jsp checkbox value not binding

I have a problem in getting checkbox's value from JSP. I used SpringMVC, and my related class as follows:
Form:

@Getter @Setter
public class Sample3Form extends AbstractForm {<
    private List<Sample1Bean> sampleList;   //all data
    private ValidPagedListHolder<Sample1Bean> samplePagedList;//1 page data
}

Sample1Bean:

@Getter @Setter
public class Sample1Bean extends AbstractOnlineDataBean {
    private String keiyakuno;
    private String keiyakunm;
    private String torisakino;

}

Parent Bean Class:

@Getter
@Setter
public abstract class AbstractOnlineDataBean extends AbstractDataBean {
    private Integer selectedIndex;
    private String maskPattern;
}

Controller:

@Controller
@RequestMapping(value = "app/sample3")
@SessionAttributes(value = "sample3Form")
@NotCheckToken
public class SampleController3 extends AbstractController {

    private static Logger logger = LoggerFactory.getLogger(UserUtil.class);

    @ModelAttribute(value = "sample3Form")
    public Sample3Form getForm() {
        return new Sample3Form();
    }
......
......
    @RequestMapping(value = "delete")
    public String delete(Model model, @Validated Sample3Form form, BindingResult result, HttpServletRequest request) throws GdcSystemException {    
        List<Sample1Bean> list = new ArrayList<Sample1Bean>();

        int size = form.getSampleList().size();
        ValidPagedListHolder<Sample1Bean> plist = form.getSamplePagedList();
        list = plist.getPageList();

        for (int i = 0 ; i < size; i ++) {
            if (form.getSampleList().get(i).getSelectedIndex() == null ) {

            } else {
                //I cannot arrive here even though I checked in JSP 
                list.add(form.getSampleList().get(i));
            }

        }
...
...

}

JSP:

<ab:form id="sample3" action="submit" modelAttribute="sample3Form">
    <table width="100%"  class="tableborder" cellpadding="0" cellspacing="0">
    ...
    ...
        <ab:tr index="${row.index}">
            <td width="2%" class="data_list_area">
                <ab:checkbox path="${sample3Form.samplePagedList.pageList[row.index].selectedIndex}" fwformat='NONE' 
                value="${sample3Form.samplePagedList.page * sample3Form.samplePagedList.pageSize + row.index }" name="checkedids"/>

            </td>            
            <td width="5%" class="data_list_area">
                <!--ab:out value="${sample3Form.samplePagedList.pageList[row.index].keiyakuno}" /-->
                <ab:label path="" fwformat='NONE'>${sample3Form.samplePagedList.pageList[row.index].keiyakuno}</ab:label>
            </td>
...
...

In above JSP, I set checkboxes and bind the checkboxes to selectedIndex property for for every row data.but i cannot get selected row in controller as follows:

form.getSampleList().get(i).getSelectedIndex()

The return value is always null. why? Could who please tell me how i can get the selectedIndex in controller?
By the way, I can get checked row in controller like below:

String[] arr = request.getParameterValues("checkedids");

Note: in JSP, added checkboxes is defined by name "checkedids".

Thanks.