@Override
  public Session open(Class<?> entityType) {
    final Map<Object, Object> store = classToStore.computeIfAbsent(entityType, key -> new ConcurrentHashMap<>());
    return new Session(entityType, keyExtractorFactory.create(entityType), store);
  }
mapping between class and its store
criteria/inmemory/src/org/immutables/criteria/inmemory/InMemoryBackend.java