AutoRefreshingCache is a MemoryCache which tries to refresh its hot entries before their actual expiration time is reached.
This class gives you a single public method (i.e. GET).
Here comes the pseudocode.


get(param)
	result = cache.get(param)
	if (result == null)
		refresh(param)
		return cache.get(param)
	if (result needs to be refreshed)
		if (try_lock(result))
			run refresh(param) in a new thread
	return result

refresh(param)
	data = calc(param)
	cache.add(key:param, value:data, expiration:exp_time)