上传的文件在bean中始终为null

Question

我正在尝试使用JSF创建文件上传器。我在各种论坛上提到过几篇文章，但没有任何效果，所以我觉得，可能还有其他一些案例并发布了。我在表单中使用enctype，我有commons-IO和commons-fileupload，我在web.xml中添加了适当的过滤器。但它仍然不起作用并抛出NullPointerException。

<h:form id="expenseForm" enctype="multipart/form-data"">
    <table class="table table-condensed table-bordered table-hover table-striped">
        <tr>
           <td><h:outputText value="#{BHMaintenanceController.getSelectedLanguageLabel('Reason For Expense')}"/></td>
           <td><p:inputText value="#{BHMaintenanceController.newExpense.reason}"/></td>
        </tr>
        <tr>
           <td><h:outputText value="#{BHMaintenanceController.getSelectedLanguageLabel('Total Expense')}"/></td>
           <td><p:inputText value="#{BHMaintenanceController.newExpense.expense}"/></td>
        </tr>
        <tr>
           <td><h:outputText value="#{BHMaintenanceController.getSelectedLanguageLabel('Upload Receipt')}"/></td>
           <td><p:fileUpload value="#{BHMaintenanceController.expense_receipt}" mode="simple" skinSimple="true"/></td>
        </tr>
    </table>
    <p:commandButton value="#{BHMaintenanceController.getSelectedLanguageLabel('File Expense')}" ajax="false" action="#{BHMaintenanceController.fileExpense()}"></p:commandButton>
</h:form>

如您所见，我已正确添加enctype格式。我还添加了commons-io-2.4和commons-fileupload-1.3.1。我正在使用primefaces 5.1和JSF 2.0。

我的web.xml

<display-name>Panzer</display-name>
<servlet>
    <servlet-name>Faces Servlet</servlet-name>
    <servlet-class>javax.faces.webapp.FacesServlet</servlet-class>
    <load-on-startup>1</load-on-startup>
</servlet>
<servlet-mapping>
    <servlet-name>Faces Servlet</servlet-name>
    <url-pattern>*.jsf</url-pattern>
</servlet-mapping>
<context-param>
    <param-name>javax.faces.DEFAULT_SUFFIX</param-name>
    <param-value>.xhtml</param-value>
</context-param>
<context-param>
    <param-name>primefaces.THEME</param-name>
    <param-value>start</param-value>
</context-param>

<context-param>
    <param-name>primefaces.UPLOADER</param-name>
    <param-value>auto|native|commons</param-value>
</context-param>

<filter>
    <filter-name>PrimeFaces FileUpload Filter</filter-name>
    <filter-class>  org.primefaces.webapp.filter.FileUploadFilter</filter-class>
</filter>
<filter-mapping>
    <filter-name>PrimeFaces FileUpload Filter</filter-name>
    <servlet-name>Faces Servlet</servlet-name>
</filter-mapping>

是否有版本问题，或者我做错了什么？