<title>JSP tag libraries with XSL</title>

XML Template Pages encourages web sites to create JSP tag libraries.
By putting the JSP creation in stylesheets, you can separate the
programming from the web pages.  The XTP pages can be simple XML or
HTML documents and simply add the custom tags as necessary.

<p/>The examples in this section use the full, verbose, XSL and JSP
syntax.  For those put off by the verbosity, the examples are
rewritten in <a href='xtp-jsp-stylescript.xtp'>StyleScript examples</a>.

<summarylist/>

<section name=mixing title="Creating JSP from XTP">

<p><resin/> creates a JSP file from an XTP file.  XTP pages can
separate the content (in XML) from the scripting (in XSL).  The XSL
stylesheets now become a palette of JSP actions you can use just by
adding tags.

<ol>
<li>Parse XTP file as HTML
<li>Find and parse XSL stylesheet
<li>Applying the stylesheet to the XTP, creating a JSP file
<li>Execute the JSP file
</ol>

<p>A trivial example is a named counter.  If the counter has an 'id'
attribute, we'll use it at the value of the application variable.

<p>XTP pages can use the counter just by adding the tag:</p>

<example title='counter.xtp'>
<eg-em>A counter example: </eg-em>&lt;ct:counter id='test'/&gt;
</example>

<p>Here the patterns to do it.  For efficiency, we've added the 
<code/cache/> directive.  The cache directive tells XTP to execute the
stylesheet only once and cache the generated JSP file.

<example title='default.xsl'>
&lt;xtp:directive.cache/&gt;

&lt;xsl:template match='counter[@id]'&gt;
  <eg-em>&lt;jsp:expression&gt;</eg-em>
     &lt;xsl:text&gt;<eg-em>application.attribute["</eg-em>&lt;/xsl:text>
     &lt;xsl:value-of select='@id'/>
     &lt;xsl:text><eg-em>"]</eg-em>&lt;/xsl:text>
  <eg-em>&lt;/jsp:expression&gt;</eg-em>
&lt;/xsl:template&gt;

&lt;xsl:template match='counter'&gt;
  <eg-em>&lt;jsp:expression&gt;</eg-em>
     <eg-em>application.attribute.counter++</eg-em>
  <eg-em>&lt;/jsp:expression&gt;</eg-em>
&lt;/xsl:template&gt;

</example>

<p>The following JSP file is the result.  <resin/> will
execute the generated JSP file to process HTTP requests.  Because
default.xsl was marked as cached, on following requests <resin/> will
merely reexecute 'gen438.jsp'.

<results title='gen438.jsp'>
A counter example: &lt;jsp:expression&gt;
  application.attribute["test"]++ 
&lt;jsp:expression&gt;
</results>

</section>

<section name=flow title="Creating JSP flow control">

XTP can also generate JSP control.  Many programmers like using tags
for programming, like Cold Fusion.  Here's how to create a basic set
of programming tags.

<ul>
<li>ct:get prints the value of a variable
<li>ct:if generates an if statement
<li>ct:iter loops
</ul>

An example use might print the HTTP headers.

<example>
<example>
&lt;ct:iter expr='request.header'/&gt;
  &lt;ct:get expr='i'/&gt; : &lt;ct:get expr='request.header[i]'/&gt;
&lt;/ct:iter&gt;
</example>
<results>
Accept : image/gif, image/png
User-Agent : Mozilla/4.5
Accept-Language : en
Host : www.caucho.com
</results>
</example>

Here is the stylesheet:

<example title='definition of ct:get'>
&lt;xsl:template match='ct:get'>
  <eg-em>&lt;jsp:expression></eg-em>&lt;xsl:value-of select='@expr'/><eg-em>&lt;/jsp:expression></eg-em>
&lt;/xsl:template>
</example>

<example title='definition of ct:if'>
&lt;xsl:template match='ct:if'>
  <eg-em>&lt;jsp:scriptlet>
    if (</eg-em>&lt;xsl:value-of select='@expr'/><eg-em>) {
  &lt;/jsp:scriptlet></eg-em>
    &lt;xsl:apply-templates/>
  <eg-em>&lt;jsp:scriptlet>}&lt;/jsp:scriptlet></eg-em>
&lt;/xsl:template>
</example>

<example title='definition of ct:iter'>
&lt;xsl:template match='ct:iter'>
  <eg-em>&lt;jsp:scriptlet>
    for (var i in </eg-em>&lt;xsl:value-of select='@expr'/><eg-em>) {
  &lt;/jsp:scriptlet></eg-em>
    &lt;xsl:apply-templates/>
  <eg-em>&lt;jsp:scriptlet>}&lt;/jsp:scriptlet></eg-em>
&lt;/xsl:template>
</example>

</section>