为什么在jsp页面上获取的常量值都为空？

claim_voucher_deal.jsp

<%@ page contentType="text/html;charset=UTF-8" language="java" %>
<%@ taglib prefix="c" uri="http://java.sun.com/jsp/jstl/core" %>
<%@ taglib prefix="spring" uri="http://www.springframework.org/tags" %>
<%@ page import="com.yuan.oa.global.Constant" %>
<jsp:include page="top.jsp"/>

<section id="content" class="table-layout animated fadeIn">
    <div class="tray tray-center">
        <div class="content-header">
            <h2> 待处理报销单 </h2>
            <p class="lead"></p>
        </div>
        <div class="admin-form theme-primary mw1000 center-block" style="padding-bottom: 175px;">
            <div class="panel  heading-border">
                <div class="panel-menu">
                    <div class="row">
                        <div class="hidden-xs hidden-sm col-md-3">
                            <div class="btn-group">
                                <button type="button" class="btn btn-default light">
                                    <i class="fa fa-refresh"></i>
                                </button>
                                <button type="button" class="btn btn-default light">
                                    <i class="fa fa-trash"></i>
                                </button>
                                <button type="button" class="btn btn-default light">
                                    <i class="fa fa-plus" onclick="javascript:window.location.href='/claim_voucher/to_add';"></i>
                                </button>
                            </div>
                        </div>
                        <div class="col-xs-12 col-md-9 text-right">
                            <div class="btn-group">
                                <button type="button" class="btn btn-default light">
                                    <i class="fa fa-chevron-left"></i>
                                </button>
                                <button type="button" class="btn btn-default light">
                                    <i class="fa fa-chevron-right"></i>
                                </button>
                            </div>
                        </div>
                    </div>
                </div>
                <div class="panel-body pn">
                    <table id="message-table" class="table admin-form theme-warning tc-checkbox-1">
                        <thead>
                        <tr class="">
                            <th class="text-center hidden-xs">Select</th>
                            <th class="hidden-xs">事由</th>
                            <th>状态</th>
                            <th class="hidden-xs">创建人</th>
                            <th class="hidden-xs">金额</th>
                            <th class="text-center">创建时间</th>
                            <th>操作</th>
                        </tr>
                        </thead>
                        <tbody>
                        <c:forEach items="${list}" var="cv">
                            <tr class="message-unread">
                                <td class="hidden-xs">
                                    <label class="option block mn">
                                        <input type="checkbox" name="mobileos" value="FR">
                                        <span class="checkbox mn"></span>
                                    </label>
                                </td>
                                <td>${cv.cause}</td>
                                <td class="hidden-xs">
                                    <span class="badge badge-warning mr10 fs11">${cv.status}</span>
                                </td>
                                <td>${cv.creater.name}</td>
                                <td class="text-center fw600">${cv.totalAmount}</td>
                                <td><spring:eval expression="cv.createTime"/></td>
                                <td>
                                    //为何这里的常量值都为空（如Constant.CLAIMVOUCHER_CREATED是个空字符串）
                                    <c:if test="${cv.status==Constant.CLAIMVOUCHER_CREATED || cv.status==Constant.CLAIMVOUCHER_BACK}">
                                        <a href="/claim_voucher/details?id=${cv.id}">修改</a>
                                        <a href="/claim_voucher/details?id=${cv.id}">提交</a>
                                    </c:if>
                                    <c:if test="${cv.status==Constant.CLAIMVOUCHER_SUBMIT || cv.status==Constant.CLAIMVOUCHER_RECHECK}">
                                        <a href="/claim_voucher/details?id=${cv.id}">打回</a>
                                        <a href="/claim_voucher/details?id=${cv.id}">审核</a>
                                    </c:if>
                                    <c:if test="${cv.status==Constant.CLAIMVOUCHER_APPROVED}">
                                        <a href="/claim_voucher/details?id=${cv.id}">打款</a>
                                    </c:if>
                                    <a href="/claim_voucher/details?id=${cv.id}">${Constant.CLAIMVOUCHER_CREATED}详细信息${cv.status}</a>
                                </td>
                            </tr>
                        </c:forEach>
                        </tbody>
                    </table>
                </div>
            </div>
        </div>
    </div>
</section>


<jsp:include page="bottom.jsp"/>

Constant.java

package com.yuan.oa.global;

import java.util.ArrayList;
import java.util.List;

//常量类
public class Constant {
    //职务
    public static final String POST_STAFF="员工";
    public static final String POST_FM="部门经理";
    public static final String POST_GM="总经理";
    public static final String POST_CASHIER="财务";
    //职务集合
    public static List<String> getPosts()
    {
        List<String> list=new ArrayList<String>();
        list.add(POST_STAFF);
        list.add(POST_FM);
        list.add(POST_GM);
        list.add(POST_CASHIER);
        return list;
    }

    //费用类型
    public static List<String> getItems()
    {
        List<String> list=new ArrayList<String>();
        list.add("交通");
        list.add("餐饮");
        list.add("住宿");
        list.add("办公");
        list.add("其他");
        return list;
    }

    //报销单状态(不要忘了加final)
    public static final String CLAIMVOUCHER_CREATED="新创建";
    public static final String CLAIMVOUCHER_SUBMIT="提交";
    public static final String CLAIMVOUCHER_APPROVED="已审核";
    public static final String CLAIMVOUCHER_BACK="已打回";
    public static final String CLAIMVOUCHER_TEAMINATED="已终止";
    public static final String CLAIMVOUCHER_RECHECK="待复审";
    public static final String CLAIMVOUCHER_PAID="已打款";

    //审核额度
    public static final double LIMIT_CHECK=5000;

    //处理方式（处理状态）：不要忘了加final
    public static final String DEAL_CREATED="创建";
    public static final String DEAL_SUBMIT="提交";
    public static final String DEAL_UPDATE="修改";
    public static final String DEAL_BACK="打回";
    public static final String DEAL_REJECT="拒绝";
    public static final String DEAL_PASS="通过";
    public static final String DEAL_PAID="打款";




}