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

问题

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.

回答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.

回答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.

回答3:

Indeed there is something different.

request.getParameter will only work for hardcoded URL parameters specified in action attribute of <form> element. In your case it does not contain any.

All other parameters will be incoded into the form itself, which you have to process by parsing HTTP request's input stream directly.

Fortunately, you are not the first and there are some good open-source libraries that take care of this.

I've been using Apache FileUpload. You create a parser and pass a request object to it and then iterate through different items. One of them will be your hidden field.

回答4:

The multi-part encoding shouldn't affect hidden text fields. It is likely something else. Can you post more of the HTML/Servlet code?

回答5:

Not sure if this helps, but I have used multipart forms in jsp pages which are submitted to a struts servlet and these pages have hidden fields which are received in my Struts Action classes (wrapped in Struts ActionForm), so I don't think there is any hard stop here.

Have you tried receiving this value as String and seeing what actually comes there?

回答6:

You'd check the servlet code itself. Are you getting the request? Can you debug the app in order to see which variables are present in the environment when you try to get the value and parse it.

回答7:

I only had the id atttribute set for the field and it didn't show up in the List items list. When I added the name attribute, it showed up.

来源：https://stackoverflow.com/questions/2827778/does-form-with-enctype-multipart-form-data-cause-problems-accessing-a-hidden-f