当显示的页面内容太长时，需要省略号代替时，通过自定义标签来实现方法

1.写一个Java类：在\src\cn\itcast\web\el下建一个EIUtils类

package cn.itcast.web.el;

public class ElUtils {

public static String subString(String source,Integer length){

if(source.length()>length){
return source.substring(0,length) + ".......";
}
return source;
}

}

2.在WEB-INF下建itcast.tld文件
<?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">

<description>A tag library exercising SimpleTag handlers.</description>
<tlib-version>1.0</tlib-version>
<short-name>SimpleTagLibrary</short-name>
<uri>/itcast</uri>

<function>
<name>subString</name>
<function-class>cn.itcast.web.el.ElUtils</function-class>
<function-signature>java.lang.String subString( java.lang.String,java.lang.Integer )</function-signature>
</function>
</taglib>

3.在页面中先通过<%@taglib uri="/WEB-INF/itcast.tld" prefix="itcast" %>
导入自定义标签，然后通过<td><c:out value="${itcast:subString(c.description,10)}" escapeXml="true"/></td>