Best approach for checking time remaining in Spring Boot Session from browser / front end?

I have a Spring Boot app using Spring Session and Spring Security. Java @ config. That's all working. I can set my timeout dynamically as I need.

What I need to do is check the remaining session time from the browser. I guess I could check the server with an ajax request every X minutes and have a servlet do a session.getLastAccessedTime() and a session.getMaxInactiveInterval() and do the math to see what time is remaining and return that. (not sure if that would refresh the lastAccessedTime though..)

But as with everything Spring-related, my first instinct is usually wrong. Ha. Plus if the browser is sitting idle for 30 minutes, seems silly to ping the server every ~minute when nothing has changed.

I'm seeing if anyone has a better solution for this. I'd think that maybe I could set a cookie with a epoch time (~1602002425) when the session will timeout. Then I can watch that value with a javascript function. But I don't see a way to do that in Spring. I tried making a CookieSerializer bean creating a custom cookie name, but there is not a way I can see to SET that value. (looks like a random base64 string value gets created for me) This is probably Spring preventing me from doing something the "wrong way". Then this cookie would need to be updated whenever the Session is refreshed as well.

I've spend a couple days looking from an answer online and all I see are ways to set that Session timeout value servers-side (which I can already so) or JavaScript solutions front-end that are not Spring specific.

End goal is to get a ~"session is about to expire.. refresh?" button to popup when client is getting close to timeout. Does anyone have an approach they've use that works for this? I feel like someone has to have solved this, but I am not seeing any posts that address this. Thanks!

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EDIT

I figured out I can use this code to set a cookie and return the lastAccessed time in the response body..

	@RequestMapping("/api/timeout")
	@ResponseBody
	public String add(HttpServletRequest request, HttpServletResponse response) throws Exception
		{
		return getLastAccessed(request, response);
		}

	String getLastAccessed(HttpServletRequest request, HttpServletResponse response)
		{
		ServletRequestAttributes attr = (ServletRequestAttributes) RequestContextHolder.currentRequestAttributes();
		HttpSession session =  attr.getRequest().getSession(false);
		long lastAccessed = session.getLastAccessedTime();

		final String cookieName = "timeout";
		final String cookieValue = String.valueOf(lastAccessed);
		final Boolean useSecureCookie = false;
		final int expiryTime = 60 * 60 * 24;  // 24h in seconds
		final String cookiePath = "/";

		Cookie cookie = new Cookie(cookieName, cookieValue);
		cookie.setSecure(useSecureCookie); 
		cookie.setMaxAge(expiryTime);  
		cookie.setPath(cookiePath);  
		response.addCookie(cookie);

		return String.valueOf(lastAccessed);
		}

But as I feared, the lastAccessed timestamp gets updated every time I check for it. I need to find a way to check that from the client browser without updating it, or another approach that I can use where the cookie either gets update with any 'other' request, or some other "Spring way" of doing this that I am unaware of currently.

Not quite an answer, and this is a bit late for your question, but i am currently trying to solve this problem by looking at using websockets to contact the client pre-emptivly .

My current approach is to have a thread that checks the timeout every n minutes, and is reset whenever a endpoint is called (detecting endpoint calls as per this thread)

https://stackoverflow.com/a/22371901

when timer gets close to 0, use websockets to let the client know

Good overview here https://developer.mozilla.org/en-US/docs/Web/API/WebSockets_API

We are using spring boot and angular, so this tutorial may also help, but it does have some angular specific stuff.

https://www.javaguides.net/2019/06/spring-boot-angular-8-websocket-example-tutorial.html

good luck if you are still trying at this.