JSP2.0自定义标签

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JSP1.0中可以通过继承TagSupport或者BodyTagSupport来实现自定义的tag处理方法。

JSP2.0中也支持另外一种更为简单的自定tag的方法，那就是直接讲JSP代码保存成*.tag或者*.tagx的标签定义文件。tag和tagx文件不仅支持经典jsp代码，各种标签模版代码，还支持xml样式的jsp指令代码。

按照约定，tag和tagx文件需要放置在WEB-INF/tags目录下。

下面是一些简单的示例：

1.简单地显示时间time.tag

<%@ tag import="java.util.*" import="java.text.*" %>
<%
  DateFormat df = DateFormat.getDateInstance(DateFormat.LONG);
  Date d = new Date(System.currentTimeMillis());
  out.println(df.format(d));
%>

<%@ taglib prefix="util" tagdir="/WEB-INF/tags" %>
<html>
  <head>
  </head>
  <body>
     Today is <util:time/>.
  </body>
</html>

2.复制字符串多少遍repeater.tag

<%@ attribute name="count" type="java.lang.Integer" required="true" %>
<%@ attribute name="value" type="java.lang.String" required="true" %>
<%!
  private String repeater(Integer count, String s) {
    int n = count.intValue();
    StringBuffer sb = new StringBuffer();
    for (int i = 0; i < n; i++) {
      sb.append(s);
    }
    return sb.toString();
  }
%>
<%
  out.println(repeater(count, value));
%>

<%@ taglib prefix="util" tagdir="/WEB-INF/tags" %>
<html>
  <head>
  </head>
  <body>
    Let's get some sleep! <util:repeater count='${3 * 10}' value='zzz'/>
  </body>
</html>

3.查找省份lookup.tag

<%@ tag import="java.util.*" %>
<%@ attribute name="cityName" required="true" %>
<%@ variable name-given="province" %>
<%@ variable name-given="population" variable-class="java.lang.Integer" %>
<%
  if ("Toronto".equals(cityName)) {
    jspContext.setAttribute("province", "Ontario");
    jspContext.setAttribute("population", new Integer(2553400));
  }
  else if ("Montreal".equals(cityName)) {
    jspContext.setAttribute("province", "Quebec");
    jspContext.setAttribute("population", new Integer(2195800));
  }
  else {
    jspContext.setAttribute("province", "Unknown");
    jspContext.setAttribute("population", new Integer(-1));
  }
%>
<jsp:doBody/>

<%@ taglib prefix="util" tagdir="/WEB-INF/tags" %>
<html>
  <head>
  </head>
  <body>
    <% pageContext.setAttribute("cityName", "Montreal"); %>
    <util:lookup cityName="${cityName}">
      ${cityName}'s province is ${province}.
      ${cityName}'s population is approximately ${population / 1000000} million.
    </util:lookup>
  </body>
</html>

上面的都是使用的经典jsp代码，下面将第3个示例使用其他代码实现：

*使用标签：

<%@ tag import="java.util.*" %>
<%@ taglib prefix="c" uri="http://java.sun.com/jsp/jstl/core" %>
<%@ attribute name="cityName" required="true" %>
<%@ variable name-given="province" %>
<%@ variable name-given="population" %>
<c:choose>
  <c:when test="cityName eq 'Toronto'>
    <c:set var="province" value="Ontario"/>
    <c:set var="population" value="2553400"/>
  </c:when>
  <c:when test="cityName eq 'Montreal'>
    <c:set var="province" value="Quebec"/>
    <c:set var="population" value="2195800"/>
  </c:when>
  <c:otherwise>
    <c:set var="province" value="Unknown"/>
    <c:set var="population" value="-1"/>
  </c:otherwise>
</c:choose>
%>
<jsp:doBody/>

<%@ taglib prefix="util" tagdir="/WEB-INF/tags" %>
<%@ taglib prefix="c" uri="http://java.sun.com/jsp/jstl/core" %>
<html>
  <head>
  </head>
  <body>
    <c:set var="cityName" value="Montreal"/>
    <util:lookup cityName="${cityName}">
      ${cityName}'s province is ${province}.
      ${cityName}'s population is approximately ${population / 1000000} million.
    </util:lookup>
  </body>
</html>

*使用jsp指令，通常这种方式生成xml格式的文件

<?xml version='1.0'?>
<jsp:root xmlns:jsp="http://java.sun.com/JSP/Page">
	<jsp:directive.tag import="java.util.*"/>
	<jsp:directive.taglib prefix="c" uri="http://java.sun.com/jsp/jstl/core"/>
	<jsp:directive.attribute name="cityName" required="true"/>
	<jsp:directive.variable name-given="province"/>
	<jsp:directive.variable name-given="population"/>
	<c:choose>
	  <c:when test="cityName eq 'Toronto'>
	    <c:set var="province" value="Ontario"/>
	    <c:set var="population" value="2553400"/>
	  </c:when>
	  <c:when test="cityName eq 'Montreal'>
	    <c:set var="province" value="Quebec"/>
	    <c:set var="population" value="2195800"/>
	  </c:when>
	  <c:otherwise>
	    <c:set var="province" value="Unknown"/>
	    <c:set var="population" value="-1"/>
	  </c:otherwise>
	</c:choose>
</jsp:root>

<?xml version='1.0'?>
<jsp:root version='2.0'
          xmlns:jsp="http://java.sun.com/JSP/Page"
          xmlns:util="urn:jsptagdir:/WEB-INF/tags"
          xmlns:c="http://java.sun.com/jsp/jstl/core">
  <jsp:directive.page contentType="text/html"/>          
	<html>
	  <head>
	  </head>
	  <body>
	    <c:set var="cityName" value="Montreal"/>
	    <util:lookup cityName="${cityName}">
	      ${cityName}'s province is ${province}.
	      ${cityName}'s population is approximately ${population / 1000000} million.
	    </util:lookup>
	  </body>
	</html>
</jsp:root>

附录：

标签文件中常用的指令：

tag	类似JSP page指令，可以用于import常用的java类库等
include	导入其他的标签定义文件
taglib	使用其他标签，如jstl, spring tag, struts tag等等
attribute	定义一个属性
variable	定义一个jsp page中可见的变量，默认范围为NESTED，表示标签内有效。可选项有AT_BEGIN和AT_END

这些指令的可选属性

body-content	标签body的处理方式，可选项： 'empty', 'tagdependent' or 'scriptless'
import	导入使用的java类库
pageEncoding	设置页面编码
isELIgnored	是否忽略el表达式
dynamic-attributes	用于存储自定义属性的map，所谓的自定义属性指：未隐式申明的变量
language	使用的脚本语言，目前必须是java
display-name	标签名
small-icon	for tools
large-icon	for tools
description	标签作用描述
example	informal description of how the tag is used

<jsp:directive:attribute>的可选属性

name	属性名
required	true or false
rtexprvalue	true or false - 指定是否支持运行时表达式
type	值类型 - 默认是java.lang.String
fragment	true or false - 值先传递给容器(false), 直接传给标签处理方法(true)
description	属性描述

<jsp:directive:variable>的可选属性

name-given	变量名（标签使用时的变量名）
name-from-attribute	Specifies the name of an attribute, whose value is the name of the variable that will be available in the calling JSP page. Exactly one of name-given or name-from-attribute must be supplied.
alias	A locally scoped variable which will store the variable's value. Used only with name-from-attribute.
variable-class	变量类.默认是java.lang.String.
declare	Indicates whether the variable is declared in the calling JSP page or tag file. Default is true. Not entirely clear what this means!
scope	变量范围，可选项 AT_BEGIN(标签后jsp page内有效), AT_END(标签后jsp page内有效) and NESTED. NESTED(默认，标签内有效)
description	变量描述