Question

How can I validate if a String is null or empty using the c tags of JSTL?

I have a variable named var1 and I can display it, but I want to add a comparator to validate it.

<c:out value="${var1}" />

I want to validate when it is null or empty (my values are string type).

Answer 1

How can I validate if a String is null or empty using the c tags of JSTL?

You can use the empty keyword in a <c:if> for this:

<c:if test="${empty var1}">
    var1 is empty or null.
</c:if>
<c:if test="${not empty var1}">
    var1 is NOT empty or null.
</c:if>

Or the <c:choose>:

<c:choose>
    <c:when test="${empty var1}">
        var1 is empty or null.
    </c:when>
    <c:otherwise>
        var1 is NOT empty or null.
    </c:otherwise>
</c:choose>

Or if you don’t need to conditionally render a bunch of tags and thus you could only check it inside a tag attribute, then you can use the EL conditional operator ${condition? valueIfTrue : valueIfFalse}:

<c:out value="${empty var1 ? 'var1 is empty or null' : 'var1 is NOT empty or null'}" />

To learn more about those ${} things (the Expression Language, which is a separate subject from JSTL), check here.

See also:

• How does EL empty operator work in JSF?