Does form with enctype="multipart/form-data" cause problems accessing a hidden field

Question

I have created a hidden form element

<form name="UploadImage" enctype="multipart/form-data" method="post" action="UploadImage">
    <label>
        </label>
    <input name="imgUploadObjId" id="imgUploadObjId" value="52" type="hidden">

    //rest of the form here

</form>

And I am trying to get the value with this line in a servlet (as I have done before):

int objId = Integer.parseInt(request.getParameter("imgUploadObjId"));

But I get this (line 33 is the line above):

java.lang.NumberFormatException: null
java.lang.Integer.parseInt(Unknown Source) java.lang.Integer.parseInt(Unknown Source) web.objects.UploadImage.doPost(UploadImage.java:33) javax.servlet.http.HttpServlet.service(HttpServlet.java:637) javax.servlet.http.HttpServlet.service(HttpServlet.java:717)

Is there something different about a form with enctype="multipart/form-data"? Or can you see some other error.

Answer 1

The servlet parses the parameters by default using application/x-www-form-urlencoded encoding. The multipart/form-data encoding however isn't supported in servlets until Servlet 3.0. The getParameter() calls will all return null.

In Servlet 3.0, you should have used HttpServletRequest#getParts() instead to get all parts of a multipart/form-data request, including normal form fields. Prior to Servlet 3.0, you should have used Apache Commons FileUpload to parse multipart/form-data requests. See also the following answer for a detailed example of both approaches: How to upload files to server using JSP/Servlet?

Note that if you aren't using any <input type="file"> field at all, then you can just leave the encoding away from the <form>. It will then default to application/x-www-form-urlencoded.

Answer 2

As a workaround, you can also add the required hidden parameters as GET parameters in the form's action attribute:

<form name="UploadImage" enctype="multipart/form-data" method="post" action="UploadImage?imgUploadObjId=52">

    //rest of the form here

</form>

this will allow the request.getParameter("imgUploadObjId") call to work.