public synchronized Attribute<X, T> getAttribute(){
        if (attribute == null){
            for (WeakReference<EntityManagerFactoryImpl> factoryRef: factories){
                EntityManagerFactoryImpl factory = factoryRef.get();
                if (factory != null){
                    factory.getDatabaseSession();
                } else {
                    factories.remove(factoryRef);
                }
            }
        }
        return attribute;
    }