Spring MVC: list of checkboxes not returned to controller on POST - spring

Question

Why doesn't the server see my list of filled checkboxes?
This question seems to be here asked many times, but the details are so different for each requester that it seems a different answer is needed each time. Here is my story.
These are my data-bearing classes. The Offer contains a list of Filter objects in the filter attribute:.
public class Offer implements Serializable {
#Id
#GeneratedValue(strategy=GenerationType.AUTO)
#Column(name="id")
private Long id = null;
#Column(name="title")
private String title = null;
[snip]
#ManyToMany(fetch=FetchType.EAGER)
#JoinTable(name = "offer_filter",
joinColumns = { #JoinColumn(name = "offer_id", nullable = false, updatable = false) },
inverseJoinColumns = { #JoinColumn(name = "filter_id", nullable = false, updatable = false) })
private List<Filter> filters;
[snip]
}
public class Filter implements Serializable {
#Id
#GeneratedValue(strategy=GenerationType.AUTO)
#Column(name="id")
private Long id;
#NotBlank
#Length(max=100)
#Column(name="text")
private String text;
[snip]
#Transient
private boolean owned = false;
[snip]
}
The simple controller sends the offerEdit.jsp page, with a fully-populated Offer object. The object contains a list of Filters. Because of the owned attribute, only one of the three Filters is pre-checked. This simulates my eventual plan, where the list of Filters is the whole universe and what the Offer owns is a subset.
Note the annotations, that the Offer has the filter list going to the web page but doesn't see it coming back.
public class OfferController {
[snip]
#RequestMapping(value = "/edit", method = RequestMethod.GET)
public String getEdit(#RequestParam("id") Long id, Model model, HttpSession session) {
Offer offerAttribute = offerService.get(id);
// At this point, offerAttribute.filters has three elements.
// Mockup -- tells web page that only the middle one of the three Filters should be checked.
List<Filter> filters = offer.getFilters();
Filter filter = filters.get(1);
filter.setOwned(true);
model.addAttribute("offerAttribute", offerAttribute);
return "offer/offerEdit";
}
#RequestMapping(value = "/edit", method = RequestMethod.POST)
public String postEdit(#RequestParam("id") Long id, #Valid #ModelAttribute("offerAttribute") Offer offerAttribute, BindingResult result, HttpSession session, Model model) {
// At this point, offerAttribute.filters is null.
if(result.hasErrors()) {
result.reject("offer.invalidFields");
return "offer/offerEdit";
}
offerAttribute.setId(id);
offerService.edit(offerAttribute);
return "redirect:/offer/list";
}
[snip]
}
The web page has this for its checkbox section. I use form:checkbox over form:checkboxes because I want to use a table,
[snip]
<form:form modelAttribute="offerAttribute" method="POST" action="${saveUrl}">
<table>
<tr>
<td></td>
<td><form:hidden path="id" /></td>
</tr>
<tr>
<td><form:label path="title">Title:</form:label></td>
<td><form:input path="title" size="80" /></td>
<td><form:errors path="title" cssClass="error" /></td>
</tr>
[snip]
</table>
<table>
</table>
<table>
<c:forEach items="${offerAttribute.filters}" var="filter">
<tr>
<td><form:checkbox
path="filters"
label="${filter.text}"
value="${filter.id}"
checked="${filter.owned ? 'true' : ''}" />
</td>
</tr>
</c:forEach>
</table>
[snip]
The displayed web page has three filter checkboxes displayed, with just the middle checkbox filled in.
For the returned list, I expect the server to get only the middle checkbox, which is just what I want.
Here is what the generated checkboxes look like as source:
<table style="border: 1px solid; width: 100%; text-align:left;">
<tr>
<td>
<input id="filters1" name="filters" type="checkbox" value="1"/>
<label for="filters1">Adults (18+) desired, please</label>
<input type="hidden" name="_filters" value="on"/>
</td>
</tr>
<tr>
<td>
<input id="filters2" name="filters" checked="true" type="checkbox" value="2"/>
<label for="filters2">Quiet audiences, please</label>
<input type="hidden" name="_filters" value="on"/>
</td>
</tr>
<tr>
<td>
<input id="filters3" name="filters" type="checkbox" value="4"/>
<label for="filters3">Filter Text First</label>
<input type="hidden" name="_filters" value="on"/>
</td>
</tr>
</table>
My checkbox is set, and in the HTML. To restate my question,
Why doesn't the checkbox value get seen in the controller's POST handler?
Thanks for any answers,
Jerome.

Answer 1

The values of checkboxes could not be directly bind to List.
To get this working you need to create a simple pojo databean that will hold the values of your form fields in jsp. And in that databean to bind the values you need to declare int[] filterId and the values of your checkboxes will bind in that array.
Hope this helps you.

Answer 2

After a lot of web research and different debugging sessions, I came up with a configuration I can live with.
In my controller I provide a model attribute of "filterList", containing List. The "offerAttribute" is an Offer object, containing List filters.
The view has this sequence:
<table>
<c:forEach items="${filterList}" var="filter" varStatus="status">
<tr>
<td><input type="checkbox" name="filters[${status.index}].id"
value="${filter.id}"
${filter.owned ? 'checked' : ''} /> ${filter.text}
</td>
</tr>
</c:forEach>
</table>
When the POST is done the ownerAttribute.filters list is just as long as the original filterList object that created the checkboxes. The checked ones contains a Filter.id value. The unchecked ones contain a null. (That is, the returned filters list is "sparse".) If the user clicks on just a few checkboxes then I must parse the returned list to find those that were chosen.
Once I know the ID values of the checked-on filters, I fetch each of them through Hibernate, put them into my reconstructed Offer object and then persist them to the database.
I realize that I'm not (currently) using the form:checkbox. But it works, and I'm pressed for time here. I'll come back later, perhaps, and see what form:checkbox can do for me.