来源：http://blog.csdn.net/i_buntu/article/details/6407893
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JSTL1.2
The
JSTL 1.2 Maintenace Release aligns with the Unified Expression Language
(EL) that is being delivered as part of the JavaServer Pages (JSP) 2.1
specification。JSTL 1.2 is part of the Java EE 5 platform. （from
http://java.sun.com/products/jsp/jstl/）
JSTL1.1
Jakarta
Taglibs hosts the Standard Taglib 1.1, an implementation of the JSP
Standard Tag Library (JSTL), version 1.1, which was developed under the
Java Community Process.
NOTE: Standard-1.1 (JSTL 1.1) requires a JSP
container that supports the Java Servlet 2.4 and JavaServer Pages 2.0
specifications. Jakarta Tomcat 5 supports the new specifications. The
Standard-1.1 taglib has been tested with Tomcat 5.0.3.
JSTL1.0
Standard-1.0
(implementation of the JSTL 1.0 specification) requires a JSP container
that supports the Java Servlet 2.3 and JavaServer Pages 1.2
specifications. Jakarta Tomcat 4 supports these specifications. The
Standard 1.0 taglib has been tested with Tomcat 4.1.24. (from
http://jakarta.apache.org/taglibs/doc/standard-doc/intro.html)
Tomcat6 实现了servlet 2.5 和JSP2.1的规范，可以支持JSTL1.2；
而Tomcat5实现了 servlet 2.4 和JSP2.0的规范，只能支持JSTL1.1
所以，只要使用JSTL1.1(standard-1.1.jar和jstl-1.1.jar)替换JSTL1.2（jstl-1.2.jar）即可在Tomcat5下正常运行！
另外，web.xml必须指定使用servlet 2.4 和JSP2.0的规范：
<web-app version="2.4" xmlns="http://java.sun.com/xml/ns/j2ee"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation="http://java.sun.com/xml/ns/j2ee http://java.sun.com/xml/ns/j2ee/web-app_2_4.xsd">
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jstl 1.1 c标签引入：http://java.sun.com/jsp/jstl/core
jstl 1.0 c标签引入： http://java.sun.com/jstl/core