jstl 自定义标签报空指针错误！

common.tld 文件  路径为WEB-INF/tld/下<?xml version="1.0" encoding="UTF-8"?>    
<taglib xmlns="http://java.sun.com/xml/ns/j2ee"   
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"   
    xsi:schemaLocation="http://java.sun.com/xml/ns/j2ee http://java.sun.com/xml/ns/j2ee/web-jsptaglibrary_2_0.xsd"   
    version="2.0">    
    <tlib-version>1.0</tlib-version>    
    <short-name>common</short-name>     
   
    <function>    
        <name>subDate</name>      
        <function-class>com.synball.hksis.util.bo.CommonService</function-class>    
        <function-signature>    
            java.lang.String subDate(java.util.Date,java.util.Date,int)    
        </function-signature>    
    </function>    
</taglib>
jsp文件
<%@taglib uri="/WEB-INF/tld/common.tld" prefix="zy"%>
${zy:subDate(each.eventlink_id.starttime,each.eventlink_id.starttime,2)}
java文件
public String subDate(Date date1, Date date2,int d){
 
}