后端向前端推送消息就需要长连接，首先想到的就是websocket。这里推荐一个比websocket更轻量级的长连接推送实现方式：SpringBoot之SseEmitter

contrller：

//@CrossOrigin // 跨域（看具体情况）
@RestController
@RequestMapping("/sse")
public class TestNotify {

    /**
     * 用于创建连接（将用户注册到server中）
     */
    @GetMapping("/connect/{userId}")
    public SseEmitter connect(@PathVariable String userId) {
        return SseEmitterServer.connect(userId);
    }

    @GetMapping("/push/{message}")
    public ResponseEntity<String> push(@PathVariable(name = "message") String message) {
        SseEmitterServer.batchSendMessage(message);
        return ResponseEntity.ok("送消息给所有人");
    }
}

server：

/**
 * 主动向前端推送信息
 * @Date 2021/4/21
 */
public class SseEmitterServer {

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

    /**
     * 当前连接数
     */
    private static AtomicInteger count = new AtomicInteger(0);

    /**
     * 使用map对象，便于根据userId来获取对应的SseEmitter，或者放redis里面
     */
    private static Map<String, SseEmitter> sseEmitterMap = new ConcurrentHashMap<>();

    /**
     * 创建用户连接并返回 SseEmitter
     * @param employeeCode 用户ID
     * @return SseEmitter
     */
    public static SseEmitter connect(String employeeCode) {
        // 设置超时时间，0表示不过期。默认30秒，超过时间未完成会抛出异常：AsyncRequestTimeoutException
        SseEmitter sseEmitter = new SseEmitter(0L);
        // 注册回调
        sseEmitter.onCompletion(completionCallBack(employeeCode));
        sseEmitter.onError(errorCallBack(employeeCode));
        sseEmitter.onTimeout(timeoutCallBack(employeeCode));
        sseEmitterMap.put(employeeCode, sseEmitter);
        // 数量+1
        count.getAndIncrement();
        logger.info("创建新的sse连接，当前用户：{}", employeeCode);
        return sseEmitter;
    }

    /**
     * 给指定用户发送信息
     * @param employeeCode
     * @param jsonMsg
     */
    public static void sendMessage(String employeeCode, String jsonMsg) {
        try {
            SseEmitter emitter = sseEmitterMap.get(employeeCode);
            if (emitter == null) {
                logger.warn("sse用户[{}]不在注册表，消息推送失败", employeeCode);
                return;
            }
            emitter.send(jsonMsg, MediaType.APPLICATION_JSON);
        } catch (IOException e) {
            logger.error("sse用户[{}]推送异常:{}", employeeCode, e.getMessage());
            removeUser(employeeCode);
        }
    }

    /**
     * 群发消息
     * @param jsonMsg
     * @param employeeCodes
     */
    public static void batchSendMessage(String jsonMsg, List<String> employeeCodes) {
        employeeCodes.forEach(employeeCode -> sendMessage(jsonMsg, employeeCode));
    }

    /**
     * 群发所有人
     * @param jsonMsg
     */
    public static void batchSendMessage(String jsonMsg) {
        sseEmitterMap.forEach((k, v) -> {
            try {
                v.send(jsonMsg, MediaType.APPLICATION_JSON);
            } catch (IOException e) {
                logger.error("用户[{}]推送异常:{}", k, e.getMessage());
                removeUser(k);
            }
        });
    }

    /**
     * 移除用户连接
     */
    public static void removeUser(String employeeCode) {
        SseEmitter emitter = sseEmitterMap.get(employeeCode);
        if(emitter != null){
            emitter.complete();
        }
        sseEmitterMap.remove(employeeCode);
        // 数量-1
        count.getAndDecrement();
        logger.info("移除sse用户：{}", employeeCode);
    }

    /**
     * 获取当前连接信息
     */
    public static List<String> getIds() {
        return new ArrayList<>(sseEmitterMap.keySet());
    }

    /**
     * 获取当前连接数量
     */
    public static int getUserCount() {
        return count.intValue();
    }

    private static Runnable completionCallBack(String employeeCode) {
        return () -> {
            logger.info("结束sse用户连接：{}", employeeCode);
            removeUser(employeeCode);
        };
    }

    private static Runnable timeoutCallBack(String employeeCode) {
        return () -> {
            logger.info("连接sse用户超时：{}", employeeCode);
            removeUser(employeeCode);
        };
    }

    private static Consumer<Throwable> errorCallBack(String employeeCode) {
        return throwable -> {
            logger.info("sse用户连接异常：{}", employeeCode);
            removeUser(employeeCode);
        };
    }

}

html（直接复制保存为html即可使用）：

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>SseEmitter</title>
</head>
<body>
<button onclick="closeSse()">关闭连接</button>
<div id="message"></div>
</body>
<script>
    let source = null;

    // 用时间戳模拟登录用户
    const userId = new Date().getTime();

	console.log(userId);
	
	
    if (!!window.EventSource) {

        // 建立连接
        source = new EventSource('http://localhost:8090/sse/connect/' + userId);

        /**
         * 连接一旦建立，就会触发open事件
         * 另一种写法：source.onopen = function (event) {}
         */
        source.addEventListener('open', function (e) {
            setMessageInnerHTML("建立连接。。。");
        }, false);

        /**
         * 客户端收到服务器发来的数据
         * 另一种写法：source.onmessage = function (event) {}
         */
        source.addEventListener('message', function (e) {
            setMessageInnerHTML(e.data);
        });


        /**
         * 如果发生通信错误（比如连接中断），就会触发error事件
         * 或者：
         * 另一种写法：source.onerror = function (event) {}
         */
        source.addEventListener('error', function (e) {
            if (e.readyState === EventSource.CLOSED) {
                setMessageInnerHTML("连接关闭");
            } else {
                console.log(e);
            }
        }, false);

    } else {
        setMessageInnerHTML("你的浏览器不支持SSE");
    }

    // 监听窗口关闭事件，主动去关闭sse连接，如果服务端设置永不过期，浏览器关闭后手动清理服务端数据
    window.onbeforeunload = function () {
        closeSse();
    };

    // 关闭Sse连接
    function closeSse() {
        source.close();
        const httpRequest = new XMLHttpRequest();
        httpRequest.open('GET', 'http://localhost:8090/sse/close/' + userId, true);
        httpRequest.send();
        console.log("close");
    }

    // 将消息显示在网页上
    function setMessageInnerHTML(innerHTML) {
        document.getElementById('message').innerHTML += innerHTML + '<br/>';
    }
</script>
</html>

原文地址：https://www.jianshu.com/p/32d9989cae6f