Tag Collaboration

Tags usually collaborate with their environment in implicit and explicit ways. Implicit collaboration is done by means of 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 through 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 9, Scripting in JSP Pages), you use the standard JSP element jsp:useBean to declare a scripting variable.

For example, tut-install/javaeetutorial5/examples/web/bookstore4/web/books/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>