Designing a TTL cache

A GC-backed TTL cache binds three mechanisms:

• WeakHashMap — the collector retires entries whose keys are reachable only from the map. (Values must not reach their own keys, or nothing dies.)
• A 60-second window — System.nanoTime() arithmetic; expired by time.
• A size bound — eviction by policy.

boolean fresh(CacheKey k) {
    CacheLine line = store.get(k);
    if (line == null) return false;
    if (line.expiry() <= System.nanoTime()) { store.remove(k); return false; }
    return true;
}

Each mechanism answers a different failure: time expires stale data, size bounds memory, weak keys release objects whose owners are gone. Remove any one and the cache leaks in that dimension. The checkpoint assembles all three — the same reasoning production caches (Caffeine and friends) apply at industrial strength.