New to EJB3, please help/explain.

Inside a session bean I declare an EntityManager as follow

@PersistenceContext(unitName="ScheduleUnit")
private EntityManager em;

and this works. But when I do this

private EntityManager em;
private EntityManagerFactory emf;

public void myFunction() {
  emf = Persistence.createEntityManagerFactory("ScheduleUnit");
  em = emf.createEntityManager();
}

I get the following error:

A JDBC Driver or DataSource class name must be specified in the ConnectionDriverName property