How to use JSTL with Struts2 ?

Answer 1

To be more comfortable with Struts2 tags and OGNL language, read and bookmark this answer.

Since Struts2 tags only evaluate OGNL expressions (and not EL Expression, as you error clearly states), you need to access the JSTL object through the PageContext attribute (in OGNL #attr.something) specified with var :

<c:forEach items="${breadcrumb.links}" var="link">
    <s:url action='%{#attr.link.url}' var="url" />
    <li>
        <a href="${url}">${link.name}</a>
    </li>
</c:forEach>

From OGNL Basics:

#attr['foo'] or #attr.foo : Access to PageContext if available, otherwise searches request/session/application respectively