尝试从 JSF2.0 中的 web.xml 访问参数时获取 null

Question

我在web.xml 中定义了一个参数为uploadDirectory，我试图从ManagedBean 之一中获取该参数。下面是web.xml的sn-p-

<!-- Primefaces file upload -->
<!-- thresholdSize specifies the maximum file size in bytes to keep uploaded 
    files in memory. If it exceeds this limit, it’ll be temporarily written to 
    disk. -->
<!-- uploadDirectory is the folder where to keep temporary files that exceed 
    thresholdSize. -->
<filter>
    <filter-name>PrimeFaces FileUpload Filter</filter-name>
    <filter-class>org.primefaces.webapp.filter.FileUploadFilter</filter-class>
    <init-param>
        <param-name>thresholdSize</param-name>
        <param-value>51200</param-value>
    </init-param>
    <init-param>
        <param-name>uploadDirectory</param-name>
        <param-value>/opt/upload/</param-value>
    </init-param>
</filter>
<filter-mapping>
    <filter-name>PrimeFaces FileUpload Filter</filter-name>
    <servlet-name>Faces Servlet</servlet-name>
</filter-mapping>

下面是ManagedBean的sn-p-

@ManagedBean
@ViewScoped
public class ProjectInfo implements Serializable {

    /**
     * 
     */
    private static final long serialVersionUID = 1L;

    private static final String destination = FacesContext.getCurrentInstance()
            .getExternalContext().getInitParameter("uploadDirectory");

为什么我得到null 为destination？

Answer 1

您定义的参数是过滤器的参数，因此它们在您的托管 bean 中不可用。例如，您可以将此过滤器子类化并添加将此参数置于请求范围内的代码，并从托管 bean 中读取此参数，或者如果您想从托管 bean（其请求未被此过滤器过滤）访问此参数，请定义他们是<context-param>。