NullPointerException when uploading a file

问题

When uploading a file, I get the following error:

Struts Problem Report Struts has detected an unhandled exception: Messages: File: java/io/File.java Line number: 317 
Stacktraces java.lang.NullPointerException java.io.File.(File.java:317) 
    example.uploadFile.execute(uploadFile.java:36) 
    sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) 
    sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
    sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.jav‌​a:43)
    java.lang.reflect.Method.invoke(Method.java:483)

index.jsp

<form action="upload" method="post" enctype="multipart/form-data">
    <label for="myFile">Upload your file</label>
    <input type="file" name="myFile" />
    <input type="submit" value="Upload"/>
</form>

uploadFile.java

package example;
import java.io.File;
import org.apache.commons.io.FileUtils;
import java.io.IOException;    
import com.opensymphony.xwork2.ActionSupport;

public class uploadFile extends ActionSupport {
   private File myFile;
   private String myFileContentType;
   private String myFileFileName;
   private String destPath;

   public String execute() {

      destPath = "C:\\Program Files\\Apache Software Foundation\\Apache Tomcat 8.0.3\\work";

      try{
         System.out.println("Src File name: " + myFile);
         System.out.println("Dst File name: " + myFileFileName);                     
         File destFile  = new File(destPath, myFileFileName);
         FileUtils.copyFile(myFile, destFile);      
      }catch(IOException e){
         return ERROR;
      }

      return SUCCESS;
   }

   public File getMyFile() {
      return myFile;`
   }
   public void setMyFile(File myFile) {
      this.myFile = myFile; this the file which user selects to upload
   }
   public String getMyFileContentType() {
      return myFileContentType; content type of the uploading file
   }
   public void setMyFileContentType(String myFileContentType) {
      this.myFileContentType = myFileContentType;`
   }
   public String getMyFileFileName() {
      return myFileFileName;
   }
   public void setMyFileFileName(String myFileFileName) {
      this.myFileFileName = myFileFileName; 
   }
}

struts.xml

<struts>
    <include file="example.xml"/>
    <constant name="struts.devMode" value="true" />
    <constant name="struts.multipart.maxSize" value="1000000" />

    <!-- Configuration for the default package. -->
    <package name="default" extends="struts-default">
       <action name="upload" class="example.uploadFile">
            <interceptor-ref name="basicStack"/>
            <interceptor-ref name="fileUpload">
                <param name="allowedTypes">image/jpeg,image/gif</param>
            </interceptor-ref>
            <result name="success">/success.jsp</result>
            <result name="error">/error.jsp</result>
        </action>
    </package>
</struts>

回答1:

To work properly, the FileUpload Interceptor must run before some other Interceptors in the basicStack;

you can check it out in the struts-default.xml:

Example 1:

<!-- Sample file upload stack -->
<interceptor-stack name="fileUploadStack">
    <interceptor-ref name="fileUpload"/>
    <interceptor-ref name="basicStack"/>
</interceptor-stack>

Example 2:

<interceptor-stack name="defaultStack">
    <interceptor-ref name="exception"/>
    <interceptor-ref name="alias"/>
    <interceptor-ref name="servletConfig"/>
    <interceptor-ref name="i18n"/>
    <interceptor-ref name="prepare"/>
    <interceptor-ref name="chain"/>
    <interceptor-ref name="scopedModelDriven"/>
    <interceptor-ref name="modelDriven"/>
    <interceptor-ref name="fileUpload"/>
    <interceptor-ref name="checkbox"/>
    <interceptor-ref name="datetime"/>
    <interceptor-ref name="multiselect"/>
    <interceptor-ref name="staticParams"/>
    <interceptor-ref name="actionMappingParams"/>
    <interceptor-ref name="params">
        <param name="excludeParams">^action:.*,^method:.*</param>
    </interceptor-ref>
    <interceptor-ref name="conversionError"/>
    <interceptor-ref name="validation">
        <param name="excludeMethods">input,back,cancel,browse</param>
    </interceptor-ref>
    <interceptor-ref name="workflow">
        <param name="excludeMethods">input,back,cancel,browse</param>
    </interceptor-ref>
    <interceptor-ref name="debugging"/>
    <interceptor-ref name="deprecation"/>
</interceptor-stack>

Then place it manually before the basicStack declaration, or use a stack (the defaultStack, or the fileUploadStack) and include the Interceptor's name before the parameter's name, for example:

<interceptor-ref name="defaultStack">
    <param name="fileUpload.allowedTypes">image/jpeg,image/gif</param>
</interceptor-ref>

or

<interceptor-ref name="fileUploadStack">
    <param name="fileUpload.allowedTypes">image/jpeg,image/gif</param>
</interceptor-ref>

来源：https://stackoverflow.com/questions/29210254/nullpointerexception-when-uploading-a-file