Java GwtUpload Servlet出错

我正在尝试实现GwtUpload库的基本示例，如图所示

在我的服务器代码中，出现以下错误：

异常java.lang.ClassCastException:org.apache.commons.fileupload.disk.DiskFileItem不能强制转换为org.apache.commons.fileupload.FileItem

我不明白为什么会这样。DiskFileItem是FileItem的一个子类，应该可以工作。我已经进入了调试器，并确认这两个类是它们看起来的样子，但是赋值失败了

更奇怪的是，当我试图在watch窗口中调用FileItem方法时，我没有遇到任何问题，但是如果我试图在代码中访问它们，我就会得到错误

以下是我的Servlet代码：

public class GwtUploadServlet extends UploadAction
{
    private static final long serialVersionUID = 1L;

    /**
     * Maintain a list with received files and their content types.
     */
    Hashtable<String, String> receivedContentTypes = new Hashtable<String, String>();

    /**
     * Maintain a list with received files.
     */
    Hashtable<String, File> receivedFiles = new Hashtable<String, File>();

    /**
     * Override executeAction to save the received files in a custom place and
     * delete this items from session.
     */
    @Override
    public String executeAction(HttpServletRequest request,
                                List<FileItem> sessionFiles) throws UploadActionException
    {
        String response = "";
        int cont = 0;
        for ( int i = 0 ; i < sessionFiles.size(); i++ )
        {
            if (false == sessionFiles.get(i).isFormField())
            {
                cont++;
                try
                {
                    // / Create a temporary file placed in the default system
                    // temp folder
                    File file = File.createTempFile("upload-", ".bin");
                    sessionFiles.get(i).write(file);

                    // / Save a list with the received files
                    receivedFiles.put(sessionFiles.get(i).getFieldName(), file);
                    receivedContentTypes.put(sessionFiles.get(i).getFieldName(),
                                             sessionFiles.get(i).getContentType());

                    // / Send a customized message to the client.
                    response += "File saved as " + file.getAbsolutePath();

                }
                catch (Exception e)
                {
                    throw new UploadActionException(e);
                }
            }
        }

        // / Remove files from session because we have a copy of them
        removeSessionFileItems(request);

        // / Send your customized message to the client.
        return response;
    }

    /**
     * Get the content of an uploaded file.
     */
    @Override
    public void getUploadedFile(HttpServletRequest request,
            HttpServletResponse response) throws IOException
    {
        String fieldName = request.getParameter(PARAM_SHOW);
        File f = receivedFiles.get(fieldName);
        if (f != null)
        {
            response.setContentType(receivedContentTypes.get(fieldName));
            FileInputStream is = new FileInputStream(f);
            copyFromInputStreamToOutputStream(is, response.getOutputStream());
        }
        else
        {
            renderXmlResponse(request, response, ERROR_ITEM_NOT_FOUND);
        }
    }

    /**
     * Remove a file when the user sends a delete request.
     */
    @Override
    public void removeItem(HttpServletRequest request, String fieldName)
            throws UploadActionException
    {
        File file = receivedFiles.get(fieldName);
        receivedFiles.remove(fieldName);
        receivedContentTypes.remove(fieldName);
        if (file != null)
        {
            file.delete();
        }
    }
}

public类GwtUploadServlet扩展了UploadAction
{
私有静态最终长serialVersionUID=1L；
/**
*维护包含已接收文件及其内容类型的列表。
*/
Hashtable receivedContentTypes=新Hashtable（）；
/**
*维护包含已接收文件的列表。
*/
Hashtable receivedFiles=新Hashtable（）；
/**
*重写executeAction以将收到的文件保存在自定义位置并
*从会话中删除此项目。
*/
@凌驾
公共字符串执行（HttpServletRequest，
列表会话文件）引发UploadActionException
{
字符串响应=”；
int cont=0；
for（int i=0；i

确保类路径上没有多个版本的commons fileupload。

确保类路径上没有多个版本的commons fileupload。

谢谢。这就是问题所在，谢谢。这就是问题所在。