@Override
        public Set<K> keys() {
            return toStream(cache.iterator())
                    .map(javax.cache.Cache.Entry::getKey)
                    .collect(Collectors.toSet());
        }
Removes all elements in the cache, but leaves the cache in a useable state.
support/jcache/src/main/java/org/apache/shiro/cache/jcache/JCacheManager.java