Is there any good reason to use <rtexprvalue>false</rtexprvalue> in JSP tags?

Question

Is there any good reason to disallow scriptlet or EL expression to be inserted as attribute value?

Let's say we have tag:

<tag>
    <name>mytag</name>
    <tag-class>org.apache.beehive.netui.tags.tree.Tree</tag-class>
    <attribute>
        <name>attr</name>
        <required>false</required>
        <rtexprvalue>false</rtexprvalue>
        <type>boolean</type>
    </attribute>
</tag>

What could be a good reason for dissallowing the below?

<my:mytag attr="${setting}" />

Answer 1

I'd say it is mostly a backwards-compatibility measure, just like the ability to turn off EL completely for a given JSP.

Maybe the tag library existed before EL, and uses the special syntax ${} for its own purposes. Maybe the attribute value frequently takes values that include a literal ${}.

Without such a setting, existing code (in the tag library or the JSP) would need to be modified in order to still work after upgrading to the latest version of the Servlet spec.

Answer 2

There might potentially be attributes (eg. id to refer to the tag itself) that it's not suitable to set at run-time. You could probably come up with a scenario where you really wanted ${} to mean plain text, or where any EL content was evaluated manually in some kind of custom context.

But I think the real reason for the switch is backwards compatibility. Custom tags predate JSP's built-in support for the EL; changing the definition underneath a taglib so that ${} now has a special meaning is likely to break existing templates' use of that tag. Safer to default to rtexprvalue-false for back-compat and let new taglibs for JSP 2.0 specify they want the new behaviour.

[yes. what Thilo said. :-)]