Tomcat provides a framework for implementing tag plugins. The plugins instruct Jasper, at translation time, to replace tag handler calls with Java scriptlets. The framework allows tag library authors to implement plugins for their tags.
Tomcat is released with plugins for several JSTL tags. Note that these plugins work with JSTL 1.1 as well as JSTL 1.0, though the examples uses JSTL 1.1 and JSP 2.0. These plugins are not complete (for instance, some item types are not handled in <c:if>). They do serve as examples to show plugins in action (just examine the generated Java files), and how they can be implemented.