我已经用websocket把连接代码和redis的数据插入到了web服务器代码中，出现了标题这样的错误，我正在做一个聊天应用程序，我应该怎么做来修复它呢？谢谢
@serverendpoint（value=“/chatroomserver”）公共类chatroomserverendpoint{

static Set<Session> users = Collections.synchronizedSet(new HashSet<>());

@OnOpen
public void handleOpen(Session session) {
    users.add(session);
}

@OnMessage
public void handleMessage(String message, Session userSession) throws IOException {
    System.out.println("tien");
    String username = (String) userSession.getUserProperties().get("username");
     Jedis jedis = new Jedis("localhost"); 
      System.out.println("Connection to server sucessfully"); 

      System.out.println("Server is running: "+jedis.ping()); 
      List<String> list = jedis.lrange("tutorial-list", 0 ,5); 
      for(int i = 0; i<list.size(); i++) { 
          System.out.println("List of stored keys:: "+list.get(i)); 

      }

    if (username == null) {
        userSession.getUserProperties().put("username", message);
        userSession.getBasicRemote().sendText("System: you are connectd as " + message);
    } else {
        for (Session session : users) {
            session.getBasicRemote().sendText(username + ": " + message);
        }
    }

}

@OnClose
public void handleClose(Session session) {
    users.remove(session);
}

@OnError
public void handleError(Throwable t) {
    t.printStackTrace();
}

}