jsonObject.getStr("uid");
        NettyConfig.getChannelMap().put(uid, ctx.channel());
        // Store user ID as channel attribute for later retrieval
        AttributeKey<String> key = AttributeKey.valueOf("userId");
        ctx.channel().attr(key).setIfAbsent(uid);
        // Reply to client
        ctx.channel().writeAndFlush(new TextWebSocketFrame("Server received your message"));
    }
    @Override
    public void handlerRemoved(ChannelHandlerContext ctx) throws Exception {
        log.info("User went offline: {}", ctx.channel().id().asLongText());
        // Remove from channel group
        NettyConfig.getChannelGroup().remove(ctx.channel());
        removeUserId(ctx);
    }
    @Override
    public void exceptionCaught(ChannelHandlerContext ctx, Throwable cause) throws Exception {
        log.info("Exception: {}", cause.getMessage());
        // Remove channel on error
        NettyConfig.getChannelGroup().remove(ctx.channel());
        removeUserId(ctx);
        ctx.close();
    }
    /** Remove user‑channel mapping */
    private void removeUserId(ChannelHandlerContext ctx) {
        AttributeKey<String> key = AttributeKey.valueOf("userId");
        String userId = ctx.channel().attr(key).get();
        NettyConfig.getChannelMap().remove(userId);
    }
}

Push Message Service and Implementation

public interface PushMsgService {
    /** Push to a specific user */
    void pushMsgToOne(String userId, String msg);
    /** Push to all users */
    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("Socket server not connected");
        }
        channel.writeAndFlush(new TextWebSocketFrame(msg));
    }
    @Override
    public void pushMsgToAll(String msg) {
        NettyConfig.getChannelGroup().writeAndFlush(new TextWebSocketFrame(msg));
    }
}

Testing the Flow

The article includes a series of screenshots that show:

Connecting to the server.

Establishing the WebSocket handshake.

Sending a message from the client.

Receiving the server’s acknowledgment.

Calling the push‑message API to broadcast a message to the frontend.

After these steps, the simple Netty example successfully pushes messages from the backend to the web client.