JSTL: check if property doesn't exist
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21-08-2019 - |
Question
I'm getting blocked on a jsp page and our 1 java engineer isn't able to help now.
There is a template called "module-review.jsp" that is loaded in 2 instances, via a normal page load an via an api that returns it as part of a json object.
There is a variable called "review.updatedDate". On a normal page view, this variable is loaded as a hashmap into the page and looks like this:
{_value=2009-07-02 11:54:30.0, class=sql-timestamp}
So if I want the date value, I use ${review.updatedDate._value}
However, when module-review.jsp is loaded by the API, the date value is returned directly as a date object, where ${review.updatedDate} returns the date value directly.
I need to have a set of conditional statements that will only display ${review.updatedDate} if ._value doesn't exist. Everything I have tried gives me errors that ._value doesn't exist, which is rather ironic.
I am currently trying to use this, but it fails on the 2nd conditional:
<c:if test="${ (not empty review.updatedDate['_value']) }">
${review.updatedDate._value}
</c:if>
<c:if test="${ (empty review.updatedDate['_value']) }">
${review.updatedDate}
</c:if>
Solution
Apart from "don't do it that way", I guess you could test the type of updatedDate:
<c:choose>
<c:when test="${review.updatedDate.class.name == 'java.util.Date'}">
Date: ${review.updatedDate}
</c:when>
<c:otherwise>
Map: ${review.updatedDate._value}
</c:otherwise>
</c:choose>
...assuming that the date is an instance of the Date class. Strangely, this approach didn't work when I tried to test for java.util.HashMap.
Perhaps a more reliable approach would be to hand the test off to a Java class:
package typetest;
import java.util.Map;
public class TypeUtil {
public static boolean isMap(Object o) {
return o instanceof Map;
}
}
This can be mapped to a custom function via a TLD file (e.g. WEB-INF/maptest.tld):
<?xml version="1.0" encoding="UTF-8"?>
<taglib xmlns="http://java.sun.com/xml/ns/javaee" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation="http://java.sun.com/xml/ns/javaee http://java.sun.com/xml/ns/javaee/web-jsptaglibrary_2_1.xsd"
version="2.1">
<tlib-version>1.0</tlib-version>
<short-name>myfn</short-name>
<uri>http://typeutil</uri>
<function>
<name>isMap</name>
<function-class>typetest.TypeUtil</function-class>
<function-signature>boolean isMap(java.lang.Object)</function-signature>
</function>
</taglib>
Sample JSP that imports the function:
<%@ taglib prefix="c" uri="http://java.sun.com/jsp/jstl/core"%>
<%@ taglib prefix="myfn" uri="http://typeutil"%>
<html>
<body>
<c:choose>
<c:when test="${myfn:isMap(review.updatedDate)}">
Map: ${review.updatedDate._value}
</c:when>
<c:otherwise>
Date: ${review.updatedDate}
</c:otherwise>
</c:choose>
</body>
</html>