Example.java

	private static final Map<String, Long> allowed = new ConcurrentHashMap<String, Long>();
	private static final AtomicLong lastCleanup = new AtomicLong();
	private static final long MAX_DURATION = TimeUnit.MINUTES.toMillis(45);
	private static final long CLEANUP_INTERVAL = TimeUnit.MINUTES.toMillis(30);
	
	public static void process(String k) {
		long n = System.currentTimeMillis();
		allowed.put(k, n);
		long m = lastCleanup.get();
		if (n - m > CLEANUP_INTERVAL) {
			if (lastCleanup.compareAndSet(m, n)) {
				long mark = n - MAX_DURATION;
				for (String k : allowed.keySet()) {
					allowed.computeIfPresent(k, (key, value) -> (value == null || value.longValue() < mark) ? null : value);
				}
			}
		}
	}