Submitting list of checkboxes to Spring controller

I'm trying to submit list of checkboxes along with file to my controller with following code:

<form:form enctype="multipart/form-data" modelAttribute="uploadItem" method="post" action="/image/upload">

    <form:label for="fileData" path="filename">Photo:</form:label>
    <input class="filePhoto" type="file" name="fileData" id="fileData" /><br>
    <input type="submit" value="Submit!" /><br>
    <c:forEach items="${tags}" var="tag">
        <form:checkbox path="tagList"  value="${tag.tagId}" label="${tag.name}"/><br>
    </c:forEach>
    <form:textarea path="description" label="description"/>
     <img id="blah" src="#" alt="Preview" width="300" />
</form:form>

My problem is that when it gets converted to my List<String>, it is in form of

null, null, null, on, null, null, on

where on are checked checkboxes and null are those unchecked. What I need is id's of those checkboxes instead of on because I don't know what exactly is on

Let me know what other information is needed. List of checkboxes is passed to controller so I don't think form snippets are relevant here, I think its more of an JSP issue