Validating Null and Empty Strings with JSTL c:if, c:choose, and EL Ternary Operator

In web application development using JavaServer Pages (JSP), it’s common to dynamically generate content based on the values of variables. Often, developers need to validate whether a string variable is null or empty before proceeding with further processing or rendering. JavaServer Pages Standard Tag Library (JSTL) provides several mechanisms to perform such validations effectively.

Introduction to JSTL and EL

Before diving into validation techniques, it’s crucial to understand the role of JSTL and Expression Language (EL). JSTL simplifies JSP page development by providing a collection of tags that encapsulate core functionalities. EL is used within JSTL tags to evaluate expressions against variables in JavaBeans components.

Using JSTL c:if for Validation

The <c:if> tag allows you to conditionally execute parts of your JSP code based on the evaluation of an expression. To check if a string variable, say var1, is null or empty, you can use the EL empty operator within <c:if>:

<c:if test="${empty var1}">
    var1 is empty or null.
</c:if>

Here, ${empty var1} evaluates to true if var1 is either null or an empty string (""). Conversely, you can check for non-null and non-empty strings using:

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

Utilizing JSTL c:choose for More Complex Logic

For more complex conditional logic, <c:choose>, along with <c:when> and <c:otherwise>, provides a structured approach:

<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>

This structure is similar to a switch-case statement in Java, where <c:when> evaluates conditions and <c:otherwise> serves as the default case.

Handling Blank Strings with JSTL Functions

Sometimes, you need to consider strings that contain only whitespace characters (e.g., spaces or tabs) as empty. To address this, use the fn:trim function from JSTL’s functions library:

<%@ taglib prefix="fn" uri="http://java.sun.com/jsp/jstl/functions"%>
<c:if test="${empty fn:trim(var1)}">
    var1 is empty or null (considering whitespace).
</c:if>

Using the EL Ternary Operator

For inline evaluations within tags, such as <c:out>, the ternary operator in EL can be used to conditionally display messages:

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

This expression succinctly checks if var1 is empty and outputs the appropriate message.

The c:out Tag with Default Values

The <c:out> tag can also be used to handle null values by specifying a default value:

<c:out default="var1 is empty or null." value="${var1}" />

If var1 is null, it outputs the specified default message.

Regular Expressions for Comprehensive Checks

For comprehensive checks that include strings with only whitespace, you can employ regular expressions. Here’s how you might use them:

<%@ taglib uri="http://java.sun.com/jsp/jstl/functions" prefix="fn" %>
<c:if test="${not empty description}">
    <c:set var="description" value="${fn:replace(description, ' ', '')}" />
    <c:if test="${not empty description}">
        The description is not blank.
    </c:if>
</c:if>

This example removes all spaces and checks if the resulting string is non-empty.

Conclusion

Validating null or empty strings in JSP using JSTL and EL provides flexibility and clarity. Whether you use <c:if>, <c:choose>, or inline ternary operators, understanding these tools enhances your ability to manage dynamic content effectively. Always consider edge cases like whitespace-only strings and incorporate regular expressions when necessary to ensure robust validation.