How to do numeric comparison but return false if can't parse to number?

Question

Broadly: I want the test in a JSTL Core <c:when> tag to return false if either:
- A variable can't be parsed as a number; or
- Comparison of the same variable to a number-literal is false.
I know that the variable won't be parsable as a number in some cases; this should not cause an error.

Details of the use-case follow...

I have the following in a JSP file on a WebSphere Portal v7 server. This JSP is rendered by a Web Content portlet configured to use an IBM Web Content Manager JSP component.

<%@ page session="false" buffer="none" %>
<%@ page trimDirectiveWhitespaces="true" %>
<%@ taglib uri="http://java.sun.com/jsp/jstl/core" prefix="c" %>
<%@ taglib uri="http://www.ibm.com/xmlns/prod/websphere/portal/v7.0/json" prefix="json" %>
<%@ taglib uri="http://www.ibm.com/xmlns/prod/websphere/portal/v7.0/portal-fmt" prefix="portal-fmt" %>
<%@ taglib uri="http://www.ibm.com/xmlns/prod/websphere/portal/v7.0/portal-core" prefix="portal-core" %>
<%@ taglib uri="http://www.ibm.com/xmlns/prod/websphere/portal/v7.0/portal-navigation" prefix="portal-navigation" %>
<%@ page import="com.isw.portal.theme.SideNav" %> <%!
    SideNav iswSideNav=SideNav.getInstance();
%>
<portal-navigation:navigation startLevel="${navTabsLevel}" stopLevel="${navTabsLevel+3}">
<%=iswSideNav.getNavHTML(wpsNavModel,wpsSelectionModel,request,response) %>
</portal-navigation:navigation>

This works consistently on normal page views.
However, when the Portal Search collection is being updated (which occurs automatically once every 6 hours and takes about 2 minutes), this JSP produces several exceptions every second.
The exceptions are always the same two repeated as below. The second exception always includes a stack trace, which I've omitted except for the line stating the NumberFormatException message.

NavigationTag E com.ibm.wps.engine.tags.NavigationTag setStartLevel EJPEJ0027E: StartLevel less than 1 is ignored.
NavigationTag E com.ibm.wps.engine.tags.NavigationTag setStartLevel EJPEJ0026E: StartLevel is not a valid number.
    java.lang.NumberFormatException: For input string: ""

As these exceptions don't seem to cause any functional problems, I want to wrap the <portal-navigation:navigation> element inside a <c:choose> element, such that the navigation is rendered when navTabsLevel is parsable as a number and that number is >= 1, but otherwise show a 1-line warning.
How do I do this without causing a "string cannot be parsed to number" error?

Answer 1

You can use <c:catch> for that.

<c:catch var="exception">
    <portal-navigation:navigation startLevel="${navTabsLevel}" ... />
</c:catch>
<c:if test="${not empty exception}">
    Handle fail.
</c:if>

Alternatively, and better, create a custom EL function like matches(), isNumber(), etc.

<c:choose>
    <c:when test="${my:isNumber(navTabsLevel)}">
        <portal-navigation:navigation startLevel="${navTabsLevel}" ... />
    </c:when>
    <c:otherwise>
        Handle fail.
    </c:otherwise>
</c:choose>

There's at least nothing available in standard JSTL functions set for that.