Entitymanager.flush() VS EntityManager.getTransaction().commit - What should I prefer?

What should I prefer when updating the database? What are the pros & cons with either method and when shall I use the one or the other?

public void disemployEmployee(Integer employeeId, Date endDate) {
    Employee employee = (Employee)em.find("Employee", employeeId);
    employee.getPeriod().setEndDate(endDate);
    em.flush();
}

public void disemployEmployee(Integer employeeId, Date endDate) {
    Employee employee = (Employee)em.find("Employee", employeeId);
    em.getTransaction().begin();
    employee.getPeriod().setEndDate(endDate);
    em.getTransaction().commit();
}