[Struts]java.lang.IllegalArgumentException: array element type mismatch大家帮忙看看

在struts里面,我用了<html:file> 他在ActionForm中是一个数组,程序没有执行到Action就抛出异常
下面是我的ActionForm代码,麻烦大家提点意见:

public class HomeworkForm extends ActionForm
{

private String date = null;
private org.apache.struts.upload.FormFile[] file = null;
private String content = null;
private String submitDate = null;

/**
* Get date
* @return String
*/
public String getDate()
{
return date;
}

/**
* Set date
* @param <code>String</code>
*/
public void setDate(String d)
{
this.date = d;
}

/**
* Get file
* @return org.apache.struts.upload.FormFile[]
*/
public org.apache.struts.upload.FormFile[] getFile()
{
return file;
}

/**
* Set file
* @param <code>org.apache.struts.upload.FormFile[]</code>
*/
public void setFile(org.apache.struts.upload.FormFile[] f)
{
this.file = f;
}

/**
* Get content
* @return String
*/
public String getContent()
{
return content;
}

/**
* Set content
* @param <code>String</code>
*/
public void setContent(String c)
{
this.content = c;
}

/**
* Get submitDate
* @return String
*/
public String getSubmitDate()
{
return submitDate;
}

/**
* Set submitDate
* @param <code>String</code>
*/
public void setSubmitDate(String s)
{
this.submitDate = s;
}

public void reset(ActionMapping mapping, HttpServletRequest request)
{

// Reset values are provided as samples only. Change as appropriate.

date = null;
file = null;
content = null;
submitDate = null;

}

public ActionErrors validate(
ActionMapping mapping,
HttpServletRequest request)
{

ActionErrors errors = new ActionErrors();
// Validate the fields in your form, adding
// adding each error to this.errors as found, e.g.

// if ((field == null) || (field.length() == 0)) {
// errors.add("field", new org.apache.struts.action.ActionError("error.field.required"));
// }
return errors;

}
}