Question

I am using Guava to cache hot data. When the data does not exist in the cache, I have to get it from database:

public final static LoadingCache<ObjectId, User> UID2UCache = CacheBuilder.newBuilder()
        //.maximumSize(2000)
        .weakKeys()
        .weakValues()
        .expireAfterAccess(10, TimeUnit.MINUTES)
        .build(
        new CacheLoader<ObjectId, User>() {
            @Override
            public User load(ObjectId k) throws Exception {
                User u = DataLoader.datastore.find(User.class).field("_id").equal(k).get();
                return u;
            }
        });

My problem is when the data does not exists in database, I want it to return null and to not do any caching. But Guava saves null with the key in the cache and throws an exception when I get it:

com.google.common.cache.CacheLoader$InvalidCacheLoadException:
CacheLoader returned null for key shisoft.

How do we avoid caching null values?

Answer 1

Just throw some Exception if user is not found and catch it in client code while using get(key) method.

new CacheLoader<ObjectId, User>() {
    @Override
    public User load(ObjectId k) throws Exception {
        User u = DataLoader.datastore.find(User.class).field("_id").equal(k).get();
        if (u != null) {
             return u;
        } else {
             throw new UserNotFoundException();
        }
    }
}

From CacheLoader.load(K) Javadoc:

Returns:  
  the value associated with key; must not be null  
Throws:  
  Exception - if unable to load the result

Answering your doubts about caching null values:

Returns the value associated with key in this cache, first loading
that value if necessary. No observable state associated with this
cache is modified until loading completes.

(from LoadingCache.get(K) Javadoc)

If you throw an exception, load is not considered as complete, so no new value is cached.

EDIT:

Note that in Caffeine, which is sort of Guava cache 2.0 and “provides an in-memory cache using a Google Guava inspired API” you can return null from load method:

 Returns:
   the value associated with key or null if not found

If you may consider migrating, your data loader could freely return when user is not found.