Can weakValues() and expireAfterAccess() be combined? - guava

Question

I want to do something like this:
CacheBuilder
.newBuilder()
.maximumSize(CONFIG.cacheMaxSize())
.expireAfterAccess(CONFIG.cacheTimeout(),
CONFIG.cacheTimeUnit())
.weakValues()
.build(cacheLoader);
The behavior I expect is that an entry will only be expired if the value is not referenced AND the expiration time has passed. Is that how this usage will work?

Answer 1

Not directly, since the weak value can be garbage collected as soon as there are no more strong references to the object. What you could do however is use a ForwardingCache backed by two separate caches, a weak-value cache and a timed-expiry cache, so that the time-based cache holds a strong reference to the object thereby keeping it in the weak-value cache. It'd look something like this:
public class WeakValuedExpiringCache<K, V> extends ForwardingCache<K, V> {
private final Cache<K, V> expiringCache;
private final Cache<K, V> weakCache;
public WeakValuedExpiringCache(CacheBuilder expiringSpec) {
expiringCache = expiringSpec.build();
weakCache = CacheBuilder.newBuilder().weakValues().build();
}
// weakCache is the canonical cache since it will hold values longer than
// expiration if there remain other strong references
protected Cache<K, V> delagate() {
return weakCache;
}
#override
public V get(K key, Callable<? extends V> valueLoader)
throws ExecutionException {
// repopulate the expiring cache if needed, and update the weak cache
V value = expiringCache.get(key, valueLoader);
weakCache.put(key, value); // don't call super.put() here
}
#Override
public void put(K key, V value) {
expiringCache.put(key, value);
super.put(key, value);
}
// Handle putAll(), cleanUp(), invalidate(), and invalidateAll() similarly
}
You can do the same thing with a ForwardingLoadingCache as well, just like .get() above you should load the value from the expiringCache and .put() it into the weakCache in the relevant loading methods.

Answer 2

No, an entry will expire if the value is not referenced OR the expiration time has passed:
public class CacheBuilderIT {
#Test
public void expireAfterAccessWithWeakValues() throws InterruptedException {
Cache<Object, Object> cache = CacheBuilder.newBuilder()
.expireAfterAccess(500, MILLISECONDS)
.weakValues()
.build();
Object key = new Object();
Object value = new Object(); // keep a strong reference to the value
cache.put(key, value);
Thread.sleep(300);
assert cache.getIfPresent(key) != null : "expiration occurred too quickly";
Thread.sleep(300);
assert cache.getIfPresent(key) != null : "last access did not reset expiration";
Thread.sleep(1000);
assert cache.getIfPresent(key) != null : "reference did not prevent expiration";
}
}
Ouptut:
java.lang.AssertionError: reference did not prevent expiration