Using JSTL

JSTL includes a wide variety of tags that fit into discrete functional areas. To reflect this, as well as to give each area its own namespace, JSTL is exposed as multiple tag libraries. The URIs for the libraries are as follows:

Core: http://java.sun.com/jsp/jstl/core

XML: http://java.sun.com/jsp/jstl/xml

Internationalization: http://java.sun.com/jsp/jstl/fmt

SQL: http://java.sun.com/jsp/jstl/sql

Functions: http://java.sun.com/jsp/jstl/functions

Table 14-2 summarizes these functional areas along with the prefixes used in this tutorial.

Table 14-2 JSTL Tags
Area	Subfunction	Prefix
Core	Variable support	`c`
	Flow control
	URL management
	Miscellaneous
XML	Core	`x`
	Flow control
	Transformation
I18n	Locale	`fmt`
	Message formatting
	Number and date formatting
Database	SQL	`sql`
Functions	Collection length	`fn`
Functions	String manipulation	`fn`

Thus, the tutorial references the JSTL core tags in JSP pages by using the following taglib directive:

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

In addition to declaring the tag libraries, tutorial examples access the JSTL API and implementation. In the Sun Java System Application Server, the JSTL TLDs and libraries are distributed in the archive <J2EE_HOME>/lib/appserv-jstl.jar. This library is automatically loaded into the classpath of all Web applications running on the Application Server, so you don't need to add it to your Web application.

Tag Collaboration

Tags usually collaborate with their environment in implicit and explicit ways. Implicit collaboration is done via a well-defined interface that allows nested tags to work seamlessly with the ancestor tag that exposes that interface. The JSTL conditional tags employ this mode of collaboration.

Explicit collaboration happens when a tag exposes information to its environment. JSTL tags expose information as JSP EL variables; the convention followed by JSTL is to use the name var for any tag attribute that exports information about the tag. For example, the forEach tag exposes the current item of the shopping cart it is iterating over in the following way:

<c:forEach var="item" items="${sessionScope.cart.items}">
  ...
</c:forEach>

In situations where a tag exposes more than one piece of information, the name var is used for the primary piece of information being exported, and an appropriate name is selected for any other secondary piece of information exposed. For example, iteration status information is exported by the forEach tag via the attribute status.

When you want to use an EL variable exposed by a JSTL tag in an expression in the page's scripting language (see Chapter 16), you use the standard JSP element jsp:useBean to declare a scripting variable.

For example, bookshowcart.jsp removes a book from a shopping cart using a scriptlet. The ID of the book to be removed is passed as a request parameter. The value of the request parameter is first exposed as an EL variable (to be used later by the JSTL sql:query tag) and then is declared as a scripting variable and passed to the cart.remove method:

<c:set var="bookId" value="${param.Remove}"/>
<jsp:useBean id="bookId" type="java.lang.String" />
<% cart.remove(bookId); %>
<sql:query var="books" 
  dataSource="${applicationScope.bookDS}">
  select * from PUBLIC.books where id = ?
  <sql:param value="${bookId}" />
</sql:query>