Spring 3 form binding object is always null

Question

I'm trying to bind a nested object with Spring 3, and I'm having issues.

JSP:

<portlet:actionURL var="formAction" />
<form:form id="add-objects-form" method="post" action="${formAction}">
   <input name = "obj.a"...>
   <input name = "obj.b"...>
   <input type = "file" multiple="multiple" name="file"/>
</form>

Form Object:

class FormObject{
private final static Logger logger = ...

private MultipartFile file
private Obj obj

...getters and setters
}

Controller:

@RequestMapping(method = RequestMethod.POST)
public void uploadDocument(@ModelAttribute FormObject formObject, BindingResult results ) {

}

formObject gets obj.a and obj.b, but file is always null.

Answer 1

Add modelAttribute="formObject" in <form:form>

Also make sure you haven't excluded debug information from classes. If you have, or you are uncertain, specify @ModelAttribute("formObject")

For handling files (multipart data) you need to specify the enctype for the form:

enctype="multipart/form-data"

Update: since you are using a js-library for fileupload, here's what to do:

• upload only picture with ajax request (don't submit anything else). Store the uploaded files in a temporary location
• the response will possibly contain the names of the temp files. Store these names in a hidden field
• then submit the form (either via ajax or via regular submit), and pass the value of the hidden field, which now contains only strings
• move the files from the temporary location to a permanent location
• files that have been uploaded, but without the form being submitted, can be cleaned by some hourly/daily job.