Should I instantiate EntityManager for every method in repository

Question

I am new to Hibernate and I was wondering if I should instantiate EntityManager for every method of my repositories or I can just use one EntityManager object for all the methods in my repository.

Here's my code:

public class ActivityRepositoryImpl extends BaseRepositoryImpl < Activity, Long > implements ActivityRepository {

    public ActivityRepositoryImpl(EntityManagerFactory entityManagerFactory) {
        super(entityManagerFactory);
    }

    @Override
    public void save(Activity activity) {
        EntityManager entityManager = entityManagerFactory.createEntityManager();
        entityManager.getTransaction().begin();
        entityManager.persist(activity);
        entityManager.getTransaction().commit();
        entityManager.close();
    }

    @Override
    public void update(Activity activity) {
        EntityManager entityManager = entityManagerFactory.createEntityManager();
        entityManager.getTransaction().begin();
        entityManager.merge(activity);
        entityManager.getTransaction().commit();
        entityManager.close();
    }
}