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 |