Access to Webwork objects from JSP 2.0 EL

To access Webwork ValueStack from third party JSP taglibs you have to expose property values to JSP.

You can use Webwork2 tag <ww:set/> to set named parameter in a JSP page, request, session or application scope. Following example, sets a request scoped parameter 'a' to list of integers:

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

After setting parameter, third party JSP taglibs can access variables, or you can use JSP 2.0 EL (Expression Language). This is convenient as short hand EL expression syntax
{{$

Unknown macro: {expression}

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

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

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

In practice, you've got to expose a lot of different variables to make effective use of third party taglibs like displaytag or wurfl. This leads to a lot of <ww:set/> tags what made me investigate how to make access to ValueStack and OGNL more transparent.

Why can't we just replace EL with OGNL?

Unfortunately, it isn't that simple. I've tinkered with JSPFactory.setDefault() to wrap around getPageContext() and create ExpressionEvaluator that would use OGNL.

This 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 it would 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.
Just import 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.

I would urge webworkers to define a set of functions that would be usable to wide community and include this in some future Webwork release.

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 WebWork2 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 webwork value stack and action context 
 * from JSP 2.0 EL taglibs.
 * 
 * @author Vjekoslav Nesek (vnesek@nmote.com)
 */
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;
	}	
}

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Access to Webwork objects from JSP 2.0 EL