JSP - 标准标记库（JSTL）教程

在本章中，我们将了解JSP中的不同标记. JavaServer Pages标准标记库(JSTL)是一组有用的JSP标记，它封装了许多JSP应用程序通用的核心功能.

JSTL支持常见的结构任务，如迭代和条件，用于处理XML文档的标记，国际化标记和SQL标记.它还提供了一个框架，用于将现有自定义标记与JSTL标记集成.

安装JSTL库

要开始使用JSP tages，您需要首先安装JSTL库.如果您使用的是Apache Tomcat容器，请按照以下两个步骤 : 去;

步骤1 : 从 Apache Standard Taglib 下载二进制发行版并解压缩压缩文件.

第2步 : 要从 Jakarta Taglibs发行版中使用标准Taglib，只需将发行版'lib'目录中的JAR文件复制到应用程序的 webapps \ROOT \WEB-INF \ lib 目录.

要使用任何库，您必须包含< taglib>在每个使用该库的JSP顶部的指令.

JSTL标记可以根据其功能进行分类，在创建JSP页面时可以使用的以下JSTL标记库组 :

核心标签组是最常用的JSTL标签.以下是在JSP中包含JSTL核心库的语法 :

<％@ taglib prefix ="c"uri ="http ://java.sun.com/jsp/jstl/core"％>

下表列出了核心JSTL标签:

S.No.	Tag & Description
1	<c:out> Like <%= ... >, but for expressions.
2	<c:set > Sets the result of an expression evaluation in a 'scope'
3	<c:remove > Removes a scoped variable (from a particular scope, if specified).
4	<c:catch> Catches any Throwable that occurs in its body and optionally exposes it.
5	<c:if> Simple conditional tag which evalutes its body if the supplied condition is true.
6	<c:choose> Simple conditional tag that establishes a context for mutually exclusive conditional operations, marked by <when> and <otherwise>.
7	<c:when> Subtag of <choose> that includes its body if its condition evalutes to 'true'.
8	<c:otherwise > Subtag of <choose> that follows the <when> tags and runs only if all of the prior conditions evaluated to 'false'.
9	<c:import> Retrieves an absolute or relative URL and exposes its contents to either the page, a String in 'var', or a Reader in 'varReader'.
10	<c:forEach > The basic iteration tag, accepting many different collection types and supporting subsetting and other functionality .
11	<c:forTokens> Iterates over tokens, separated by the supplied delimeters.
12	<c:param> Adds a parameter to a containing 'import' tag's URL.
13	<c:redirect > Redirects to a new URL.
14	<c:url> Creates a URL with optional query parameters

JSTL格式标签用于格式化和显示国际化网站的文本，日期，时间和数字。以下是在JSP中包含格式化库的语法:

<%@ taglib prefix = "fmt" uri = "http://java.sun.com/jsp/jstl/fmt" %>

下表列出了格式化JSTL标记:

S.No.	Tag & Description
1	<fmt:formatNumber> To render numerical value with specific precision or format.
2	<fmt:parseNumber> Parses the string representation of a number, currency, or percentage.
3	<fmt:formatDate> Formats a date and/or time using the supplied styles and pattern.
4	<fmt:parseDate> Parses the string representation of a date and/or time
5	<fmt:bundle> Loads a resource bundle to be used by its tag body.
6	<fmt:setLocale> Stores the given locale in the locale configuration variable.
7	<fmt:setBundle> Loads a resource bundle and stores it in the named scoped variable or the bundle configuration variable.
8	<fmt:timeZone> Specifies the time zone for any time formatting or parsing actions nested in its body.
9	<fmt:setTimeZone> Stores the given time zone in the time zone configuration variable
10	<fmt:message> Displays an internationalized message.
11	<fmt:requestEncoding> Sets the request character encoding

JSTL SQL标记库提供用于与关系数据库（RDBMS）（如Oracle，mySQL或Microsoft SQL Server）交互的标记。

以下是在JSP中包含JSTL SQL库的语法:

<%@ taglib prefix = "sql" uri = "http://java.sun.com/jsp/jstl/sql" %>

下表列出了SQL JSTL标记:

S.No.	Tag & Description
1	<sql:setDataSource> Creates a simple DataSource suitable only for prototyping
2	<sql:query> Executes the SQL query defined in its body or through the sql attribute.
3	<sql:update> Executes the SQL update defined in its body or through the sql attribute.
4	<sql:param> Sets a parameter in an SQL statement to the specified value.
5	<sql:dateParam> Sets a parameter in an SQL statement to the specified java.util.Date value.
6	<sql:transaction > Provides nested database action elements with a shared Connection, set up to execute all statements as one transaction.

JSTL XML标记提供了一种以JSP为中心的方法来创建和操作XML文档。以下是在JSP中包含JSTL XML库的语法。

JSTL XML标记库具有用于与XML数据交互的自定义标记。这包括解析XML，转换XML数据以及基于XPath表达式的流控制。

<%@ taglib prefix = "x" 
   uri = "http://java.sun.com/jsp/jstl/xml" %>

在继续执行示例之前，您需要将以下两个与XML和XPath相关的库复制到<Tomcat安装目录\lib中:

下表列出了各种JSTL函数:

S.No.	Tag & Description
1	<x:out> Like <%= ... >, but for XPath expressions.
2	<x:parse> Used to parse the XML data specified either via an attribute or in the tag body.
3	<x:set > Sets a variable to the value of an XPath expression.
4	<x:if > Evaluates a test XPath expression and if it is true, it processes its body. If the test condition is false, the body is ignored.
5	<x:forEach> To loop over nodes in an XML document.
6	<x:choose> Simple conditional tag that establishes a context for mutually exclusive conditional operations, marked by <when> and <otherwise> tags.
7	<x:when > Subtag of <choose> that includes its body if its expression evalutes to 'true'.
8	<x:otherwise > Subtag of <choose> that follows the <when> tags and runs only if all of the prior conditions evaluates to 'false'.
9	<x:transform > Applies an XSL transformation on a XML document
10	<x:param > Used along with the transform tag to set a parameter in the XSLT stylesheet

JSTL包含许多标准函数，其中大多数是常见的字符串操作函数。以下是在JSP中包含JSTL Functions库的语法:

<%@ taglib prefix = "fn" 
   uri = "http://java.sun.com/jsp/jstl/functions" %>

下表列出了各种JSTL函数:

S.No.	Function & Description
1	fn:contains() Tests if an input string contains the specified substring.
2	fn:containsIgnoreCase() Tests if an input string contains the specified substring in a case insensitive way.
3	fn:endsWith() Tests if an input string ends with the specified suffix.
4	fn:escapeXml() Escapes characters that can be interpreted as XML markup.
5	fn:indexOf() Returns the index withing a string of the first occurrence of a specified substring.
6	fn:join() Joins all elements of an array into a string.
7	fn:length() Returns the number of items in a collection, or the number of characters in a string.
8	fn:replace() Returns a string resulting from replacing in an input string all occurrences with a given string.
9	fn:split() Splits a string into an array of substrings.
10	fn:startsWith() Tests if an input string starts with the specified prefix.
11	fn:substring() Returns a subset of a string.
12	fn:substringAfter() Returns a subset of a string following a specific substring.
13	fn:substringBefore() Returns a subset of a string before a specific substring.
14	fn:toLowerCase() Converts all of the characters of a string to lower case.
15	fn:toUpperCase() Converts all of the characters of a string to upper case.
16	fn:trim() Removes white spaces from both ends of a string.