struts2 报错：java.lang.NullPointerException: Source must not be null

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今天使用strut2+ajax进行异步上传时出错，控制台没有打印信息，ajax回调函数中alert返回值得到如下信息，大概就是空指针的意思。
实际上是上传的input name值和action变量名不一致造成。input name为myfile，变量为myFile.
最主要的还不是这个，变量可以不一样，重要的是set方法。setter和getter方法是自动生成的，set方法应该是setMyfile,然而生成的是setMyFile,找不到对应的set方法了，自然报错了。这个错误也是太难找了。
一个字母的大小写造成的错误！注意这个地方！

HTTP Status 500 -

type Exception report

message

description The server encountered an internal error () that prevented it from fulfilling this request.

exception

java.lang.NullPointerException: Source must not be null 
org.apache.commons.io.FileUtils.copyFile(FileUtils.java:761) 
org.apache.commons.io.FileUtils.copyFile(FileUtils.java:731) 
com.etoak.action.UploadAction.execute(UploadAction.java:44) 
sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) 
sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57) 
sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) 
java.lang.reflect.Method.invoke(Method.java:606) 
com.opensymphony.xwork2.DefaultActionInvocation.invokeAction(DefaultActionInvocation.java:450) 
com.opensymphony.xwork2.DefaultActionInvocation.invokeActionOnly(DefaultActionInvocation.java:289) 
com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:252) 
org.apache.struts2.interceptor.debugging.DebuggingInterceptor.intercept(DebuggingInterceptor.java:256) 
com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:246) 
com.opensymphony.xwork2.interceptor.DefaultWorkflowInterceptor.doIntercept(DefaultWorkflowInterceptor.java:167) 
com.opensymphony.xwork2.interceptor.MethodFilterInterceptor.intercept(MethodFilterInterceptor.java:98) 
com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:246) 
com.opensymphony.xwork2.validator.ValidationInterceptor.doIntercept(ValidationInterceptor.java:265) 
org.apache.struts2.interceptor.validation.AnnotationValidationInterceptor.doIntercept(AnnotationValidationInterceptor.java:68) 
com.opensymphony.xwork2.interceptor.MethodFilterInterceptor.intercept(MethodFilterInterceptor.java:98) 
com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:246) 
com.opensymphony.xwork2.interceptor.ConversionErrorInterceptor.intercept(ConversionErrorInterceptor.java:138) 
com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:246) 
com.opensymphony.xwork2.interceptor.ParametersInterceptor.doIntercept(ParametersInterceptor.java:239) 
com.opensymphony.xwork2.interceptor.MethodFilterInterceptor.intercept(MethodFilterInterceptor.java:98) 
com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:246) 
com.opensymphony.xwork2.interceptor.ParametersInterceptor.doIntercept(ParametersInterceptor.java:239) 
com.opensymphony.xwork2.interceptor.MethodFilterInterceptor.intercept(MethodFilterInterceptor.java:98) 
com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:246) 
com.opensymphony.xwork2.interceptor.StaticParametersInterceptor.intercept(StaticParametersInterceptor.java:191) 
com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:246) 
org.apache.struts2.interceptor.MultiselectInterceptor.intercept(MultiselectInterceptor.java:73) 
com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:246) 
org.apache.struts2.interceptor.CheckboxInterceptor.intercept(CheckboxInterceptor.java:91) 
com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:246) 
org.apache.struts2.interceptor.FileUploadInterceptor.intercept(FileUploadInterceptor.java:325) 
com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:246) 
com.opensymphony.xwork2.interceptor.ModelDrivenInterceptor.intercept(ModelDrivenInterceptor.java:100) 
com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:246) 
com.opensymphony.xwork2.interceptor.ScopedModelDrivenInterceptor.intercept(ScopedModelDrivenInterceptor.java:141) 
com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:246) 
com.opensymphony.xwork2.interceptor.ChainingInterceptor.intercept(ChainingInterceptor.java:145) 
com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:246) 
com.opensymphony.xwork2.interceptor.PrepareInterceptor.doIntercept(PrepareInterceptor.java:171) 
com.opensymphony.xwork2.interceptor.MethodFilterInterceptor.intercept(MethodFilterInterceptor.java:98) 
com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:246) 
com.opensymphony.xwork2.interceptor.I18nInterceptor.intercept(I18nInterceptor.java:161) 
com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:246) 
org.apache.struts2.interceptor.ServletConfigInterceptor.intercept(ServletConfigInterceptor.java:164) 
com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:246) 
com.opensymphony.xwork2.interceptor.AliasInterceptor.intercept(AliasInterceptor.java:193) 
com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:246) 
com.opensymphony.xwork2.interceptor.ExceptionMappingInterceptor.intercept(ExceptionMappingInterceptor.java:189) 
com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:246) 
org.apache.struts2.impl.StrutsActionProxy.execute(StrutsActionProxy.java:54) 
org.apache.struts2.dispatcher.Dispatcher.serviceAction(Dispatcher.java:563) 
org.apache.struts2.dispatcher.ng.ExecuteOperations.executeAction(ExecuteOperations.java:77) 
org.apache.struts2.dispatcher.ng.filter.StrutsPrepareAndExecuteFilter.doFilter(StrutsPrepareAndExecuteFilter.java:99)