无法通过@PersistenceContext注入EntityManager - NullPointerException

Question

我正在尝试创建一个DataManager，用作我的Beans的外观。 不幸的是，我无法让EntityManager工作。 我试图通过@PersistenceContext注入它，但是当使用EntityManager时，我总是得到一个NullPointerException。

@ApplicationScoped
public class DataManager {

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

  /*
   * Adds a new employee to the database.
   * @param employee - the employee object that will be added.
   * @return true if transaction was successful.
   */
  public boolean createEmployee(Employee employee){     
    try {
        if (employee != null) {
            if (em.createNamedQuery("Employee.findByMail").setParameter("mail",employee.getMail()).getResultList().isEmpty()) {
                em.persist(employee);
                return true;
            }
        }
    } catch (Exception e) {
        e.printStackTrace();
    } 
    return false;
  }

}