You could define a TLD file "myFunctions.tld" containing this:
<function>
<name>abs</name>
<function-class>java.lang.Math</function-class>
<function-signature>double abs (double)</function-signature>
</function>
and then write this in your
JSP:
<%@ taglib prefix="math" uri="/WEB-INF/myFunctions.tld" %>
${math:abs(myDoubleVariable)}