@Override
    protected Map<String, T> bulkRestore(final Collection<String> storageKeys) throws ResourceIOException {
        final Map<String, T> resultMap = new HashMap<>();
        for (final String storageKey: storageKeys) {
            final T storageObject = cache.get(storageKey);
            if (storageObject != null) {
                resultMap.put(storageKey, storageObject);
            }
        }
        return resultMap;
    }
Constructs a storage backend using the provided Ehcache
with the given configuration options, but using an alternative
cache entry serialization strategy.
@param cache where to store cached origin responses
@param config cache storage configuration options - note that
  the setting for max object size <b>will be ignored</b> and
  should be configured in the Ehcache instead.
@param serializer alternative serialization mechanism
httpclient5-cache/src/main/java/org/apache/hc/client5/http/impl/cache/ehcache/EhcacheHttpCacheStorage.java