XML Tags

A key aspect of dealing with XML documents is to be able to easily access their content. XPath, a W3C recommendation since 1999, provides an easy notation for specifying and selecting parts of an XML document. The JSTL XML tag set, listed in Table 6-4, is based on XPath.

Table 6-4 XML Tags
Area	Function	Tags
XML	Core	`out` `parse` `set`
	Flow Control	`choose` `when` `otherwise` `forEach` `if`
	Transformation	`transform` `param`

The XML tags use XPath as a local expression language; XPath expressions are always specified using attribute select. This means that only values specified for select attributes are evaluated using the XPath expression language. All other attributes are evaluated using the rules associated with the global expression language.

In addition to the standard XPath syntax, the JSTL XPath engine supports the following scopes to access Web application data within an XPath expression:

$foo

$param:

$header:

$cookie:

$initParam:

$pageScope:

$requestScope:

$sessionScope:

$applicationScope:

These scopes are defined in exactly the same way as their counterparts in the JSTL expression language discussed in Implicit Objects. Table 6-5 shows some examples of using the scopes.

Table 6-5 Example XPath Expressions
XPath Expression	Result
`$sessionScope:profile`	The session-scoped attribute named `profile`
`$initParam:mycom.productId`	The `String` value of the `mycom.productId` context parameter

The XML tags are illustrated in another version (bookstore5) of the Duke's Bookstore application. This version replaces the database with an XML representation (books.xml) of the bookstore database. To build and install this version of the application, follow the directions in The Example JSP Pages replacing bookstore4 with bookstore5.

Since the XML tags require an XPath evaluator, which is provided in two libraries, jaxen-full.jar and saxpath.jar, included with the JSTL tag library.

Core Tags

The core XML tags provide basic functionality to easily parse and access XML data.

The parse tag parses an XML document and saves the resulting object in the scoped attribute specified by attribute var. In bookstore5, the XML document is parsed and saved to a context attribute in parseBooks.jsp, which is included by all JSP pages that need access to the document:

<c:if test="${applicationScope:booklist == null}" > 
  <c:import url="/books.xml" var="xml" />
  <x:parse xml="${xml}" var="booklist" scope="application" />
</c:if>

The out and set tags parallel the behavior described in Expression Tags for the XPath local expression language. The out tag evaluates an XPath expression on the current context node and outputs the result of the evaluation to the current JspWriter object.

The set tag evaluates an XPath expression and sets the result into a JSP scoped attribute specified by attribute var.

The JSP page bookdetails.jsp selects a book element whose id attribute matches the request parameter bookId and sets the abook attribute. The out tag then selects the book's title element and outputs the result.

As you have just seen, x:set stores an internal XML representation of a node retrieved using an XPath expression; it doesn't convert the selected node into a String and store it. Thus, x:set is primarily useful for storing parts of documents for later retrieval.

If you want to store a String, you need to use x:out within c:set. The x:out tag converts the node to a String, and c:set> then stores the String as a scoped attribute. For example, bookdetails.jsp stores a scoped attribute containing a book price, which is later fed into a fmt tag, as follows:

<c:set var="price">
  <x:out select="$abook/price"/>
</c:set>
<h4><fmt:message key="ItemPrice"/>: 
  <fmt:formatNumber value="${price}" type="currency"/>

The other option, which is more direct but requires that the user have more knowledge of XPath, is to coerce the node to a String manually using XPath's string function.

Flow Control Tags

The XML flow control tags parallel the behavior described in Flow Control Tags for the XPath expression language.

The JSP page catalog.jsp uses the forEach tag to display all the books contained in booklist as follows:

<x:forEach var="book" 
  select="$applicationScope:booklist/books/*">
  <tr>
    <c:set var="bookId">
      <x:out select="$book/@id"/>
    </c:set>=
    <td bgcolor="#ffffaa"> 
      <c:url var="url"
      value="/bookdetails" >
        <c:param name="bookId" value="${bookId}" />
        <c:param name="Clear" value="0" />
      </c:url>
      <a href="<c:out value='${url}'/>">
      <strong><x:out select="$book/title"/>&nbsp;
      </strong></a></td> 
    <td bgcolor="#ffffaa" rowspan=2> 
      <c:set var="price">
        <x:out select="$book/price"/>
      </c:set>
      <fmt:formatNumber value="${price}" type="currency"/> 
      &nbsp;
    </td> 
    <td bgcolor="#ffffaa" rowspan=2> 
    <c:url var="url" value="/catalog" >
      <c:param name="Add" value="${bookId}" />
    </c:url> 
    <p><strong><a href="<c:out value='${url}'/>">&nbsp;
      <fmt:message key="CartAdd"/>&nbsp;</a>
    </td>
  </tr> 
  <tr> 
    <td bgcolor="#ffffff"> 
    &nbsp;&nbsp;<fmt:message key="By"/> <em>
      <x:out select="$book/firstname"/>&nbsp;
      <x:out select="$book/surname"/></em></td></tr>
</x:forEach>

Transformation Tags

The transform tag applies a transformation, specified by a XSLT stylesheet set by the attribute xslt, to an XML document, specified by the attribute xml. If the xml attribute is not specified, the input XML document is read from the tag's body content.

The param subtag can be used along with transform to set transformation parameters. The attributes name and value are used to specify the parameter. The value attribute is optional. If it is not specified the value is retrieved from the tag's body.