How to show a alert and then forward to an Action class with Struts?

Hello!

I hope some one got an idea how to solve this problem.
(Btw I use Struts 1.3.10 in a war.)

I got this task, an action-class (StartAction) is checking for a value.
If value exist then redirect to success, else show an alert, like:

        if(valueFomDataBase==null)
        {
            return mapping.findForward("alertmessage");
        }
        else
        {
            return mapping.findForward("success");
        }

I did a quick sketch of struts-config.xml to test this, looking like:

    <action-mappings>
        <action path="/helloWorld" type="strutsexample.action.StartAction" name="helloWorldForm">
            <forward name="success" path="/index.jsp"/>
            <forward name="alertmessage" path="/alertmessage.jsp"/>
        </action>
       
        <action path="/setValue" type="strutsexample.action.SetValueAction" name="helloWorldForm">
            <forward name="success" path="/setvalue.jsp"/>
        </action>
       
        <action path="/done" type="strutsexample.action.DoneAction" name="helloWorldForm">
            <forward name="success" path="/done.jsp"/>
        </action>
    </action-mappings>

The tricky part is how to create the alertmessage.jsp, it should just be a blank page but first showing a javascript-alert saying something like "A value is missing, please click ok and fill in"

When user clicks the ok-button at the alert-message the page should forward or redirect to "/setValue"

If someone got a nice idea how to solve this, perhaps with just pure javascript or in some nice struts-pattern please let me know.

I have been googling but I just find javascripts the shows alert when user submits a form.

Best regards
Fredrik