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     <a class="edit-on-gh" href="https://github.com/apache/struts-site/edit/master/source/tag-developers/access-to-valuestack-from-jsps.md" title="Edit this page on GitHub">Edit on GitHub</a>

     <h1 id="access-to-valuestack-from-jsps">Access to ValueStack from JSPs</h1>

 <p>To access the ValueStack from third-party JSP taglibs, expose property values to JSP using the <code class="highlighter-rouge">&lt;s:set/&gt;</code> tag.</p>

 <p><strong>Set a request scoped parameter ‘a’ to list of integers</strong></p>

 <pre><code class="language-jsp">&lt;s:set name="'a'" value="{ 1, 2, 3, 4 }" scope="request"/&gt;
 </code></pre>

 <p>After setting parameters, third-party JSP taglibs can access variables or use JSP 2.0 EL (Expression Language).
 This is convenient as short hand EL expression syntax <code class="highlighter-rouge">${expression}</code> can be used in a text or inside of tag attributes:</p>

 <div class="highlighter-rouge"><div class="highlight"><pre class="highlight"><code>a[0] = ${a[0]}

 &lt;sample:tag value="${a[1]}"/&gt;
 </code></pre></div></div>

 <p>In practice, several variables must be exposed to make effective use of third party taglibs like <a href="http://displaytag.sourceforge.net/11/">DisplayTag</a>.
 Unfortunately, this approach leads to a lot of <code class="highlighter-rouge">&lt;s:set/&gt;</code> tags.</p>

 <blockquote>
   <p>Unfortunately, it isn’t that simple. we tinkered with <code class="highlighter-rouge">JSPFactory.setDefault()</code> to wrap around <code class="highlighter-rouge">getPageContext()</code>
 and create <code class="highlighter-rouge">ExpressionEvaluator</code> that would use OGNL. This strategy works in practice, but code generated by Jasper2
 doesn’t call <code class="highlighter-rouge">JSPFactory.getPageContext().getExpressionEvaluator()</code> but goes directly to static method that is
 hardwired to Jakarta Commons-EL implementation.</p>
 </blockquote>

 <blockquote>
   <table>
     <tbody>
       <tr>
         <td>Even if this approach did work, it wouldn’t be <em>clean</em> as <code class="highlighter-rouge">JSPFactory.setDefault()</code> should only be called by JSP implementation.</td>
       </tr>
     </tbody>
   </table>
 </blockquote>

 <p>There is a simple, if not elegant, solution available in JSP 2.0 EL, for exposing ValueStack to OGNL. It is possible
 to create custom functions that can be called from EL expressions. Functions have to be ‘public static’ and specified
 in a TLD file.</p>

 <p>To use a function, import the TLD in a JSP file where you’ve want to use a function. For example, you could access
 Action properties by evaluating OGNL expression by a function ‘vs’ (for valuestack) in EL.</p>

 <pre><code class="language-jsp">&lt;%@ taglib uri="/WEB-INF/tld/wwel.tld" prefix="x" %&gt;

 a[0] = ${x:vs('a[0]')}
 a[0] * 4 = ${x:vs('a[0] * 4')}

 Current action name: ${x:name()}
 Top of ValueStack: ${x:top()}
 </code></pre>

 <p>To use this code you’ve got to add <code class="highlighter-rouge">wwel.tld</code> and <code class="highlighter-rouge">Functions.java</code> to your webapp project.</p>

 <blockquote>
   <p>If someone were interested, it would be helpful for a developer (like you!) to define a set of functions that we could
 include in a future release of the framework.</p>
 </blockquote>

 <p><strong>wwel.tld</strong></p>

 <div class="language-xml highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="cp">&lt;?xml version="1.0"?&gt;</span>
 <span class="nt">&lt;taglib</span> <span class="na">xmlns=</span><span class="s">"http://java.sun.com/xml/ns/j2ee"</span>
 	<span class="na">xmlns:xsi=</span><span class="s">"http://www.w3.org/2001/XMLSchema-instance"</span>
 	<span class="na">xsi:schemaLocation=</span><span class="s">"http://java.sun.com/xml/ns/j2ee http://java.sun.com/xml/ns/j2ee/web-jsptaglibrary_2_0.xsd"</span>
 	<span class="na">version=</span><span class="s">"2.0"</span><span class="nt">&gt;</span>

 <span class="nt">&lt;description&gt;</span>
 This taglib enables access to the ValueStack
 from JSP 2.0 Expression Language
 <span class="nt">&lt;/description&gt;</span>

 <span class="nt">&lt;tlib-version&gt;</span>1.0<span class="nt">&lt;/tlib-version&gt;</span>

 <span class="nt">&lt;short-name&gt;</span>wwel<span class="nt">&lt;/short-name&gt;</span>

 <span class="nt">&lt;function&gt;</span>
 	<span class="nt">&lt;name&gt;</span>vs<span class="nt">&lt;/name&gt;</span>
 	<span class="nt">&lt;function-class&gt;</span>com.nmote.wwel.Functions<span class="nt">&lt;/function-class&gt;</span>
 	<span class="nt">&lt;function-signature&gt;</span>
 		java.lang.Object findOnValueStack(java.lang.String)
 	<span class="nt">&lt;/function-signature&gt;</span>
 <span class="nt">&lt;/function&gt;</span>

 <span class="nt">&lt;function&gt;</span>
 	<span class="nt">&lt;name&gt;</span>name<span class="nt">&lt;/name&gt;</span>
 	<span class="nt">&lt;function-class&gt;</span>com.nmote.wwel.Functions<span class="nt">&lt;/function-class&gt;</span>
 	<span class="nt">&lt;function-signature&gt;</span>
 		java.lang.Object getActionName()
 	<span class="nt">&lt;/function-signature&gt;</span>
 <span class="nt">&lt;/function&gt;</span>

 <span class="nt">&lt;function&gt;</span>
 	<span class="nt">&lt;name&gt;</span>top<span class="nt">&lt;/name&gt;</span>
 	<span class="nt">&lt;function-class&gt;</span>com.nmote.wwel.Functions<span class="nt">&lt;/function-class&gt;</span>
 	<span class="nt">&lt;function-signature&gt;</span>
 		java.lang.Object getTopOfValueStack()
 	<span class="nt">&lt;/function-signature&gt;</span>
 <span class="nt">&lt;/function&gt;</span>

 <span class="nt">&lt;/taglib&gt;</span>
 </code></pre></div></div>

 <p><strong>Functions.java</strong></p>

 <div class="language-java highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="kn">package</span> <span class="n">com</span><span class="o">.</span><span class="na">nmote</span><span class="o">.</span><span class="na">wwel</span><span class="o">;</span>

 <span class="kn">import</span> <span class="nn">com.opensymphony.xwork.ActionContext</span><span class="o">;</span>

 <span class="cm">/**
  * Utility functions for accessing value stack and action context
  * from JSP 2.0 EL taglibs.
  */</span>
 <span class="kd">public</span> <span class="kd">class</span> <span class="nc">Functions</span> <span class="o">{</span>

 	<span class="kd">public</span> <span class="kd">static</span> <span class="n">Object</span> <span class="nf">findOnValueStack</span><span class="o">(</span><span class="n">String</span> <span class="n">expr</span><span class="o">)</span> <span class="o">{</span>
 		<span class="n">ActionContext</span> <span class="n">a</span> <span class="o">=</span> <span class="n">ActionContext</span><span class="o">.</span><span class="na">getContext</span><span class="o">();</span>
 		<span class="n">Object</span> <span class="n">value</span> <span class="o">=</span> <span class="n">a</span><span class="o">.</span><span class="na">getValueStack</span><span class="o">().</span><span class="na">findValue</span><span class="o">(</span><span class="n">expr</span><span class="o">);</span>
 		<span class="k">return</span> <span class="n">value</span><span class="o">;</span>
 	<span class="o">}</span>

 	<span class="kd">public</span> <span class="kd">static</span> <span class="n">Object</span> <span class="nf">getTopOfValueStack</span><span class="o">()</span> <span class="o">{</span>
 		<span class="n">ActionContext</span> <span class="n">a</span> <span class="o">=</span> <span class="n">ActionContext</span><span class="o">.</span><span class="na">getContext</span><span class="o">();</span>
 		<span class="n">Object</span> <span class="n">value</span> <span class="o">=</span> <span class="n">a</span><span class="o">.</span><span class="na">getValueStack</span><span class="o">().</span><span class="na">peek</span><span class="o">();</span>
 		<span class="k">return</span> <span class="n">value</span><span class="o">;</span>
 	<span class="o">}</span>

 	<span class="kd">public</span> <span class="kd">static</span> <span class="n">Object</span> <span class="nf">getActionName</span><span class="o">()</span> <span class="o">{</span>
 		<span class="n">ActionContext</span> <span class="n">a</span> <span class="o">=</span> <span class="n">ActionContext</span><span class="o">.</span><span class="na">getContext</span><span class="o">();</span>
 		<span class="n">Object</span> <span class="n">value</span> <span class="o">=</span> <span class="n">a</span><span class="o">.</span><span class="na">getName</span><span class="o">();</span>
 		<span class="k">return</span> <span class="n">value</span><span class="o">;</span>
 	<span class="o">}</span>
 <span class="o">}</span>
 </code></pre></div></div>

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	<a class="edit-on-gh" href="https://github.com/apache/struts-site/edit/master/source/tag-developers/access-to-valuestack-from-jsps.md" title="Edit this page on GitHub">Edit on GitHub</a>

	<h1 id="access-to-valuestack-from-jsps">Access to ValueStack from JSPs</h1>

	<p>To access the ValueStack from third-party JSP taglibs, expose property values to JSP using the <code class="highlighter-rouge"><s:set/></code> tag.</p>

	<p><strong>Set a request scoped parameter ‘a’ to list of integers</strong></p>

	<pre><code class="language-jsp"><s:set name="'a'" value="{ 1, 2, 3, 4 }" scope="request"/>
	</code></pre>

	<p>After setting parameters, third-party JSP taglibs can access variables or use JSP 2.0 EL (Expression Language).
	This is convenient as short hand EL expression syntax <code class="highlighter-rouge">${expression}</code> can be used in a text or inside of tag attributes:</p>

	<div class="highlighter-rouge"><div class="highlight"><pre class="highlight"><code>a[0] = ${a[0]}

	<sample:tag value="${a[1]}"/>
	</code></pre></div></div>

	<p>In practice, several variables must be exposed to make effective use of third party taglibs like <a href="http://displaytag.sourceforge.net/11/">DisplayTag</a>.
	Unfortunately, this approach leads to a lot of <code class="highlighter-rouge"><s:set/></code> tags.</p>

	<blockquote>
	<p>Unfortunately, it isn’t that simple. we tinkered with <code class="highlighter-rouge">JSPFactory.setDefault()</code> to wrap around <code class="highlighter-rouge">getPageContext()</code>
	and create <code class="highlighter-rouge">ExpressionEvaluator</code> that would use OGNL. This strategy works in practice, but code generated by Jasper2
	doesn’t call <code class="highlighter-rouge">JSPFactory.getPageContext().getExpressionEvaluator()</code> but goes directly to static method that is
	hardwired to Jakarta Commons-EL implementation.</p>
	</blockquote>

	<blockquote>
	<table>
	<tbody>
	<tr>
	<td>Even if this approach did work, it wouldn’t be <em>clean</em> as <code class="highlighter-rouge">JSPFactory.setDefault()</code> should only be called by JSP implementation.</td>
	</tr>
	</tbody>
	</table>
	</blockquote>

	<p>There is a simple, if not elegant, solution available in JSP 2.0 EL, for exposing ValueStack to OGNL. It is possible
	to create custom functions that can be called from EL expressions. Functions have to be ‘public static’ and specified
	in a TLD file.</p>

	<p>To use a function, import the TLD in a JSP file where you’ve want to use a function. For example, you could access
	Action properties by evaluating OGNL expression by a function ‘vs’ (for valuestack) in EL.</p>

	<pre><code class="language-jsp"><%@ taglib uri="/WEB-INF/tld/wwel.tld" prefix="x" %>

	a[0] = ${x:vs('a[0]')}
	a[0] * 4 = ${x:vs('a[0] * 4')}

	Current action name: ${x:name()}
	Top of ValueStack: ${x:top()}
	</code></pre>

	<p>To use this code you’ve got to add <code class="highlighter-rouge">wwel.tld</code> and <code class="highlighter-rouge">Functions.java</code> to your webapp project.</p>

	<blockquote>
	<p>If someone were interested, it would be helpful for a developer (like you!) to define a set of functions that we could
	include in a future release of the framework.</p>
	</blockquote>

	<p><strong>wwel.tld</strong></p>

	<div class="language-xml highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="cp"><?xml version="1.0"?></span>
	<span class="nt"><taglib</span> <span class="na">xmlns=</span><span class="s">"http://java.sun.com/xml/ns/j2ee"</span>
	<span class="na">xmlns:xsi=</span><span class="s">"http://www.w3.org/2001/XMLSchema-instance"</span>
	<span class="na">xsi:schemaLocation=</span><span class="s">"http://java.sun.com/xml/ns/j2ee http://java.sun.com/xml/ns/j2ee/web-jsptaglibrary_2_0.xsd"</span>
	<span class="na">version=</span><span class="s">"2.0"</span><span class="nt">></span>

	<span class="nt"><description></span>
	This taglib enables access to the ValueStack
	from JSP 2.0 Expression Language
	<span class="nt"></description></span>

	<span class="nt"><tlib-version></span>1.0<span class="nt"></tlib-version></span>

	<span class="nt"><short-name></span>wwel<span class="nt"></short-name></span>

	<span class="nt"><function></span>
	<span class="nt"><name></span>vs<span class="nt"></name></span>
	<span class="nt"><function-class></span>com.nmote.wwel.Functions<span class="nt"></function-class></span>
	<span class="nt"><function-signature></span>
	java.lang.Object findOnValueStack(java.lang.String)
	<span class="nt"></function-signature></span>
	<span class="nt"></function></span>

	<span class="nt"><function></span>
	<span class="nt"><name></span>name<span class="nt"></name></span>
	<span class="nt"><function-class></span>com.nmote.wwel.Functions<span class="nt"></function-class></span>
	<span class="nt"><function-signature></span>
	java.lang.Object getActionName()
	<span class="nt"></function-signature></span>
	<span class="nt"></function></span>

	<span class="nt"><function></span>
	<span class="nt"><name></span>top<span class="nt"></name></span>
	<span class="nt"><function-class></span>com.nmote.wwel.Functions<span class="nt"></function-class></span>
	<span class="nt"><function-signature></span>
	java.lang.Object getTopOfValueStack()
	<span class="nt"></function-signature></span>
	<span class="nt"></function></span>

	<span class="nt"></taglib></span>
	</code></pre></div></div>

	<p><strong>Functions.java</strong></p>

	<div class="language-java highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="kn">package</span> <span class="n">com</span><span class="o">.</span><span class="na">nmote</span><span class="o">.</span><span class="na">wwel</span><span class="o">;</span>

	<span class="kn">import</span> <span class="nn">com.opensymphony.xwork.ActionContext</span><span class="o">;</span>

	<span class="cm">/**
	* Utility functions for accessing value stack and action context
	* from JSP 2.0 EL taglibs.
	*/</span>
	<span class="kd">public</span> <span class="kd">class</span> <span class="nc">Functions</span> <span class="o">{</span>

	<span class="kd">public</span> <span class="kd">static</span> <span class="n">Object</span> <span class="nf">findOnValueStack</span><span class="o">(</span><span class="n">String</span> <span class="n">expr</span><span class="o">)</span> <span class="o">{</span>
	<span class="n">ActionContext</span> <span class="n">a</span> <span class="o">=</span> <span class="n">ActionContext</span><span class="o">.</span><span class="na">getContext</span><span class="o">();</span>
	<span class="n">Object</span> <span class="n">value</span> <span class="o">=</span> <span class="n">a</span><span class="o">.</span><span class="na">getValueStack</span><span class="o">().</span><span class="na">findValue</span><span class="o">(</span><span class="n">expr</span><span class="o">);</span>
	<span class="k">return</span> <span class="n">value</span><span class="o">;</span>
	<span class="o">}</span>

	<span class="kd">public</span> <span class="kd">static</span> <span class="n">Object</span> <span class="nf">getTopOfValueStack</span><span class="o">()</span> <span class="o">{</span>
	<span class="n">ActionContext</span> <span class="n">a</span> <span class="o">=</span> <span class="n">ActionContext</span><span class="o">.</span><span class="na">getContext</span><span class="o">();</span>
	<span class="n">Object</span> <span class="n">value</span> <span class="o">=</span> <span class="n">a</span><span class="o">.</span><span class="na">getValueStack</span><span class="o">().</span><span class="na">peek</span><span class="o">();</span>
	<span class="k">return</span> <span class="n">value</span><span class="o">;</span>
	<span class="o">}</span>

	<span class="kd">public</span> <span class="kd">static</span> <span class="n">Object</span> <span class="nf">getActionName</span><span class="o">()</span> <span class="o">{</span>
	<span class="n">ActionContext</span> <span class="n">a</span> <span class="o">=</span> <span class="n">ActionContext</span><span class="o">.</span><span class="na">getContext</span><span class="o">();</span>
	<span class="n">Object</span> <span class="n">value</span> <span class="o">=</span> <span class="n">a</span><span class="o">.</span><span class="na">getName</span><span class="o">();</span>
	<span class="k">return</span> <span class="n">value</span><span class="o">;</span>
	<span class="o">}</span>
	<span class="o">}</span>
	</code></pre></div></div>

	</section>
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