Guava CacheLoader Misses

Question

I'm at a loss here.

I've had really good luck manually loading/inserting into a Guava cache, but I decided it should be more "thread safe" (via the CacheLoader).

When I refactored to use the CacheLoader.load style syntax my cache stopped working. Every get() is a cache miss now, and kicks off a call to load().

Am I missing something really simple?

Cache:

public class DatabasePropertyCache implements RemovalListener<DatabasePropertyCache.Key, Optional<Object>> {

    //InitializeOnDemand style singleton
    //  http://en.wikipedia.org/wiki/Singleton_pattern#Initialization_On_Demand_Holder_Idiom
    private static class Singleton {
        static DatabasePropertyCache INSTANCE = new DatabasePropertyCache();
    }
    public static DatabasePropertyCache getInstance() {
        return Singleton.INSTANCE;
    }
    private DatabasePropertyCache() {
        logger = Logger.getLogger(DatabasePropertyCache.class);
        cache = CacheBuilder
                    .newBuilder()
                    .expireAfterWrite(24, TimeUnit.HOURS)
                    .removalListener(this)
                    .build( 
                        new CacheLoader<Key, Optional<Object>>() {

                            @Override
                            public Optional<Object> load(Key key) throws Exception {
                                Object propertyValue = [performs a query]
                                return Optional.fromNullable(propertyValue);
                            }
                        });
    }

    private final LoadingCache<Key, Optional<Object>> cache;
    private final Logger logger;

    public void onRemoval(RemovalNotification<Key, Optional<Object>> arg0) {
        if(logger.isDebugEnabled()) {
            logger.debug(String.format("Cached database property expiring %s", arg0.getKey().toString()));
        }
    }

    /**
     * Returns a cached value. If it has never been cache before, it will be fetched from the database.
     * 
     * @param value
     * @param locale
     * @param dataset
     * @return
     */
    public Object getObject(String id, DatabasePropertiesEnum property, Locale locale) {
        Key key = new Key(id, property, locale);

        try {
            return cache.get(key).orNull();
        }
        catch (ExecutionException eex) {
            logger.warn(String.format("Error fetching database property %s", key.toString()), eex);
        }
        return null;
    }

    protected class Key {

        private static final String TOSTRING_TEMPLATE = "DatabasePropertyCache.Key[dataset=%s, locale=%s, property=%s]";

        private String id;
        private DatabasePropertiesEnum property;
        private Locale locale;

        public Key(String id, DatabasePropertiesEnum property, Locale locale) {
            this.id= id;
            this.property = property;
            this.locale = locale;
        }

        @Override
        public int hashCode() {
            return new HashCodeBuilder()
                    .append(this.id)
                    .append(this.property)
                    .append(this.locale)
                    .toHashCode();
        }

        public String getId() {
            return id;
        }

        public DatabasePropertiesEnum getProperty() {
            return property;
        }

        public Locale getLocale() {
            return locale;
        }

        @Override
        public String toString() {
            return String.format(TOSTRING_TEMPLATE, id, locale.toString(), property.name());
        }
    }
}

Usage:

Object cacheHit = DatabasePropertyCache.getInstance().getObject("some_id", DatabasePropertiesEnum.SOME_KEY, [users current locale]);

Answer 1

Key class must correctly implement equals() and hashcode().

You could use the Guava EqualsTester to test your equals and hashCode:

Answer 2

It looks like your Key class needs to define an equals() method.