Check File Existence and display alert message

Question

Hi,
I have a servlet which uploads file to server. Below is my code and is working file. Now, I want to check if the file uploading by the user is already exists. If so , I have to display a message to user "The file you are trying to upload already exists. Do you want to overwrite" with YES or NO actions.
How can I display this alert message from user.
//item.write(tosave);
          FileItemFactory factory = new DiskFileItemFactory();
          boolean isMultipart = FileUpload.isMultipartContent(request);
          System.out.println(isMultipart);
          Map fileDetails = new HashMap();
          ServletFileUpload upload = new ServletFileUpload(factory);
          List items = upload.parseRequest(request);
          Iterator iter = items.iterator();
          while (iter.hasNext()) {
               item = (FileItem) iter.next();
                    if (item.isFormField()) {
                                   String fieldName = item.getFieldName();     
                                   String fieldValue = item.getString();
                                   fileDetails.put("fileName",item.getString());
                    else {
                              fileDetails.put("path",item.getName());
                              cfile=new File(item.getName());
                              onlyFileName = cfile.getName();
               if (! folder.exists()) folder.mkdirs();
               if(cfile.exists()){  //THIS CHECK THE FILE EXISTENCE.  HOW CAN I DISPLAY ALERT MESSAGE TO USER HERE.
                    System.out.println("File Already Exists Path is ");     
Can anyone suggest me on this.

Answer 1

It won't work like that because JavaScript will run either before the form is submitted or after the page has been generated. In either case, you can't really communicate between the server side code and client side code.
What you can do is use Ajax to communicate with a servlet that checks for the duplicate file and responds accordingly. You can then perform the appropriate action on the page. This should be very simple to setup, take a look at this basic tutorial [1].
[1] http://www.w3schools.com/ajax/default.asp
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