分三步：
一、在WEB-INF 下建一个名为tlds的文件夹，再建一个tld文件，如 formatTime.tld，内容为：
代码如下：

?xml version="1.0" encoding="ISO-8859-1" ?
!DOCTYPE taglib PUBLIC "-//Sun Microsystems, Inc.//DTD JSP Tag Library 1.1//EN"
"http://java.sun.com/j2ee/dtds/web-jsptaglibrary_1_1.dtd"
taglib
tlibversion1.0/tlibversion
jspversion1.1/jspversion
shortnamentuc/shortname
tag
nameformatTimeAsString/name
tagclasscom.ufinity.taglibTest.FormatTimeTag/tagclass
bodycontentempty/bodycontent
attribute
nametimestamp/name
/attribute
attribute
nameformat/name
/attribute
attribute
nameshowTH/name
/attribute
attribute
namestyle/name
/attribute
/tag
/taglib

二、建一个class，内容为：
代码如下：

package com.ufinity.taglibTest;
import java.io.IOException;
import java.text.DateFormat;
import java.text.ParseException;
import java.text.SimpleDateFormat;
import java.util.Calendar;
import java.util.Date;
import java.util.Locale;
import javax.servlet.jsp.tagext.TagSupport;
/**
* Description of the class
*
* @author Wangqy
* @version 1.0
* @since 2009-8-25
*/
public class FormatTimeTag extends TagSupport {
/**
* serialVersionUID long
*/
private static final long serialVersionUID = 8757501937718830491L;
private String timestamp;
private String format;
private String showTH;
private String style;
public int doEndTag()
{
try
{
String info = this.convertDateTime(timestamp, format, Boolean.parseBoolean(showTH), style);
pageContext.getOut().println(info);
}
catch (IOException e) {
}
return EVAL_PAGE;
}
private String convertDateTime(String dateTime, String formater, boolean showTH, String caseStyle) {
String timePosted = null;
SimpleDateFormat dateFm = null;
DateFormat format = new SimpleDateFormat("yyyyMMddHHmmss");
Date formatTime = null;
try {
formatTime = format.parse(dateTime);
} catch (ParseException e) {
return null;
}
Calendar calendar = Calendar.getInstance();
calendar.setTime(formatTime);
if (showTH) {
int day = calendar.get(Calendar.DAY_OF_MONTH);
String daySuffix = "th";
if ((day % 10) == 1) {
daySuffix = ((day / 10) == 1) ? "th" : "st";
} else if ((day % 10) == 2) {
daySuffix = (day == 12) ? "th" : "nd";
} else if ((day % 10) == 3) {
daySuffix = (day == 13) ? "th" : "rd";
}
formater = formater.substring(0, formater.indexOf(" ")) + "'"
+ daySuffix + "'"
+ formater.substring(formater.indexOf(" "));
dateFm = new SimpleDateFormat(formater, Locale.ENGLISH);
} else {
dateFm = new SimpleDateFormat(formater, Locale.ENGLISH);
}
timePosted = dateFm.format(formatTime);
if (caseStyle.equals("Upper")) {
timePosted = timePosted.toUpperCase();
} else if (caseStyle.equals("Lower")) {
timePosted = timePosted.toLowerCase();
}
return timePosted;
}
public void setFormat(String format) {
this.format = format;
}
public void setShowTH(String showTH) {
this.showTH = showTH;
}
public void setStyle(String style) {
this.style = style;
}
public void setTimestamp(String timestamp) {
this.timestamp = timestamp;
}
}

三、建个jsp页面测试下：
代码如下：

%@ page language="java" pageEncoding="utf-8"%
%@ taglib uri="WEB-INF/tlds/formatTime.tld" prefix="tf" %
html
head
meta http-equiv="Content-Type" content="text/html; charset=gb2312" /
title自定义标签示例/title
/head
body
p
转换“20090403132233”
/p
format:dd MMMM yyyy showTH:true style:Upper 转换后为：tf:formatTimeAsString timestamp="20090403132233" format="dd MMMM yyyy" showTH="true" style="Upper"/br/
format:dd MMMM yyyy showTH:true style:Lower 转换后为：tf:formatTimeAsString timestamp="20090403132233" format="dd MMMM yyyy" showTH="true" style="Lower"/br/
format:dd MMMM yyyy showTH:false style:Upper 转换后为：tf:formatTimeAsString timestamp="20090403132233" format="dd MMMM yyyy" showTH="false" style="Upper"/br/
format:dd MMMM yyyy showTH:true style:"" 转换后为：tf:formatTimeAsString timestamp="20090403132233" format="dd MMMM yyyy" showTH="true" style=""/br/
/body
/html

ok了。这里timestamp是一个给定值，如果是通过${}标签动态取的话，需要将tld文件的
代码如下：

attribute
nametimestamp/name
/attribute

修改为
代码如下：

attribute
nametimestamp/name
rtexprvaluetrue/rtexprvalue
/attribute