Jsf 如何设置javax.servlet.http.Part的默认值

我有一个由三个视图组成的面流。但是，如果不使用上载文件，则在第一个视图中会出现NullpointerException。代码如下：

Wizard.xhtml

---

在没有选择上传文件的情况下，我有没有办法设置默认值？

抛开所有JSF抽象性，你问的问题基本上与BalusC完全相同，我理解安全约束，但问题是如果用户在进入下一个屏幕之前没有选择文件，如何防止faces流中出现NullPointerException。我尝试过空检查，但它们不起作用，因为JSF生命周期的UpdateModel阶段存在异常

        <div class="form-group">
            <h:outputLabel value="ID Number" for="idnumber" styleClass="control-label col-md-2"/>
            <div class="control col-md-4">
                <h:inputText id="idnumber" value="#{flowScope.idNo}" 
                         class="form-control" required="true">
                    <f:passThroughAttribute name="placeHolder" value="ID Number"/>
                </h:inputText>
            </div>

            <h:outputLabel value="Upload Photo" for="photo" styleClass="control-label col-md-2" />

            <div class="control col-md-4">
                <h:inputFile id="photo" value="#{flowScope.filePart}" 
                         validator="#{myValidator.validateFile}" 
                         class="form-control"
                              required="false"  >   
                </h:inputFile> 
            </div>

private void assignFlowScopeValues() {

 Map<Object, Object> flowScope =       FacesContext.getCurrentInstance().getApplication().getFlowHandler().getCurrentFlowScope();  
    //Basic personal data

    applicant.setIdNo((String) flowScope.get("idNo"));

    MyFileUtils myFileUtils = new MyFileUtils();
    String fileName = myFileUtils.writeImageToStore((Part)flowScope.get("filePart"));

    applicant.setPhoto(fileName);
}

javax.faces.component.UpdateModelException: java.lang.NullPointerException
at javax.faces.component.UIInput.updateModel(UIInput.java:867)
at javax.faces.component.UIInput.processUpdates(UIInput.java:749)
at javax.faces.component.UIForm.processUpdates(UIForm.java:281)
at javax.faces.component.UIComponentBase.processUpdates(UIComponentBase.java:1286)
at javax.faces.component.UIComponentBase.processUpdates(UIComponentBase.java:1286)
at javax.faces.component.UIViewRoot.processUpdates(UIViewRoot.java:1254)
at com.sun.faces.lifecycle.UpdateModelValuesPhase.execute(UpdateModelValuesPhase.java:78)
at com.sun.faces.lifecycle.Phase.doPhase(Phase.java:101)
at com.sun.faces.lifecycle.LifecycleImpl.execute(LifecycleImpl.java:198)
at javax.faces.webapp.FacesServlet.service(FacesServlet.java:646)
at org.apache.catalina.core.StandardWrapper.service(StandardWrapper.java:1682)
at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:318)
at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:160)
at org.apache.catalina.core.StandardPipeline.doInvoke(StandardPipeline.java:734)
at org.apache.catalina.core.StandardPipeline.invoke(StandardPipeline.java:673)
at com.sun.enterprise.web.WebPipeline.invoke(WebPipeline.java:99)
at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:174)
at org.apache.catalina.connector.CoyoteAdapter.doService(CoyoteAdapter.java:357)
at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:260)
at com.sun.enterprise.v3.services.impl.ContainerMapper.service(ContainerMapper.java:188)
at org.glassfish.grizzly.http.server.HttpHandler.runService(HttpHandler.java:191)
at org.glassfish.grizzly.http.server.HttpHandler.doHandle(HttpHandler.java:168)
at org.glassfish.grizzly.http.server.HttpServerFilter.handleRead(HttpServerFilter.java:189)
at org.glassfish.grizzly.filterchain.ExecutorResolver$9.execute(ExecutorResolver.java:119)
at org.glassfish.grizzly.filterchain.DefaultFilterChain.executeFilter(DefaultFilterChain.java:288)
at org.glassfish.grizzly.filterchain.DefaultFilterChain.executeChainPart(DefaultFilterChain.java:206)
at org.glassfish.grizzly.filterchain.DefaultFilterChain.execute(DefaultFilterChain.java:136)
at org.glassfish.grizzly.filterchain.DefaultFilterChain.process(DefaultFilterChain.java:114)
at org.glassfish.grizzly.ProcessorExecutor.execute(ProcessorExecutor.java:77)
at org.glassfish.grizzly.nio.transport.TCPNIOTransport.fireIOEvent(TCPNIOTransport.java:838)
at org.glassfish.grizzly.strategies.AbstractIOStrategy.fireIOEvent(AbstractIOStrategy.java:113)
at org.glassfish.grizzly.strategies.WorkerThreadIOStrategy.run0(WorkerThreadIOStrategy.java:115)
at org.glassfish.grizzly.strategies.WorkerThreadIOStrategy.access$100(WorkerThreadIOStrategy.java:55)
at org.glassfish.grizzly.strategies.WorkerThreadIOStrategy$WorkerThreadRunnable.run(WorkerThreadIOStrategy.java:135)
at org.glassfish.grizzly.threadpool.AbstractThreadPool$Worker.doWork(AbstractThreadPool.java:564)
at org.glassfish.grizzly.threadpool.AbstractThreadPool$Worker.run(AbstractThreadPool.java:544)
at java.lang.Thread.run(Thread.java:744)
Caused by: java.lang.NullPointerException
at java.util.concurrent.ConcurrentHashMap.put(ConcurrentHashMap.java:1124)
at javax.el.MapELResolver.setValue(MapELResolver.java:267)
at com.sun.faces.el.DemuxCompositeELResolver._setValue(DemuxCompositeELResolver.java:255)
at com.sun.faces.el.DemuxCompositeELResolver.setValue(DemuxCompositeELResolver.java:281)
at com.sun.el.parser.AstValue.setValue(AstValue.java:238)
at com.sun.el.ValueExpressionImpl.setValue(ValueExpressionImpl.java:294)
at org.jboss.weld.el.WeldValueExpression.setValue(WeldValueExpression.java:64)
at com.sun.faces.facelets.el.TagValueExpression.setValue(TagValueExpression.java:131)
at javax.faces.component.UIInput.updateModel(UIInput.java:832)
... 36 more