JSTL 學習筆記

JSP Standard Tag Library (JSTL) 的規範完成於2002年7月，隨後Apache Taglibs Projects在不久的幾天中提交了一個參考實現。

JSTL 的出現是爲了解決程序員一直渴望有一個標準的標籤庫的需求，同時也爲開發JSP帶來了很大的便利。

JSTL 1.0提供了一系列基於JSP 1.2 API的標籤庫，下表列舉了一些標籤庫的信息：

Description	Prefix	Default URI
Core	c	http://java.sun.com/jstl/core
XML Processing	x	http://java.sun.com/jstl/xml
I18N & Formatting	fmt	http://java.sun.com/jstl/fmt
Database Access	sql	http://java.sun.com/jstl/sql

如果要使用JSTL，那麼需要加入如下聲明段：

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

除了標籤庫之外，JSTL 1.0還定義了一個所謂的Expression Language (EL)表達式語言。
EL用於訪問運行時數據而出現，如果學過JavaScript，你會發現EL在表達上與其很類似。
${myObj.myProperty}$
${myObj["myProperty"]}$
${myObj[varWithTheName]}$
以上的語句用來檢索一個對象的內部值，都是等價的。如果是訪問一個數組或者列表：
${myList[2]}$
${myList[aVar + 1]}$

EL的支持的操作符：

Operator	Description
.	Access a property
[]	Access an array/list element
()	Group a subexpression
+	Addition
-	Subtraction or negation of a number
/ or div	Division
% or mod	Modulo (remainder)
== or eq	Test for equality
!= or ne	Test for inequality
< or lt	Test for less than
> or gt	Test for greater than
<= or le	Test for less than or equal
>= or gt	Test for greater than or equal
&& or and	Test for logical AND
|| or or	Test for logical OR
! or not	Unary Boolean complement
empty	Test for empty value (null, empty string, or an empty collection)

支持的字面量：

Literal Type	Description
String	Enclosed with single or double quotes. A quote of the same type within the string must be escaped with backslash: (/' in a string enclosed with single quotes; /" in a string enclosed with double quotes). The backslash character must be escaped as // in both cases.
Integer	An optional sign (+ or -) followed by digits between 0 and 9.
Floating Point	The same as an integer literal, except that a dot is used as the separator for the fractional part and an exponent can be specified as e or E, followed by an integer literal.
Boolean	true or false.
Null	null.

支持的內建對象：

Variable	Description
param	A collection of all request parameters as a single string value for each parameter.
paramValues	A collection of all request parameters as a string array value for each parameter.
header	A collection of all request headers as a single string value for each header.
headerValues	A collection of all request headers as a string array value for each header.
cookie	A collection of all request cookies as a single javax.servlet.http.Cookie instance value for each cookie.
initParams	A collection of all application init parameters as a single string value for each parameter.
pageContext	An instance of the javax.servlet.jspPageContext class.
pageScope	A collection of all page scope objects.
requestScope	A collection of all request scope objects.
sessionScope	A collection of all session scope objects.
applicationScope	A collection of all application scope objects.

如果你要訪問GET參數：${param.listType}
如果你要訪問HTTP頭信息：${header['User-Agent']}

訪問Session或者Request內部包含對象：

${sessionScope.customer}
${requestScope.customer}

一些例子：
First name: <c:out value="${customer.firstName}" />
<c:out value="First name: ${customer.firstName}" />
First name: <c_rt:out value="<%= customer.getFirstName() %>" /> 
都是等價的。

控制流程和迭代操作也是JSTL的一個特性，個人認爲迭代標籤是當Java 1.5沒出來之前對於Java語言的最好補充。
迭代操作使用的forEach標籤：
<c:forEach items="${forecasts.rows}" var="city">
   City: <c:out value="${city.name}" />
   Tomorrow's high: <c:out value="${city.high}" />
   Tomorrow's low: <c:out value="${city.low}" />
</c:forEach>

Choose標籤：

<c:choose>
  <c:when test="${param.first > 0}">
     <a href="foreach.jsp?first=<c:out value="${param.first - noOfRows}"/>">
                Previous Page</a>
  </c:when>
  <c:otherwise>
     Previous Page
  </c:otherwise>
</c:choose>
<c:choose>
  <c:when test="${param.first + noOfRows < forecasts.rowsCount}">
     <a href="foreach.jsp?first=<c:out value="${param.first + noOfRows}"/>">
                Next Page</a>
  </c:when>
  <c:otherwise>
     Next Page
  </c:otherwise>
</c:choose>

URL操作：

<c:url var="previous" value="foreach.jsp">
  <c:param name="first" value="${param.first - noOfRows}" />
</c:url>

特別提及的URL操作：
<c:url value="/images/logo.gif" />
<a href="<c:out value="${previous}"/>">Previous Page</a>

這裏的"/images/logo.gif"不是從網站的根目錄起始，而是自動調整到所對應的context根目錄，非常實用。