EntityManagerFactory numa aplicação web

import javax.persistence.EntityManagerFactory;
import javax.persistence.Persistence;

public class EntityManagerProvider {

    private static EntityManagerFactory emf = null;

    private EntityManagerProvider()
    {
    }

    public static EntityManagerFactory getEntityManagerFactory()
    {
        if (emf == null)
        {
            emf = Persistence.createEntityManagerFactory("PU");
        }
        return emf;
    }

}

emf = EntityManagerProvider.getEntityManagerFactory();