SpringBoot使用WebSocket实现向前端推送消息功能

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

public class WebsocketUtil {
  /**
   * 记录当前在线的Session
   */
  private static final Map<String, Session> ONLINE_SESSION = new ConcurrentHashMap<>();

  /**
   * 添加session
   * @param userId
   * @param session
   */
  public static void addSession(String userId, Session session){
    // 此处只允许一个用户的session链接。一个用户的多个连接，我们视为无效。
    ONLINE_SESSION.putIfAbsent ( userId, session );
  }

  /**
   * 关闭session
   * @param userId
   */
  public static void removeSession(String userId){
    ONLINE_SESSION.remove ( userId );
  }

  /**
   * 给单个用户推送消息
   * @param session
   * @param message
   */
  public static void sendMessage(Session session, String message){
    if(session == null){
      return;
    }
    // 同步
    RemoteEndpoint.Async async = session.getAsyncRemote ();
    async.sendText ( message );
  }

  /**
   * 向所有在线人发送消息
   * @param message
   */
  public static void sendMessageForAll(String message) {
    //jdk8 新方法
    ONLINE_SESSION.forEach((sessionId, session) -> sendMessage(session, message));
  }
}

/**
 * websocket接口处理类
 */
@Component
@ServerEndpoint(value = "/chat/{userId}")
public class WebsocketController {
    /**
     * 连接事件，加入注解
     * @param userId
     * @param session
     */
    @OnOpen
    public void onOpen(@PathParam(value = "userId") String userId, Session session) {
        String message = "[" + userId + "]加入聊天室！！";
        // 添加到session的映射关系中
        WebsocketUtil.addSession(userId, session);
        // 广播通知，某用户上线了
//        WebsocketUtil.sendMessageForAll(message);
    }

    /**
     * 连接事件，加入注解
     * 用户断开链接
     *
     * @param userId
     * @param session
     */
    @OnClose
    public void onClose(@PathParam(value = "userId") String userId, Session session) {
        String message = "[" + userId + "]退出了聊天室...";
        // 删除映射关系
        WebsocketUtil.removeSession(userId);
        // 广播通知，用户下线了
        WebsocketUtil.sendMessageForAll(message);
    }

    /**
     * 当接收到用户上传的消息
     *
     * @param userId
     * @param session
     */
    @OnMessage
    public void onMessage(@PathParam(value = "userId") String userId, Session session, String message) {
        String msg = "[" + userId + "]:" + message;
        System.out.println("接收到信息：" + msg);
        // 直接广播
        WebsocketUtil.sendMessageForAll(msg);
    }

    /**
     * 处理用户活连接异常
     *
     * @param session
     * @param throwable
     */
    @OnError
    public void onError(Session session, Throwable throwable) {
        try {
            session.close();
        } catch (IOException e) {
            e.printStackTrace();
        }
        throwable.printStackTrace();
    }
}

@PostMapping("/send")
public void send(@RequestParam("id")  String id,@RequestParam("message")String message) {
    Session session = ONLINE_SESSION.get(id);
    WebsocketUtil.sendMessage(session,message);
    System.out.println("发送成功");
}

/**
 *AppUtils类
 *
 * @author qiancj
 * @since 2020-06-30 13:02
 */
@Component("appUtils")
public class AppUtils {

    /**
     * 应用上下文
     */
    private static ApplicationContext context;

    @Resource
    public void setApplicationContext(ApplicationContext c) {
        context = c;
    }

    /**
     * 获取配置参数
     *
     * @param property 配置参数名
     * @return 配置参数值
     */
    public static String getProperty(String property) {
        return context.getEnvironment().getProperty(property);
    }

    /**
     * 获取bean对象
     *
     * @param requiredType Class
     * @param <T>          T
     * @return bean对象
     */
    public static <T> T getBean(Class<T> requiredType) {
        return context.getBean(requiredType);
    }

    /**
     * 获取bean对象
     *
     * @param name         bean name
     * @param requiredType Class
     * @param <T>          T
     * @return bean对象
     */
    public static <T> T getBean(String name, Class<T> requiredType) {
        return context.getBean(requiredType);
    }

}

MessageService messageService = AppUtils.getBean(MessageService.class);