Java 空<；f：选择项目>；在JSF2.0中

我正在使用JSF 2.0 facelets，我正在尝试从数据库中获取selected items值，我尝试了@PostConstruct方法，但它抛出了一个例外，我将很快编写所有内容

我尝试了相同的数据库连接和实体，运行良好

我还应该提到，我在这里使用字符串发送到selectItems，在我使用SelectItem List之前，它也不起作用

有趣的是，当我在while循环中错误地插入了一个try-And-catch块时（你稍后会看到），它一次又一次地抛出异常，这意味着列表不是空的，只是没有到达页面

那是什么原因

我的jsf代码

<h:selectOneMenu >
  <f:selectItems value="#{settingsBean.countryItems}" />
</h:selectOneMenu>

@请求范围 公共类设置Bean{

/**
 * Creates a new instance of SettingsBean
 */
 private String favorits;
private String hobbies;
private String aboutme;
private String education;
private String country;
private String listCountry;
private List<String> countryItems;
private List<Countries> countries;

@EJB
CountriesFacadeLocal cin = new CountriesFacade();


public SettingsBean() {

}
@PostConstruct
public void init()
{
    countries = cin.getAll();
    if(!countries.isEmpty())
    {
        int i =0;
        ListIterator iterator = countries.listIterator();
      while(iterator.hasNext())
      {
          listCountry = countries.get(i).getCountry();
            countryItems.add(i, listCountry);
      }                    
    }
}

---

原因是SettingsBean第56行中的NullPointerException。

---

我不知道这行写了什么，但检查代码时，可能是类变量

countryItems

从未初始化过。

不要将EJB注释与新的 作为 CountriesFacadolocal cin=新CountriesFacade（）；

---

如果您决定使用容器逻辑作为EJB，您应该坚持这样做。否则您的字段根本没有初始化（既不是EJB也不是new）因此为空。

@Developer106:您不理解答案。您已将

countryItems

声明为类变量

private List countryItems；

，但其默认值为

null

。您需要使用

new ArrayList（）对其进行初始化

或其他。您试图在

null

引用上调用的任何方法都会抛出

NullPointerException

，就像

add（）

方法一样。

/**
 * Creates a new instance of SettingsBean
 */
 private String favorits;
private String hobbies;
private String aboutme;
private String education;
private String country;
private String listCountry;
private List<String> countryItems;
private List<Countries> countries;

@EJB
CountriesFacadeLocal cin = new CountriesFacade();


public SettingsBean() {

}
@PostConstruct
public void init()
{
    countries = cin.getAll();
    if(!countries.isEmpty())
    {
        int i =0;
        ListIterator iterator = countries.listIterator();
      while(iterator.hasNext())
      {
          listCountry = countries.get(i).getCountry();
            countryItems.add(i, listCountry);
      }                    
    }
}

@Override
    public List<Countries> getAll()
    {
        try{
        em.flush();
        Query query = em.createNamedQuery("Countries.findAll");
        List<Countries> countryList = query.getResultList();
        if(countryList!= null)
        {
            return countryList;
        }
        else
            return null;
        }
        catch(Exception e)
        {
            return null;
        }
    }

org.jboss.weld.exceptions.WeldException: WELD-000049 Unable to invoke [method] @PostConstruct public com.readme.user.signup.SettingsBean.init() on com.readme.user.signup.SettingsBean@606e86
    at org.jboss.weld.bean.AbstractClassBean.defaultPostConstruct(AbstractClassBean.java:595)
    at org.jboss.weld.bean.ManagedBean$ManagedBeanInjectionTarget.postConstruct(ManagedBean.java:200)
    at org.jboss.weld.bean.ManagedBean.create(ManagedBean.java:340)
    at org.jboss.weld.context.AbstractContext.get(AbstractContext.java:121)
    at org.jboss.weld.bean.proxy.ContextBeanInstance.getInstance(ContextBeanInstance.java:99)
    at org.jboss.weld.bean.proxy.ProxyMethodHandler.invoke(ProxyMethodHandler.java:87)
    at com.readme.user.signup.org$jboss$weld$bean-web-ManagedBean-class_com$readme$user$signup$SettingsBean_$$_WeldClientProxy.getFavorits(org$jboss$weld$bean-web-ManagedBean-class_com$readme$user$signup$SettingsBean_$$_WeldClientProxy.java)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.lang.reflect.Method.invoke(Method.java:601)
    at javax.el.BeanELResolver.getValue(BeanELResolver.java:302)
    at com.sun.faces.el.DemuxCompositeELResolver._getValue(DemuxCompositeELResolver.java:176)
    at com.sun.faces.el.DemuxCompositeELResolver.getValue(DemuxCompositeELResolver.java:203)
    at com.sun.el.parser.AstValue.getValue(AstValue.java:116)
    at com.sun.el.parser.AstValue.getValue(AstValue.java:163)
    at com.sun.el.ValueExpressionImpl.getValue(ValueExpressionImpl.java:219)
    at org.jboss.weld.el.WeldValueExpression.getValue(WeldValueExpression.java:55)
    at com.sun.faces.facelets.el.TagValueExpression.getValue(TagValueExpression.java:109)
    at javax.faces.component.ComponentStateHelper.eval(ComponentStateHelper.java:194)
    at javax.faces.component.ComponentStateHelper.eval(ComponentStateHelper.java:182)
    at javax.faces.component.UIOutput.getValue(UIOutput.java:169)
    at com.sun.faces.renderkit.html_basic.HtmlBasicInputRenderer.getValue(HtmlBasicInputRenderer.java:205)
    at com.sun.faces.renderkit.html_basic.HtmlBasicRenderer.getCurrentValue(HtmlBasicRenderer.java:355)
    at com.sun.faces.renderkit.html_basic.HtmlBasicRenderer.encodeEnd(HtmlBasicRenderer.java:164)
    at javax.faces.component.UIComponentBase.encodeEnd(UIComponentBase.java:875)
    at com.sun.faces.renderkit.html_basic.HtmlBasicRenderer.encodeRecursive(HtmlBasicRenderer.java:312)
    at com.sun.faces.renderkit.html_basic.GridRenderer.renderRow(GridRenderer.java:185)
    at com.sun.faces.renderkit.html_basic.GridRenderer.encodeChildren(GridRenderer.java:129)
    at javax.faces.component.UIComponentBase.encodeChildren(UIComponentBase.java:845)
    at javax.faces.component.UIComponent.encodeAll(UIComponent.java:1756)
    at javax.faces.render.Renderer.encodeChildren(Renderer.java:168)
    at javax.faces.component.UIComponentBase.encodeChildren(UIComponentBase.java:845)
    at javax.faces.component.UIComponent.encodeAll(UIComponent.java:1756)
    at javax.faces.component.UIComponent.encodeAll(UIComponent.java:1759)
    at com.sun.faces.application.view.FaceletViewHandlingStrategy.renderView(FaceletViewHandlingStrategy.java:401)
    at com.sun.faces.application.view.MultiViewHandler.renderView(MultiViewHandler.java:131)
    at javax.faces.application.ViewHandlerWrapper.renderView(ViewHandlerWrapper.java:288)
    at com.sun.faces.lifecycle.RenderResponsePhase.execute(RenderResponsePhase.java:121)
    at com.sun.faces.lifecycle.Phase.doPhase(Phase.java:101)
    at com.sun.faces.lifecycle.LifecycleImpl.render(LifecycleImpl.java:139)
    at javax.faces.webapp.FacesServlet.service(FacesServlet.java:594)
    at org.apache.catalina.core.StandardWrapper.service(StandardWrapper.java:1539)
    at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:281)
    at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:175)
    at org.apache.catalina.core.StandardPipeline.doInvoke(StandardPipeline.java:655)
    at org.apache.catalina.core.StandardPipeline.invoke(StandardPipeline.java:595)
    at com.sun.enterprise.web.WebPipeline.invoke(WebPipeline.java:98)
    at com.sun.enterprise.web.PESessionLockingStandardPipeline.invoke(PESessionLockingStandardPipeline.java:91)
    at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:162)
    at org.apache.catalina.connector.CoyoteAdapter.doService(CoyoteAdapter.java:330)
    at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:231)
    at com.sun.enterprise.v3.services.impl.ContainerMapper.service(ContainerMapper.java:174)
    at com.sun.grizzly.http.ProcessorTask.invokeAdapter(ProcessorTask.java:828)
    at com.sun.grizzly.http.ProcessorTask.doProcess(ProcessorTask.java:725)
    at com.sun.grizzly.http.ProcessorTask.process(ProcessorTask.java:1019)
    at com.sun.grizzly.http.DefaultProtocolFilter.execute(DefaultProtocolFilter.java:225)
    at com.sun.grizzly.DefaultProtocolChain.executeProtocolFilter(DefaultProtocolChain.java:137)
    at com.sun.grizzly.DefaultProtocolChain.execute(DefaultProtocolChain.java:104)
    at com.sun.grizzly.DefaultProtocolChain.execute(DefaultProtocolChain.java:90)
    at com.sun.grizzly.http.HttpProtocolChain.execute(HttpProtocolChain.java:79)
    at com.sun.grizzly.ProtocolChainContextTask.doCall(ProtocolChainContextTask.java:54)
    at com.sun.grizzly.SelectionKeyContextTask.call(SelectionKeyContextTask.java:59)
    at com.sun.grizzly.ContextTask.run(ContextTask.java:71)
    at com.sun.grizzly.util.AbstractThreadPool$Worker.doWork(AbstractThreadPool.java:532)
    at com.sun.grizzly.util.AbstractThreadPool$Worker.run(AbstractThreadPool.java:513)
    at java.lang.Thread.run(Thread.java:722)
Caused by: java.lang.reflect.InvocationTargetException
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.lang.reflect.Method.invoke(Method.java:601)
    at org.jboss.weld.util.reflection.SecureReflections$13.work(SecureReflections.java:305)
    at org.jboss.weld.util.reflection.SecureReflectionAccess.run(SecureReflectionAccess.java:54)
    at org.jboss.weld.util.reflection.SecureReflectionAccess.runAsInvocation(SecureReflectionAccess.java:163)
    at org.jboss.weld.util.reflection.SecureReflections.invoke(SecureReflections.java:299)
    at org.jboss.weld.introspector.jlr.WeldMethodImpl.invoke(WeldMethodImpl.java:193)
    at org.jboss.weld.bean.AbstractClassBean.defaultPostConstruct(AbstractClassBean.java:591)
    ... 66 more
Caused by: java.lang.NullPointerException
    at com.readme.user.signup.SettingsBean.init(SettingsBean.java:56)
    ... 76 more