NPE on POST form submit multipart/form-data

Question

I am using Jsps/Servlets to allow users to upload images (as outlined here: How to upload files to server using JSP/Servlet?)

Ordinarily everything works ok (I have the MultipartConfig annotation on the appropriate servlet).

I got one case where a NullPointerException was thrown when trying to retrieve the part from the HttpServletRequest: request.getPart().

According to the docs, this happens when the request doesn't contain the "requested part" (http://docs.oracle.com/javaee/6/api/javax/servlet/http/HttpServletRequest.html#getPart(java.lang.String)).

So, the only way I could recreate this issue was manipulating the html (via my browser's html editing tool) and manually erasing the name attribute of the file input element (ie: changing the name to an empty string).

I find it very unlikely that this is what the user did to cause the exception to be thrown. Are there any other reasons that this error might have occurred? I'd like to figure out what the cause was.

Thank you.

UPDATE Relevant html: (uses bootstrap collapse to display file input element) (The data attributes are for javascript - maybe something with the quotations is messing with the name attribute?)

<form role="form" action="/fileUploadServlet" method="post"
enctype="multipart/form-data"
data-min-length='${minLength}' 
data-max-length='${maxLength}'
data-msg="${valMsg}"
data-size="${maxSize}">
    <a href="#input_collapse" class="btn btn-default" data-toggle="collapse">
         Upload File
    </a>
    <span class="collapse" id="input_collapse">
         <input type="file" name="someFile" id="some_file">
    </span>
    ...[other form fields]
</form>

Relevant Servlet code: (calling getSubmittedFileName() throws the NPE)

@MultipartConfig(maxFileSize = 1024*1024*5)
public class FileUploadServlet
{
...
try {
     Part filePart = req.getPart("someFile");
     ...filePart.getSubmittedFileName();

And where is your code? Do you expect us to have a crystal ball, or some magic sense to be able to "debug" your code without having it? — Jozef Chocholacek, Jan 17 '17 at 10:02
In which scenario does the issue occur and could you post the issue details along with your servlet code? — Shivam Aggarwal, Jan 17 '17 at 12:52
Hey, the issue occurs when the user submits the form. The servlet code above is the first thing that is checked and it throws the `NPE` (meaning the rest of the fields that are entered aren't relevant)...Thanks. — theyuv, Jan 17 '17 at 17:17

score 1 · Accepted Answer · answered Jan 19 '17 at 12:56

This was caused by some javascript validation that I had set up.

Specifically: on form submit, checking file size, on Internet Explorer, causes NPE mentioned in the question.

The javascript

$("#form").submit(function(){
    if ($("#some_file").files[0]) {
    ...
    }
});

I don't know why this happens. It doesn't happen in Firefox. To fix this, I checked for validity upon change of the element rather than on form submit.

This isn't an ideal solution. I'm still trying to find out why this caused the error on Internet Explorer.

NPE on POST form submit multipart/form-data

1 Answers1