@Override
    @Nullable
    public V putIfAbsent(@Nullable K key, @Nullable V value) {
        return put(key, value, false);
    }
Return a {@link Reference} to the {@link Entry} for the specified {@code key},
or {@code null} if not found.

@param key         the key (can be {@code null})
@param restructure types of restructure allowed during this call
@return the reference, or {@code null} if not found
ttl-core/src/main/java/com/alibaba/ttl3/internal/util/ConcurrentReferenceHashMap.java