Loading multiple entities by id efficiently in Hibernate

What you're asking to do here is for Hibernate to do special case handling for your Criteria, which is kind of a lot to ask.

You'll have to do it yourself, but it's not hard. Using SessionFactory.getCache(), you can get a reference to the actual storage for cached objects. Do something like the following:

for (Long id : allRequiredIds) {
  if (!sessionFactory.getCache().containsEntity(Song.class, id)) {
    idsToQueryDatabaseFor.add(id)
  } else {
    songs.add(session.get(Song.class, id));
  }
}

List<Song> fetchedSongs = session.createCriteria(Song.class).add(Restrictions.in("id",idsToQueryDatabaseFor).list();
songs.addAll(fetchedSongs);

Then the Songs from the cache get retrieved from there, and the ones that are not get pulled with a single select.

If you know that the IDs exist, you can use load(..) to create a proxy without actually hitting the DB:

Return the persistent instance of the given entity class with the given identifier, obtaining the specified lock mode, assuming the instance exists.

List<Song> list = new ArrayList<>(ids.size());
for (Integer id : ids)
  list.add(session.load(Song.class, id, LockOptions.NONE));

Once you access a non-identifier accessor, Hibernate will check the caches and fallback to DB if needed, using batch-fetching if configured.

If the ID doesn't exists, a ObjectNotFoundException will occur once the object is loaded. This might be somewhere in your code where you wouldn't really expect an exception - you're using a simple accessor in the end. So either be 100% sure the ID exists or at least force a ObjectNotFoundException early where you'd expect it, e.g. right after populating the list.