@Override
    @Nonnull
    public Lock getLock(K key) {
        return LocalLock.getLock(key, this);
    }
<strong>IMPORTANT:</strong> Unlike the standard {@link Map#keySet()} implementation, the set returned from
this method cannot be modified.

@return an unmodifiable set view of the keys contained in this map
xmppserver/src/main/java/org/jivesoftware/util/cache/CaffeineCache.java