set var value from input field value

I started short time ago with JSP, JSTL, HTML and JavaScript so here is my problem:

I need to set the val开发者_如何学编程ue of a var the value of an input hidden. Other option is if it possible to compare using

<c:if test="....">

the value of a variable that I sent with the request with the value of the hidden input.

Thanks.

Update

I've been trying but can't make it work.

I have this field that contains the id of and object. I also have the list with the objects so what I have to do is find the object related to that ID.

<input type="text" name="id1" />

but if I do this:

<c:set var="dd" value="${param.id1}" />
<input type="text" value="${dd}" />

The input text is empty but the text related to id1 displays 850 (i.e. the value is dinamic)

Any suggestion why is not working?

Update 2

I need the "multipart/form-data" because in the form I need to upload a picture. I understand how to get the parameters from Java, but since I'm not using the server but the JSP pages, there's any way to do it? Just need to read that input element and save it in a variable.

You can access request parameters by implicit ${param} variable.

E.g. http://example.com/context/page.jsp?foo=bar in combination with

<c:if test="${param.foo == 'bar'}">
   The foo's param value is bar!
</c:if>
<c:if test="${param.foo != 'bar'}">
   The foo's param value is not bar, it is: ${param.foo}
</c:if>

would show the first condition.

If you actually want to retain some hidden input element in subsequent requests (which wasn't really made clear in your question), then all you basically need to do is:

<input type="hidden" name="foo" value="${param.foo}">

Update: as per your update: you need to give the input element a name as well. Thus, e.g.

<input type="text" name="id1" value="${param.id1}" />

This way it's available by request.getParameter("id1") and inherently also ${param.id1}. Do you see it now?

Update 2: as per your comment here: certainly this is related to enctype="multipart/form-data". With this encoding, the request parameters aren't in the parameter map anymore, but instead in the request body, because of the mixup with binary data (file uploads). It's going to be a long story to explain it all, but basically you need to parse the request yourself. If you're on Servlet 2.5 or older, then the Apache Commons FileUpload is very helpful here. Read especially "User Guide" and "Frequently Asked Questions" over there to see code examples and to learn how to use it the right way (also in MSIE!). You can even decide to abstract the FileUpload away so that you can stick using HttpServletRequest#getParameter() and ${param} the usual way, also see this article.

If you're already on Servlet 3.0, then you can just make use of HttpServletRequest#getParts(). You can even abstract it away so that you can stick using HttpServletRequest#getParameter() and ${param} the usual way, also see this article.

Update 3: Oh, you really don't want to use JSP to do all the processing. There it is not for. It's high time to learn Servlet. Besides, when using a Filter which puts all parameters from the request body back in the request parameter map (as described in the both articles), you also don't necessarily need a Servlet after all.