Declaring Tag Libraries

This section explains how to use XML namespaces to declare tag libraries.

In standard syntax, the taglib directive declares tag libraries used in a JSP page. Here is an example of a taglib directive:

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

This syntax is not allowed in JSP documents. To declare a tag library in a JSP document, you use the xmlns attribute, which is used to declare namespaces according to the XML standard:

...
xmlns:c="http://java.sun.com/jsp/jstl/core"
...

The value that identifies the location of the tag library can take three forms:

• A plain URI that is a unique identifier for the tag library. The container tries to match it against any <taglib-uri> elements in the application’s web.xml file or the <uri> element of tag library descriptors (TLDs) in JAR files in /WEB-INF/lib/ or TLDs under WEB-INF.

• A URN of the form urn:jsptld:path.

• A URN of the form urn:jsptagdir:path.

The URN of the form urn:jsptld:path points to one tag library packaged with the application:

xmlns:u="urn:jsptld:/WEB-INF/tlds/my.tld"

The URN of the form urn:jsptagdir:path must start with /WEB-INF/tags/ and identifies tag extensions (implemented as tag files) installed in the /WEB-INF/tags/ directory or a subdirectory of it:

xmlns:u="urn:jsptagdir:/WEB-INF/tags/mytaglibs/"

You can include the xmlns attribute in any element in your JSP document, just as you can in an XML document. This capability has many advantages:

• It follows the XML standard, making it easier to use any XML document as a JSP document.

• It allows you to scope prefixes to an element and override them.

• It allows you to use xmlns to declare other namespaces and not just tag libraries.

The books.jspx page declares the tag libraries it uses with the xmlns attributes in the root element, books:

<books
    xmlns:jsp="http://java.sun.com/JSP/Page"
    xmlns:c="http://java.sun.com/jsp/jstl/core"
>

In this way, all elements within the books element have access to these tag libraries.

As an alternative, you can scope the namespaces:

<books>
...
    <jsp:useBean xmlns:jsp="http://java.sun.com/JSP/Page"
                        id="bookDB"
                         class="database.BookDB"
                         scope="page">
         <jsp:setProperty name="bookDB"
                         property="database" value="${bookDBAO}" />
    </jsp:useBean>
    <c:forEach xmlns:c="http://java.sun.com/jsp/jstl/core"
                    var="book" begin="0" items="${bookDB.books}">
                    ...
    </c:forEach>
</books>

In this way, the tag library referenced by the jsp prefix is available only to the jsp:useBean element and its subelements. Similarly, the tag library referenced by the c prefix is only available to the c:forEach element.

Scoping the namespaces also allows you to override the prefix. For example, in another part of the page, you could bind the c prefix to a different namespace or tag library. In contrast, the jsp prefix must always be bound to http://java.sun.com/JSP/Page, the JSP namespace.