springmvc+jqgrid绑定时报空指针

controller:

@RequestMapping(value="/housesData")
public @ResponseBody JqGridPage showHousesData(JqGridPage jgp,SearchInfoVo vo){
try {
int count = housesInfoService.queryResultsCount(vo);
jgp.execute(count);
List list = housesInfoService.queryByPage(vo, jgp.getFirstResult(), jgp.getMaxResults());
jgp.setData(list);
} catch (Exception e) {
e.printStackTrace();
}
return jgp;
}

页面脚本：

$("#u0_img").jqGrid({
url: "/tenant_love/back/housesData",
type: "POST",
pgbuttons:true ,
datatype: "json",
mtype : "POST" ,
colNames: [
"获取日期","标题","房源类型"
],
colModel: [
{
name: "createdTime",
index: "createdTime",
sortable:false,
width : 200,
formatter:"date",
formatoptions: {srcformat:'Y-m-d',newformat:'Y-m-d'}
}
,{
name: "housesName",
index: "housesName",
sortable:false,
width : 600,
},{
name:"needType",
index:"needType",
sortable:false,
width : 200
}
],
jsonReader: {
root: "data",
repeatitems: false
},
rowNum: 10,
rownumbers:true,
height: 360,
width: $("#u0_img").parent().innerWidth()-4,
scrollOffset: 0,
forceFit: true,
pager: "#u0pager",
viewrecords: true,
beforeRequest: housesQuery
});

    function housesQuery() {
        var postData = $("#u0_img").jqGrid("getGridParam", "postData");
        postData["page"]=1;
    }

工具类：

public class JqGridPage {

/**
 * how many rows we want to have into the grid
 */
private Integer rows;

/**
 * the requested page
 */
private Integer page;

/**
 * index row - i.e. user click to sort
 */
private String sidx;

/**
 * the direction
 */
private String sord;

/**
 * total pages
 */
private Integer total;

/**
 * records count
 */
private Integer records;

/**
 * data rows
 */
private ArrayList data;

private int firstResult;

private int maxResults;

private int result;

private String houseSumPrice;

public void execute(int count) {

    records = count;

    total = records / rows;
    if (records % rows != 0)
        total += 1;

    firstResult = page * rows - rows;
    maxResults = rows;

}

//get,set方法省略

请大神帮我看下，以上代码出了什么问题，运行时总是报空指针异常，说是JqGridPage的属性未赋值成功，也就是未绑定成功？