JSF转换器给出空指针异常

我已经阅读了所有与我的问题有关的帖子，但我找不到解决办法。我已将转换器创建到我的国家/地区实体，但当我提交表单时，转换器会抛出ConverterException：

Cannot convert 105 to Country java.lang.NullPointerException 105

其中105是我在表格中选择的国家实体的ID

<h:outputText value="Country (*): " />
<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>

CreateDao源代码：

@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) {
    return em.find(Country.class, id);
}

public Country getById(long id) {
    TypedQuery<Country> query = em.createNamedQuery(Country.FIND_BY_ID, Country.class).setParameter("id", id);
    return query.getSingleResult();
}
}

并修改DAO以打印与国家实体相关的内容：

 public Country find(Long id) {
    Country c = em.find(Country.class, id);
    System.out.println(c.getCountry());
    return c;
 }

我在控制台中看到：

FINE: SELECT ID, COUNTRY FROM COUNTRY WHERE (ID = ?)    bind => [1 parameter bound]
FINEST: Connection released to connection pool [read].
INFO: Italy

在浏览器中：

java.lang.NullPointerException
at Converters.CountryConverter.getAsObject(CountryConverter.java:46)
at com.sun.faces.renderkit.html_basic.HtmlBasicInputRenderer.getConvertedValue(HtmlBasicInputRenderer.java:171)
at com.sun.faces.renderkit.html_basic.MenuRenderer.convertSelectOneValue(MenuRenderer.java:202)
at com.sun.faces.renderkit.html_basic.MenuRenderer.getConvertedValue(MenuRenderer.java:319)
at javax.faces.component.UIInput.getConvertedValue(UIInput.java:1030)
at javax.faces.component.UIInput.validate(UIInput.java:960)
at javax.faces.component.UIInput.executeValidate(UIInput.java:1233)
at javax.faces.component.UIInput.processValidators(UIInput.java:698)
at javax.faces.component.UIComponentBase.processValidators(UIComponentBase.java:1214)
at javax.faces.component.UIComponentBase.processValidators(UIComponentBase.java:1214)
at javax.faces.component.UIForm.processValidators(UIForm.java:253)
at javax.faces.component.UIComponentBase.processValidators(UIComponentBase.java:1214)
at javax.faces.component.UIComponentBase.processValidators(UIComponentBase.java:1214)
at javax.faces.component.UIComponentBase.processValidators(UIComponentBase.java:1214)
at javax.faces.component.UIViewRoot.processValidators(UIViewRoot.java:1172)
at com.sun.faces.lifecycle.ProcessValidationsPhase.execute(ProcessValidationsPhase.java:76)
at com.sun.faces.lifecycle.Phase.doPhase(Phase.java:101)
at com.sun.faces.lifecycle.LifecycleImpl.execute(LifecycleImpl.java:118)
at javax.faces.webapp.FacesServlet.service(FacesServlet.java:593)

---

createDao.find（Long.valueOf（105））

无法编译，您需要一个

String

@AlexandreLavoie 105当前正在作为

String

到达（请参见

value

的类型）。@XtremeBiker我的意思是您可能正在使用

createDao.find（Long.valueOf（“105”）

而不是

createDao.find（Long.valueOf（105））

是的，明白了，我刚看到问题的最后一行；-）@Omar，

@FacesConverter

不需要，只要转换器一直是

@ManagedBean

。如果不是，它可以被称为

converter=“countryConverter”

，如果它是

@facescoverter

。无论如何，@Enrico对于

NullPointerException

您必须调试转换器，但是它似乎没有机会抛出一个空指针，是吗？

FINE: SELECT ID, COUNTRY FROM COUNTRY WHERE (ID = ?)    bind => [1 parameter bound]
FINEST: Connection released to connection pool [read].
INFO: Italy

java.lang.NullPointerException
at Converters.CountryConverter.getAsObject(CountryConverter.java:46)
at com.sun.faces.renderkit.html_basic.HtmlBasicInputRenderer.getConvertedValue(HtmlBasicInputRenderer.java:171)
at com.sun.faces.renderkit.html_basic.MenuRenderer.convertSelectOneValue(MenuRenderer.java:202)
at com.sun.faces.renderkit.html_basic.MenuRenderer.getConvertedValue(MenuRenderer.java:319)
at javax.faces.component.UIInput.getConvertedValue(UIInput.java:1030)
at javax.faces.component.UIInput.validate(UIInput.java:960)
at javax.faces.component.UIInput.executeValidate(UIInput.java:1233)
at javax.faces.component.UIInput.processValidators(UIInput.java:698)
at javax.faces.component.UIComponentBase.processValidators(UIComponentBase.java:1214)
at javax.faces.component.UIComponentBase.processValidators(UIComponentBase.java:1214)
at javax.faces.component.UIForm.processValidators(UIForm.java:253)
at javax.faces.component.UIComponentBase.processValidators(UIComponentBase.java:1214)
at javax.faces.component.UIComponentBase.processValidators(UIComponentBase.java:1214)
at javax.faces.component.UIComponentBase.processValidators(UIComponentBase.java:1214)
at javax.faces.component.UIViewRoot.processValidators(UIViewRoot.java:1172)
at com.sun.faces.lifecycle.ProcessValidationsPhase.execute(ProcessValidationsPhase.java:76)
at com.sun.faces.lifecycle.Phase.doPhase(Phase.java:101)
at com.sun.faces.lifecycle.LifecycleImpl.execute(LifecycleImpl.java:118)
at javax.faces.webapp.FacesServlet.service(FacesServlet.java:593)