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To access the ValueStack from third-party JSP taglibs, expose property values to JSP using the <s:set tag.

<s:set name="'a'" value="{ 1, 2, 3, 4 }" scope="request"/>

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
${expression}

can be used in a text or inside of tag attributes:

a[0] = ${a[0]}

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

In practice, several variables must be exposed to make effective use of third party taglibs like DisplayTag. Unfortunately, this approach leads to a lot of <ww:set/> tags.

Why can't we just replace EL with OGNL?
Unfortunately, it isn't that simple. we tinkered with JSPFactory.setDefault() to wrap around getPageContext() and create ExpressionEvaluator that would use OGNL. This strategy works in practice, but code generated by Jasper2 doesn't call JSPFactory.getPageContext().getExpressionEvaluator() but goes directly to static method that is hardwired to Jakarta Commons-EL implementation.

Even if this approach did work, it wouldn't be clean as JSPFactory.setDefault() should only be called by JSP implementation.

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.
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.

<%@ 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()}

To use this code you've got to add wwel.tld and Functions.java to your webapp project.

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.
wwel.tld
<?xml version="1.0"?>
<taglib xmlns="http://java.sun.com/xml/ns/j2ee"
	xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
	xsi:schemaLocation="http://java.sun.com/xml/ns/j2ee http://java.sun.com/xml/ns/j2ee/web-jsptaglibrary_2_0.xsd"
	version="2.0">

<description>
This taglib enables access to the ValueStack
from JSP 2.0 Expression Language
</description>

<tlib-version>1.0</tlib-version>

<short-name>wwel</short-name>

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

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

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

</taglib>
Functions.java
package com.nmote.wwel;

import com.opensymphony.xwork.ActionContext;

/**
 * Utility functions for accessing value stack and action context
 * from JSP 2.0 EL taglibs.
 */
public class Functions {

	public static Object findOnValueStack(String expr) {
		ActionContext a = ActionContext.getContext();
		Object value = a.getValueStack().findValue(expr);
		return value;
	}

	public static Object getTopOfValueStack() {
		ActionContext a = ActionContext.getContext();
		Object value = a.getValueStack().peek();
		return value;
	}

	public static Object getActionName() {
		ActionContext a = ActionContext.getContext();
		Object value = a.getName();
		return value;
	}
}