JSF selectonemenu 无法实例化类

Question

我想在我的 JSF 应用程序中创建一个选择一个菜单对象。每当我尝试填写列表时，我都会收到"无法实例化类"异常。这是我的豆子：

@ManagedBean(name = "districtController")
@SessionScoped
public class DistrictController implements Serializable {
    private District current;
    private DataModel items = null;
    @EJB
    private TestoBeans.DistrictFacade ejbFacade;
    private PaginationHelper pagination;
    private int selectedItemIndex;

    public DistrictController() {
        Query q = em.createQuery("select d.dName from District d",District.class);
        List<String> values = q.getResultList();
        for(String d:values){
            System.out.println(d);
        }
    }
    @PersistenceContext(unitName = "TestoIBBPU")
    private EntityManager em;
    @Resource
    private javax.transaction.UserTransaction utx;
    public District getSelected() {
        if (current == null) {
            current = new District();
            selectedItemIndex = -1;
        }
        return current;
    }
    private DistrictFacade getFacade() {
        return ejbFacade;
    }
    public PaginationHelper getPagination() {
        if (pagination == null) {
            pagination = new PaginationHelper(10) {
                @Override
                public int getItemsCount() {
                    return getFacade().count();
                }
                @Override
                public DataModel createPageDataModel() {
                    return new ListDataModel(getFacade().findRange(new int[]{getPageFirstItem(), getPageFirstItem() + getPageSize()}));
                }
            };
        }
        return pagination;
    }
    public String prepareList() {
        recreateModel();
        return "List";
    }
    public String prepareView() {
        current = (District) getItems().getRowData();
        selectedItemIndex = pagination.getPageFirstItem() + getItems().getRowIndex();
        return "View";
    }
    public String prepareCreate() {
        current = new District();
        selectedItemIndex = -1;
        return "Create";
    }
    public String create() {
        try {
            getFacade().create(current);
            JsfUtil.addSuccessMessage(ResourceBundle.getBundle("/Bundle").getString("DistrictCreated"));
            return prepareCreate();
        } catch (Exception e) {
            JsfUtil.addErrorMessage(e, ResourceBundle.getBundle("/Bundle").getString("PersistenceErrorOccured"));
            return null;
        }
    }
    public String prepareEdit() {
        current = (District) getItems().getRowData();
        selectedItemIndex = pagination.getPageFirstItem() + getItems().getRowIndex();
        return "Edit";
    }
    public String update() {
        try {
            getFacade().edit(current);
            JsfUtil.addSuccessMessage(ResourceBundle.getBundle("/Bundle").getString("DistrictUpdated"));
            return "View";
        } catch (Exception e) {
            JsfUtil.addErrorMessage(e, ResourceBundle.getBundle("/Bundle").getString("PersistenceErrorOccured"));
            return null;
        }
    }
    public String destroy() {
        current = (District) getItems().getRowData();
        selectedItemIndex = pagination.getPageFirstItem() + getItems().getRowIndex();
        performDestroy();
        recreatePagination();
        recreateModel();
        return "List";
    }
    public String destroyAndView() {
        performDestroy();
        recreateModel();
        updateCurrentItem();
        if (selectedItemIndex >= 0) {
            return "View";
        } else {
            // all items were removed - go back to list
            recreateModel();
            return "List";
        }
    }
    private void performDestroy() {
        try {
            getFacade().remove(current);
            JsfUtil.addSuccessMessage(ResourceBundle.getBundle("/Bundle").getString("DistrictDeleted"));
        } catch (Exception e) {
            JsfUtil.addErrorMessage(e, ResourceBundle.getBundle("/Bundle").getString("PersistenceErrorOccured"));
        }
    }
    private void updateCurrentItem() {
        int count = getFacade().count();
        if (selectedItemIndex >= count) {
            // selected index cannot be bigger than number of items:
            selectedItemIndex = count - 1;
            // go to previous page if last page disappeared:
            if (pagination.getPageFirstItem() >= count) {
                pagination.previousPage();
            }
        }
        if (selectedItemIndex >= 0) {
            current = getFacade().findRange(new int[]{selectedItemIndex, selectedItemIndex + 1}).get(0);
        }
    }
    public DataModel getItems() {
        if (items == null) {
            items = getPagination().createPageDataModel();
        }
        return items;
    }
    private void recreateModel() {
        items = null;
    }
    private void recreatePagination() {
        pagination = null;
    }
    public String next() {
        getPagination().nextPage();
        recreateModel();
        return "List";
    }
    public String previous() {
        getPagination().previousPage();
        recreateModel();
        return "List";
    }
    public SelectItem[] getItemsAvailableSelectMany() {
        return JsfUtil.getSelectItems(ejbFacade.findAll(), false);
    }
    public SelectItem[] getItemsAvailableSelectOne() {
        return JsfUtil.getSelectItems(ejbFacade.findAll(), true);
    }
    @FacesConverter(forClass = District.class)
    public static class DistrictControllerConverter implements Converter {
        public Object getAsObject(FacesContext facesContext, UIComponent component, String value) {
            if (value == null || value.length() == 0) {
                return null;
            }
            DistrictController controller = (DistrictController) facesContext.getApplication().getELResolver().
                    getValue(facesContext.getELContext(), null, "districtController");
            return controller.ejbFacade.find(getKey(value));
        }
        java.lang.Integer getKey(String value) {
            java.lang.Integer key;
            key = Integer.valueOf(value);
            return key;
        }
        String getStringKey(java.lang.Integer value) {
            StringBuffer sb = new StringBuffer();
            sb.append(value);
            return sb.toString();
        }
        public String getAsString(FacesContext facesContext, UIComponent component, Object object) {
            if (object == null) {
                return null;
            }
            if (object instanceof District) {
                District o = (District) object;
                return getStringKey(o.getId());
            } else {
                throw new IllegalArgumentException("object " + object + " is of type " + object.getClass().getName() + "; expected type: " + District.class.getName());
            }
        }
    }
    private Map<String,String> district = new HashMap<String,String>();
    public Map<String, String> getDistrict() {
        return district;
    }
    public void setDistrict(Map<String, String> district) {
        this.district = district;
    }
    public void persist(Object object) {
        try {
            utx.begin();
            em.persist(object);
            utx.commit();
        } catch (Exception e) {
            Logger.getLogger(getClass().getName()).log(Level.SEVERE, "exception caught", e);
            throw new RuntimeException(e);
        }
    }
}

我怎样才能做到这一点？感谢您的任何想法。

Answer 1

不能在构造函数中使用实体管理器。由于注射尚未发生

而是将此代码移动到带批注的方法@PostConstruct

@PostConstruct
public init() {
    Query q = em.createQuery("select d.dName from District d",District.class);
    districtValues = q.getResultList();
    for(String d:districtValues){
        System.out.println(d);
    }
}

方法

注释@PostConstruct将在注入 EM 和 EJB 后调用。

学习依赖注入时的一个提示是保持示例非常小，并且只让基本的端到端功能正常工作。