注入DAO会产生NullPointerException

Question

使用以下Bean，我填写了一个包含国家/地区的表格：

@ManagedBean  
@RequestScoped  
public class CreateUser {  
    @EJB  
    private ParticipantDAO participantDAO;  
    @EJB  
    private CountryDAO countryDAO;  
    private List<Country> countries = new ArrayList<Country>();  
   . . .  
   . . .  
   . . .  
    @PostConstruct  
    public void init() {  
        countries = countryDAO.getAllCountries();  
    }  

在表格中，我必须使用转换器：

 <h:selectOneMenu id="country" value="#{createUser.user.country}" required="true" requiredMessage="Please select a country." converter="#{countryConverter}" >
        <f:selectItem itemValue="#{null}" itemLabel="-- select one --" />
        <f:selectItems value="#{createUser.countries}" var="country" itemValue="#{country}" itemLabel="#{country.country}" />
 </h:selectOneMenu>

转换器给出NullPointerException似乎是因为它无法注入CountryDAO:

@ManagedBean
@RequestScoped
@FacesConverter(forClass = Country.class)
public class CountryConverter implements Converter {
@EJB
private CountryDAO countryDAO;
@Override
public String getAsString(FacesContext context, UIComponent component, Object value) {
    if (!(value instanceof Country)) {
        return null;
    }
    return String.valueOf(((Country) value).getId());
}
@Override
public Object getAsObject(FacesContext context, UIComponent component, String value) {
    if (value == null || value.isEmpty()) {
        return null;
    }
    try {
        System.out.println("Converter Value: " + value);
        Country c = countryDAO.find(Long.valueOf(value));
        System.out.println("Converter: " + c.getCountry());
        return c;
    } catch (Exception e) {
        throw new ConverterException(new FacesMessage(String.format("Cannot convert %s to Country %s %d", value, e.toString(), Long.valueOf((value)))), e);
    }
} 
}

在控制台中，我看到了"Converted Value"消息，但没有看到应该由createDAO.find方法打印的"CountryDAO find"。

@Stateless
@LocalBean
public class CountryDAO {
public CountryDAO() {
}
@PersistenceContext
private EntityManager em;
@Resource
SessionContext context;
public List<Country> getAllCountries() {
    TypedQuery<Country> query = em.createNamedQuery(Country.FIND_ALL, Country.class);
    return query.getResultList();
}
public Country find(Long id) {
    System.out.println("CountryDAO find");
    Country c = em.find(Country.class, id);
    System.out.println(c.getCountry());
    return c;
}

我尝试了用JEE6将EJB注入JSF转换器的解决方案（我不知道我是否将代码放在了正确的位置）。我把它放在转换器中（并获得NullPointerException）：

@ManagedBean
@FacesConverter(forClass = Country.class)
public class CountryConverter implements Converter {
 //    @EJB
//    private CountryDAO countryDAO;
private InitialContext ic;
private CountryDAO countryDAO;
@Override
public String getAsString(FacesContext context, UIComponent component, Object value) {
    if (!(value instanceof Country)) {
        return null;
    }
    return String.valueOf(((Country) value).getId());
}
@Override
public Object getAsObject(FacesContext context, UIComponent component, String value) {
    if (value == null || value.isEmpty()) {
        return null;
    }
    System.out.println("Converter Value: " + value);
    try {
        try {
            ic = new InitialContext();
            countryDAO = (CountryDAO) ic.lookup("java:global/DAO/CountryDAO");
        } catch (NamingException e) {
        }
        Country c = countryDAO.find(Long.valueOf(value));
        System.out.println("Converter: " + c.getCountry());
        return c;
    } catch (Exception e) {
        throw new ConverterException(new FacesMessage(String.format("Cannot convert %s to Country %s %d", value, e.toString(), Long.valueOf((value)))), e);
    }
}

Answer 1

我在其他项目中也遇到了同样的问题，当时我正在为一些素数面组件使用转换器。我用CDI的方法解决了这个问题。

您所要做的就是用@Named注释转换器类（并通过@inject（JEE6）注入DAO类，而不是用JEE5-@EJB）。

您使用绑定属性引用转换器，如下所示：<f:converter binding="#{countryConverter}" />