需求:页面每分钟获取最新告警。
问题:最开始前端使用定时器每一分钟去服务端取告警，这样导致session永远不会超时。
解决:使用websocket协议来推送告警。
pom中增加：

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

增加配置类,使用ServerEndpoint创建websocket endpoint

@Configuration
public class WebSocketConfig {

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

后端具体实现类

@ServerEndpoint("/alarm")
@Component
@Slf4j
public class AlarmSocket {

    @Autowired
    IndexService indexService;

    /**
     * 用于存放所有在线客户端
     */
    private static Map<String, Session> clients = new ConcurrentHashMap<>();

    @OnOpen
    public void onOpen(Session session) {
        log.info("有新的客户端上线: {}", session.getId());
        clients.put(session.getId(), session);
    }

    @OnClose
    public void onClose(Session session) {
        String sessionId = session.getId();
        log.info("有客户端离线: {}", sessionId);
        clients.remove(sessionId);
    }

    @OnError
    public void onError(Session session, Throwable throwable) {
        String sessionId = session.getId();
        if (clients.get(sessionId) != null) {
            log.info("发生了错误,移除客户端: {}", sessionId);
            clients.remove(sessionId);
        }
        throwable.printStackTrace();
    }

    @OnMessage
    public void onMessage(String message, Session session) {
        log.info("收到客户端发来的消息: {}", message);
    }

    /**
     * 每一分钟群发一次告警消息
     */
    @Scheduled(cron="0 0/1 * * * ?")
    private void sendAll() {
        String msg=JSON.toJSONString(R.ok(indexService.getTopAlarm()));
        for (Map.Entry<String, Session> sessionEntry : clients.entrySet()) {
            try {
                sessionEntry.getValue().getAsyncRemote().sendText(msg);
            }catch (Exception e){
                log.error("发送告警消息失败,删除session"+sessionEntry.getValue());
                try {
                    clients.remove(sessionEntry.getKey());
                }catch (Exception e1){
                    log.error("根据sessionId删除失败，清除sessionMap");
                    clients.clear();
                }
            }
        }
    }

}

前端代码

function startSocket() {
        let url='ws://localhost:8088/monitor/alarm';
        //用于告警定时刷新,解决session不超时问题
        var webSocket = null;
        if ('WebSocket' in window) {
            webSocket = new WebSocket(url);
        }
        else {
            alert('Not support websocket')
        }
        webSocket.onerror = function (event) {
            onError(event)
        };

        webSocket.onopen = function (event) {
            onOpen(event)
        };

        webSocket.onmessage = function (event) {
            onMessage(event)
        };

        function onMessage(event) {
            var res = eval('(' + event.data + ')');
            console.log(res);
        }

        function onOpen(event) {
            console.log('Connection established');
            start();
        }

        function onError(event) {
            console.log('socket error==='+event.data);
        }

        function start() {
            webSocket.send('你好');
            return false;
        }
    }