1. Add spring-websocket Dependency

<dependency>
    <groupId>org.springframework</groupId>
    <artifactId>spring-websocket</artifactId>
</dependency>

2. Enable WebSocket Support

/**
 * Enable WebSocket support
 */
@Configuration
public class WebsocketConfig {

    @Bean
    public ServerEndpointExporter serverEndpointExporter() {
        return new ServerEndpointExporter();
    }
}

3. WebSocket Utility Class

/**
 * WebSocket utility class
 */
@Slf4j
public class WebsocketUtil {

    private WebsocketUtil() {
    }

    /**
     * Server stores client session objects
     * key:clientId value:session
     */
    public static final Map<String, Session> WEBSOCKET_CLIENT_SESSION_MAP = Maps.newConcurrentMap();

    /**
     * Server sends message to all clients
     *
     * @param message
     */
    public static void sendMessageToAllClient(String message) {
        if (WEBSOCKET_CLIENT_SESSION_MAP.isEmpty()) {
            log.info("Currently no connected clients, message push ignored.");
            return;
        }
        WEBSOCKET_CLIENT_SESSION_MAP.forEach((sessionId, session) -> {
            try {
                session.getBasicRemote().sendText(message);
            } catch (Exception e) {
                log.error("Server failed to send message to client [{}].", sessionId, e);
            }
        });
    }
}

4. WebSocket Server

/**
 * WebSocket server
 */
@Slf4j
@Component
@ServerEndpoint("/websocket")
public class WebsocketServer {

    private String sessionId;

    /**
     * WebSocket connection established
     *
     * @param session
     * @throws IOException
     */
    @OnOpen
    public void onOpen(Session session) throws IOException {
        this.sessionId = session.getId();

        if (WebsocketUtil.WEBSOCKET_CLIENT_SESSION_MAP.containsKey(this.sessionId)) {
            WebsocketUtil.WEBSOCKET_CLIENT_SESSION_MAP.remove(this.sessionId).close();
        }

        WebsocketUtil.WEBSOCKET_CLIENT_SESSION_MAP.put(this.sessionId, session);

        log.info("Client [{}] established WebSocket connection.", this.sessionId);
    }

    /**
     * Close socket connection
     *
     * @param session
     */
    @OnClose
    public void onClose(Session session) {
        if (WebsocketUtil.WEBSOCKET_CLIENT_SESSION_MAP.containsKey(session.getId())) {
            WebsocketUtil.WEBSOCKET_CLIENT_SESSION_MAP.remove(session.getId());
            log.info("Client [{}] disconnected WebSocket connection.", session.getId());
        }
    }

    /**
     * Message received
     *
     * @param message
     */
    @OnMessage
    public void OnMessage(String message) {
        log.info("Received message from client {}: [{}].", this.sessionId, message);
    }

    /**
     * Connection error
     *
     * @param session
     * @param error
     */
    @OnError
    public void OnError(Session session, Throwable error) {
        log.error("WebSocket connection error,", error);
    }
}

5. Testing

Recommended Chrome plugin: Simple Web Socket Client