用struts做报表 报表打开没有内容给警告说dataSource为空

警告:Data source value for data source [...（省略）] was null
配置文件：
<package name="report" extends="struts-default,jasperreports-default" namespace="/report">
<action name="reportAction" class="com.gao.Action.reportAction" method="report">
<result type="jasper">
<param name="location">/jasper/category.jasper</param>
<param name="dataSource">${merList}</param>
<param name="format">${reportFormat}</param>
<param name="reportDate">${reportDate}</param>
</result>
</action>

</package>


其中merList 是从数据库中取出的记录
while (rs.next()){
Category category = new Category();
category.setID(rs.getInt(1));
category.setParentID(rs.getInt(2));
category.setCateName(rs.getString(3));
merList.add(category);
}